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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
After my airconditioner failed the other day, and being reluctant to pay
a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. |
#2
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. As drawn, Q1 is intended to limit current in the 8R2 emitter resistor, when the main switch is functional. The event that killed the main switch and fused the emitter resistor, could have fried Q1 in the process. I think you'll find that Q1 is intentionally inverted (collector and emitter swapped). When it functions, it resembles a temperature-dependant voltage reference of ~ 1V4. This is one of the simpler self-oscillating flyback circuits popular below 25W (in switch-mode power supplies) since about 1970. Without the appropriate tools, training and test procedures, you cannot ensure that this unit, even restored to an apparently functional condition, is safe to use. If you know the supply's output requirements, you would probably be better off replacing the whole assembly. RL |
#3
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
legg wrote:
On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. As drawn, Q1 is intended to limit current in the 8R2 emitter resistor, when the main switch is functional. The event that killed the main switch and fused the emitter resistor, could have fried Q1 in the process. I think you'll find that Q1 is intentionally inverted (collector and emitter swapped). When it functions, it resembles a temperature-dependant voltage reference of ~ 1V4. This is one of the simpler self-oscillating flyback circuits popular below 25W (in switch-mode power supplies) since about 1970. Without the appropriate tools, training and test procedures, you cannot ensure that this unit, even restored to an apparently functional condition, is safe to use. If you know the supply's output requirements, you would probably be better off replacing the whole assembly. RL The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. Sylvia. |
#4
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Sylvia Else wrote:
legg wrote: On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. As drawn, Q1 is intended to limit current in the 8R2 emitter resistor, when the main switch is functional. The event that killed the main switch and fused the emitter resistor, could have fried Q1 in the process. I think you'll find that Q1 is intentionally inverted (collector and emitter swapped). When it functions, it resembles a temperature-dependant voltage reference of ~ 1V4. This is one of the simpler self-oscillating flyback circuits popular below 25W (in switch-mode power supplies) since about 1970. Without the appropriate tools, training and test procedures, you cannot ensure that this unit, even restored to an apparently functional condition, is safe to use. If you know the supply's output requirements, you would probably be better off replacing the whole assembly. RL The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. Sylvia. Hmm.... I further surmise that the failure of Q1 doesn't explain the failure of the 8.2 Ohm resistor, because even with its specified maximum beta of 40, the switching transistor wouldn't pass enough current to burn out the resistor. The implication is that some other event has done for the both of them, but surprisingly not destroyed the switching transistor. sigh I thought I'd understood the sequence of events, but clearly not Sylvia. |
#5
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"Sylvia Else" The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. I further surmise that the failure of Q1 doesn't explain the failure of the 8.2 Ohm resistor, because even with its specified maximum beta of 40, the switching transistor wouldn't pass enough current to burn out the resistor. The implication is that some other event has done for the both of them, but surprisingly not destroyed the switching transistor. sigh I thought I'd understood the sequence of events, but clearly not ** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible. Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession. Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer). Then cross you fingers and try it again. If all is well, coat the PCB is clear acrylic lacquer to help it survive. ...... Phil |
#6
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Phil Allison wrote:
"Sylvia Else" The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. I further surmise that the failure of Q1 doesn't explain the failure of the 8.2 Ohm resistor, because even with its specified maximum beta of 40, the switching transistor wouldn't pass enough current to burn out the resistor. The implication is that some other event has done for the both of them, but surprisingly not destroyed the switching transistor. sigh I thought I'd understood the sequence of events, but clearly not ** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible. Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession. Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer). Then cross you fingers and try it again. If all is well, coat the PCB is clear acrylic lacquer to help it survive. It's true that I mentioned brushing away a dead spider. I wish I'd noted exactly where the spider was. It conceivably was around the 200K resistor, which could explain both the failures, and the demise of the spider. Anyone know the resistance of a small spider before it dies from shock? Sylvia. |
#7
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"Sylvia Else" Phil Allison wrote: ** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible. Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession. Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer). Then cross you fingers and try it again. If all is well, coat the PCB is clear acrylic lacquer to help it survive. It's true that I mentioned brushing away a dead spider. I wish I'd noted exactly where the spider was. It conceivably was around the 200K resistor, which could explain both the failures, and the demise of the spider. Anyone know the resistance of a small spider before it dies from shock? ** If you see one with three red stripes across its abdomen - means it is 2.2 kohms, +/- 20 % ............. ...... Phil |
#8
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"Phil Allison" wrote in message ... "Sylvia Else" Phil Allison wrote: ** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible. Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession. Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer). Then cross you fingers and try it again. If all is well, coat the PCB is clear acrylic lacquer to help it survive. It's true that I mentioned brushing away a dead spider. I wish I'd noted exactly where the spider was. It conceivably was around the 200K resistor, which could explain both the failures, and the demise of the spider. Anyone know the resistance of a small spider before it dies from shock? ** If you see one with three red stripes across its abdomen - means it is 2.2 kohms, +/- 20 % ............. Power rating? And what's the pinout? Dave. |
#9
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
David L. Jones wrote:
"Phil Allison" wrote in message ... "Sylvia Else" Phil Allison wrote: ** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible. Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession. Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer). Then cross you fingers and try it again. If all is well, coat the PCB is clear acrylic lacquer to help it survive. It's true that I mentioned brushing away a dead spider. I wish I'd noted exactly where the spider was. It conceivably was around the 200K resistor, which could explain both the failures, and the demise of the spider. Anyone know the resistance of a small spider before it dies from shock? ** If you see one with three red stripes across its abdomen - means it is 2.2 kohms, +/- 20 % ............. Power rating? And what's the pinout? The power rating is certainly an issue. At 2.2 kohms, it would be dissipating about 50W. I would expect to find it splattered around PS cover, an effect which is noticeably absent. At higher resistances, significantly more power would be dissipated in it than in the 8.2 ohm resistor. It's hard to see how the resistor can burn out without there being obvious signs of arachnid distress. Sylvia. |
#10
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"David L. Jerkoff" ** If you see one with three red stripes across its abdomen - means it is 2.2 kohms, +/- 20 % ............. Power rating? ** Depends on the size of the damn spider - ****wit. And what's the pinout? ** Mobile octal - of course. ..... Phil |
#11
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Sylvia Else wrote in message
u... After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. Someone else , last month, asked a similar question here.I have now found the text book that I found useful for understanding SMPSs Simplified design of Switching Power Supplies by John D Lenk, 1995 -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#12
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
N_Cook wrote:
Sylvia Else wrote in message u... After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. Someone else , last month, asked a similar question here.I have now found the text book that I found useful for understanding SMPSs Simplified design of Switching Power Supplies by John D Lenk, 1995 Thanks for the reference. Sylvia. |
#13
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
On Sun, 04 Jan 2009 14:41:04 +1100, Sylvia Else
wrote: legg wrote: On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. As drawn, Q1 is intended to limit current in the 8R2 emitter resistor, when the main switch is functional. The event that killed the main switch and fused the emitter resistor, could have fried Q1 in the process. I think you'll find that Q1 is intentionally inverted (collector and emitter swapped). When it functions, it resembles a temperature-dependant voltage reference of ~ 1V4. This is one of the simpler self-oscillating flyback circuits popular below 25W (in switch-mode power supplies) since about 1970. Without the appropriate tools, training and test procedures, you cannot ensure that this unit, even restored to an apparently functional condition, is safe to use. If you know the supply's output requirements, you would probably be better off replacing the whole assembly. RL The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. Sylvia. You seem to have access to some tools and a fair understanding of basic electronics. There's nothing preventing you from undertaking a repair by component substitution, for your own interest's sake, providing you are aware of, and use, use safe procedures around powered circuitry. This assembly was designed to fail in a safe manner - no fire, explosion or shock hazard was intended to result from a single fault resulting in the fusing of the emitter resistor. However, this does not mean that the repaired unit will meet the same standards. The events that occurred may have affected the safety isolation of magnetic isolators in such a way that a second similar event may not be as benign. If the main switch is not damaged (a big if), damage to the magnetic component's internal insulation is also unlikely. With the introduction of foreign particles, anything is possible. It should at least be subjected to a basic hipot stress test procedure, after the repaired and cleaned assembly is burned in and still warm. RL |
#14
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"No legg to stand on " This assembly was designed to fail in a safe manner - no fire, explosion or shock hazard was intended to result from a single fault resulting in the fusing of the emitter resistor. However, this does not mean that the repaired unit will meet the same standards. The events that occurred may have affected the safety isolation of magnetic isolators in such a way that a second similar event may not be as benign. ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! Internal insulation is functional ONLY !!! So fuuuuuuccckk ooofffffff IMBECILE ...... Phil |
#15
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Sylvia Else wrote:
After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. http://webpages.charter.net/jamie_5" |
#16
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Sylvia Else wrote:
Phil Allison wrote: "Sylvia Else" The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. I further surmise that the failure of Q1 doesn't explain the failure of the 8.2 Ohm resistor, because even with its specified maximum beta of 40, the switching transistor wouldn't pass enough current to burn out the resistor. The implication is that some other event has done for the both of them, but surprisingly not destroyed the switching transistor. sigh I thought I'd understood the sequence of events, but clearly not ** In an earlier post you state the PCB had been subjected to water ingress and insect attack - so almost anything is possible. Odds are, the switching transistor was forced hard on by the above, taking out the 8.2 ohms and Q1 in quick succession. Relace whatever parts you find are damaged, clean the PCB carefully with detergent and a brush (as you would washing glass ware etc in the sink ) rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair dryer). Then cross you fingers and try it again. If all is well, coat the PCB is clear acrylic lacquer to help it survive. It's true that I mentioned brushing away a dead spider. I wish I'd noted exactly where the spider was. It conceivably was around the 200K resistor, which could explain both the failures, and the demise of the spider. Anyone know the resistance of a small spider before it dies from shock? Sylvia. To me it looks like a safety circuit that is designed that requires you to pull the plug , so that Q1 being a thyristor in my case of thinking will clamp down on the circuit from an over voltage, and remain that way until you pull the plug and wait. The 8.2R could just be burnt out or it maybe located in a thermo area intentionally to burn out (thermo Fuse) in which case, Q1 would also clamp. Either scenario will prevent the coil to be energized, if that is what T1 is? Since you haven't supplied any component part numbers, it's hard to say really, if that is what's going on. -- "I'd rather have a bottle in front of me than a frontal lobotomy" http://webpages.charter.net/jamie_5" |
#17
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"Sylvia Else" wrote in message u... After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. **What are the type numbers of the two transistors? -- Trevor Wilson www.rageaudio.com.au |
#18
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"Trevor Wilson" wrote in message ... "Sylvia Else" wrote in message u... After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. **What are the type numbers of the two transistors? **Scratch that. You already told me. -- Trevor Wilson www.rageaudio.com.au |
#19
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
On Sun, 04 Jan 2009 16:05:58 +1100, Sylvia Else
wrote: Sylvia Else wrote: legg wrote: On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. As drawn, Q1 is intended to limit current in the 8R2 emitter resistor, when the main switch is functional. The event that killed the main switch and fused the emitter resistor, could have fried Q1 in the process. I think you'll find that Q1 is intentionally inverted (collector and emitter swapped). When it functions, it resembles a temperature-dependant voltage reference of ~ 1V4. This is one of the simpler self-oscillating flyback circuits popular below 25W (in switch-mode power supplies) since about 1970. Without the appropriate tools, training and test procedures, you cannot ensure that this unit, even restored to an apparently functional condition, is safe to use. If you know the supply's output requirements, you would probably be better off replacing the whole assembly. RL The main switching transistor appears to be intact. I haven't removed it from the circuit, but the voltage/current plot given by a component tester feature on my oscilloscope gives the expected traces for the base collector and base emitter junctions, allowing for the presence of the diode across the base emitter junction. Sylvia. Hmm.... I further surmise that the failure of Q1 doesn't explain the failure of the 8.2 Ohm resistor, because even with its specified maximum beta of 40, the switching transistor wouldn't pass enough current to burn out the resistor. The implication is that some other event has done for the both of them, but surprisingly not destroyed the switching transistor. sigh I thought I'd understood the sequence of events, but clearly not Sylvia. In the self-oscillatin flyback, base current is supplied regeneratively by the feedback winding, to provide over-drive for saturated switching. Beta is assumed to be somewhere between 4 and 12. Note that the drive connection is drawn across the EB jn, so fusing of the resistor would not turn the switch off - the voltage on the emitter and base terinals could then be quite large - popping locally connected parts like Q1. Some re-examination of the actual board connections and further examination of the feedback section (usually not very elaborate) may offer further insight. RL |
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Understanding a split-mode power supply.
Jamie wrote:
Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Sylvia. |
#21
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Understanding a split-mode power supply.
Phil Allison wrote:
"No legg to stand on " This assembly was designed to fail in a safe manner - no fire, explosion or shock hazard was intended to result from a single fault resulting in the fusing of the emitter resistor. However, this does not mean that the repaired unit will meet the same standards. The events that occurred may have affected the safety isolation of magnetic isolators in such a way that a second similar event may not be as benign. ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! Leaving aside the abuse, I confess this was my sentiment as well. And not only that, neither the outside nor inside units even get touched in the normal way of things. The entire board could go up in flames, if it were inflammable, and still cause nothing more than a bad smell. Since it's outside even the resulting fumes wouldn't be a threat. Sylvia. |
#22
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Understanding a split-mode power supply.
On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else
wrote: Jamie wrote: Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Sylvia. Looking at the package's flat surface, with leads pointing down, the pin-out of 2SC1815 is ECB. RL |
#23
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Understanding a split-mode power supply.
Sylvia Else wrote:
Jamie wrote: Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Sylvia. Ok, now we're getting some where. Just replace the 8.2R and transistor.. The tranny went because it's only rated for 50 Vceo, also the the Vbe was taking a little hard this way with that 200KR driving it through the 560R. when the 8.2R opened, it allowed the voltage to get high and simply shorted it. That's my story and I'm sticking to it! The larger transistor is most likely ok. http://webpages.charter.net/jamie_5" |
#24
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Understanding a split-mode power supply.
"legg" wrote in message ... On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else wrote: Jamie wrote: Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Sylvia. Looking at the package's flat surface, with leads pointing down, the pin-out of 2SC1815 is ECB. RL i.e. absolutely standard pinning for just about any and all Japanese TO92 transistors starting 2SA, B, C, or D ... FWIW, the '1815 is about the commonest general purpose small signal NPN Jappo transistor to be found anywhere, and anything similarly general purpose will sub for it. It's generally not hard to rearrange the leads of a differently pinned device, with a bit of sleeving on one or two legs to stop them touching. Arfa |
#25
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Understanding a split-mode power supply.
On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else
wrote: The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Why stress? Replace with the same part# - WES have them in Sydney. |
#26
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Understanding a split-mode power supply.
Jamie wrote:
Sylvia Else wrote: Jamie wrote: Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Sylvia. Ok, now we're getting some where. Just replace the 8.2R and transistor.. The tranny went because it's only rated for 50 Vceo, also the the Vbe was taking a little hard this way with that 200KR driving it through the 560R. when the 8.2R opened, it allowed the voltage to get high and simply shorted it. That's my story and I'm sticking to it! Semi-plausible, though the small transistor should have been able to sink enough current through the 200K to prevent its Vce rising outside its limits. Sylvia. |
#27
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Understanding a split-mode power supply.
Arfa Daily wrote:
"legg" wrote in message ... On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else wrote: Jamie wrote: Sylvia Else wrote: After my airconditioner failed the other day, and being reluctant to pay a technician to come and fix it, I've been taking a look at its electronics board. It's clear that its power supply circuit has failed. From the board itself I've inferred this partial circuit: http://members.optusnet.com.au/sylviae/smps.jpg I am pretty sure there are no other components connected to the transistor labelled Q1, and it is this transistor that has failed. The failure mode is a short (a few ohms, polarity insensitive) from base to emitter. The collector is open circuit. The transistor is thus unable to sink enough current to prevent the switching transistor from turning on, and as a result the 8.2 Ohm fusible resistor has also failed. It seems moderately likely that by replacing these two components I can get the board working again. The circuit nevertheless puzzles me. The function of Q1 appears to be to bias the switching transistor. But this seems to rely somewhat on the characteristics of the two transistors, which I would have thought was asking for trouble. In particular, it looks to me as if Q1 could simply prevent the switching transistor from ever conducting, and nothing would happen. Is this an accepted technique? Or have I misunderstood the purpose of Q1? BTW, this is from the external unit of a nine year old Daikin split system. Sylvia. How about Q1 being a thyristor as a crow bar on the bias of that second transistor? In which case, your symbol is incorrect. And would show the reason why you're getting low ohm reading is my guess on what you call the base-emitter and Collector being opened which is actually the M1 terminal etc.. That's just a guess of course. The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. Sylvia. Looking at the package's flat surface, with leads pointing down, the pin-out of 2SC1815 is ECB. RL i.e. absolutely standard pinning for just about any and all Japanese TO92 transistors starting 2SA, B, C, or D ... Well, to avoid P&P costs, I need to use what I already have, or what I can buy from DSE or Jaycar. FWIW, the '1815 is about the commonest general purpose small signal NPN Jappo transistor to be found anywhere, and anything similarly general purpose will sub for it. It's generally not hard to rearrange the leads of a differently pinned device, with a bit of sleeving on one or two legs to stop them touching. Yes, I'd already concluded that given its position in the circuit, any vaguely similar type will suffice, and that rearranging the leads is practical, if not very elegant. I've obtained a suitable resistor (not fuisible, but OK to test the solution) and will give that a go later. Shame it's getting so hot outside. Sylvia. |
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Understanding a split-mode power supply.
On Mon, 05 Jan 2009 09:28:54 +1100, Sylvia Else
wrote: Phil Allison wrote: "No legg to stand on " This assembly was designed to fail in a safe manner - no fire, explosion or shock hazard was intended to result from a single fault resulting in the fusing of the emitter resistor. However, this does not mean that the repaired unit will meet the same standards. The events that occurred may have affected the safety isolation of magnetic isolators in such a way that a second similar event may not be as benign. ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! Leaving aside the abuse, I confess this was my sentiment as well. And not only that, neither the outside nor inside units even get touched in the normal way of things. The entire board could go up in flames, if it were inflammable, and still cause nothing more than a bad smell. Since it's outside even the resulting fumes wouldn't be a threat. Sylvia. I see that specific poster's dialog in your response, only. His actual postings get filtered autiomatically here. The unit is unlikely to go up in flames, for good reasons. As a technician, you'd be advised to follow industry standards in repair of any line-isolating assembly. One doesn't always know the n'th reason why something was done. A hipot is not impossible to arrange, locally, if you don't have the equipment yourself. RL |
#29
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Understanding a split-mode power supply.
legg wrote:
On Mon, 05 Jan 2009 09:28:54 +1100, Sylvia Else wrote: Phil Allison wrote: "No legg to stand on " This assembly was designed to fail in a safe manner - no fire, explosion or shock hazard was intended to result from a single fault resulting in the fusing of the emitter resistor. However, this does not mean that the repaired unit will meet the same standards. The events that occurred may have affected the safety isolation of magnetic isolators in such a way that a second similar event may not be as benign. ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! Leaving aside the abuse, I confess this was my sentiment as well. And not only that, neither the outside nor inside units even get touched in the normal way of things. The entire board could go up in flames, if it were inflammable, and still cause nothing more than a bad smell. Since it's outside even the resulting fumes wouldn't be a threat. Sylvia. I see that specific poster's dialog in your response, only. His actual postings get filtered autiomatically here. The unit is unlikely to go up in flames, for good reasons. As a technician, you'd be advised to follow industry standards in repair of any line-isolating assembly. One doesn't always know the n'th reason why something was done. A hipot is not impossible to arrange, locally, if you don't have the equipment yourself. RL My test repair with a non-fuisible resistor and an NPN transistor from my stock seems to have worked (rather to my surprise), so I'll have to source a fusible resistor now. I'll take your comment about a hi-pot on board, though in fact I've only worked on the high voltage side of the system - the transformer hasn't been touched. Still, I suppose insulation breakdown in the transformer could be implicated in the original failure. Sylvia. |
#30
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Understanding a split-mode power supply.
"Sylvia Else" "No legg to stand on " ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! My test repair with a non-fusible resistor and an NPN transistor from my stock seems to have worked (rather to my surprise), so I'll have to source a fusible resistor now. ** WES Components in Ashfield have them. Code: " 1F8.2 " for 24 cents each. I'll take your comment about a hi-pot on board, though in fact I've only worked on the high voltage side of the system - the transformer hasn't been touched. Still, I suppose insulation breakdown in the transformer could be implicated in the original failure. ** Ignore "legg" - he is an ignorant ****. There is no such industry practice as he alludes to in the repair of SMPSs incorporated inside products. Where the PSU is a module and plenty of stock is available at LOW cost to a repair organisation - of course they prefer to just replace it, no matter what the fault. Any excuse in the book is then dragged out to justify that. Where such replacement is NOT possible or would be uneconomic - it get repaired just like you have done. BTW: What does "legg" have to say about the way your SMPS was installed in the outside unit of an air-con with no protection against water ingress, condensation or invasion by any and all insect and rodent life ? What does that do for the " safety isolation " angle - eh ??? Big joke. ...... Phil |
#31
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Understanding a split-mode power supply.
Phil Allison wrote:
"Sylvia Else" "No legg to stand on " ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! My test repair with a non-fusible resistor and an NPN transistor from my stock seems to have worked (rather to my surprise), so I'll have to source a fusible resistor now. ** WES Components in Ashfield have them. Code: " 1F8.2 " for 24 cents each. I'll take your comment about a hi-pot on board, though in fact I've only worked on the high voltage side of the system - the transformer hasn't been touched. Still, I suppose insulation breakdown in the transformer could be implicated in the original failure. ** Ignore "legg" - he is an ignorant ****. There is no such industry practice as he alludes to in the repair of SMPSs incorporated inside products. Where the PSU is a module and plenty of stock is available at LOW cost to a repair organisation - of course they prefer to just replace it, no matter what the fault. Any excuse in the book is then dragged out to justify that. Where such replacement is NOT possible or would be uneconomic - it get repaired just like you have done. BTW: What does "legg" have to say about the way your SMPS was installed in the outside unit of an air-con with no protection against water ingress, condensation or invasion by any and all insect and rodent life ? What does that do for the " safety isolation " angle - eh ??? He might be unaware, since I posted that in a separate thread to different groups. That thread was basically an opportunity for me to winge about the quality of the circuit board, and give a heads up to anyone considering buying an A/C unit. I only posted this thread here when I had found something definitely wrong with the PS, other than that it simply didn't work. Legg FYI, that other thread referred to the fact that the board had obviously suffered somewhat from being inadequately protected from the environment and small creatures. I found a dead spider, some of its web, and a good deal of dirt on the board. http://members.optusnet.com.au/sylviae/cbCrop.jpg Sylvia. |
#32
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Understanding a split-mode power supply.
"Sylvia Else" "Sylvia Else" "No legg to stand on " Legg FYI, that other thread referred to the fact that the board had obviously suffered somewhat from being inadequately protected from the environment and small creatures. I found a dead spider, some of its web, and a good deal of dirt on the board. http://members.optusnet.com.au/sylviae/cbCrop.jpg ** Looks well cleaned up. Perfectly OK to use in any device that has only FUNCTIONAL insulation requirements. ( As opposed to "double insulation " or Class 2 safety insulation requirements ) ...... Phil |
#33
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Understanding a split-mode power supply.
On 2009-01-04, Sylvia Else wrote:
The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out. 2SC1815 http://www.toshiba.com/taec/componen...nc/50/6455.pdf |
#34
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Understanding a split-mode power supply.
Ok, as I mentioned elsewhere in this thread, I managed to get the A/C
working again by replacing Q1 and the 8.2 ohm resistor. Actually, I'd misread it, and when I looked more carefully, it realised it was 6.2 ohm, as is the other one on the board with the same markings (i.e., a not blown one). Nice standard value that. So now I have to replace the resistor with a fusible, but sourcing a 6.2 ohm fusible is problematic. The situation is not made any easier because if the switching transistor fails by shorting out, then after it blows the fusible will have 340V across it. From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt 340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3 the power dissipation, and will handle twice as much. Therefore the 4.7 ohm will go open circuit before the 1.5 ohm could get hot enough to be a problem. Does this stand up? Sylvia. |
#35
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Understanding a split-mode power supply.
On Mon, 05 Jan 2009 14:34:36 +1100, Sylvia Else
wrote: Phil Allison wrote: "Sylvia Else" "No legg to stand on " ** Jesus Christ - get ****ing real you asinine ****ing WOG ****** !! The WHOLE damn AC unit is metal encased, installed & MAINS earthed !!!! My test repair with a non-fusible resistor and an NPN transistor from my stock seems to have worked (rather to my surprise), so I'll have to source a fusible resistor now. ** WES Components in Ashfield have them. Code: " 1F8.2 " for 24 cents each. I'll take your comment about a hi-pot on board, though in fact I've only worked on the high voltage side of the system - the transformer hasn't been touched. Still, I suppose insulation breakdown in the transformer could be implicated in the original failure. ** Ignore "legg" - he is an ignorant ****. There is no such industry practice as he alludes to in the repair of SMPSs incorporated inside products. Where the PSU is a module and plenty of stock is available at LOW cost to a repair organisation - of course they prefer to just replace it, no matter what the fault. Any excuse in the book is then dragged out to justify that. Where such replacement is NOT possible or would be uneconomic - it get repaired just like you have done. BTW: What does "legg" have to say about the way your SMPS was installed in the outside unit of an air-con with no protection against water ingress, condensation or invasion by any and all insect and rodent life ? What does that do for the " safety isolation " angle - eh ??? He might be unaware, since I posted that in a separate thread to different groups. That thread was basically an opportunity for me to winge about the quality of the circuit board, and give a heads up to anyone considering buying an A/C unit. I only posted this thread here when I had found something definitely wrong with the PS, other than that it simply didn't work. Legg FYI, that other thread referred to the fact that the board had obviously suffered somewhat from being inadequately protected from the environment and small creatures. I found a dead spider, some of its web, and a good deal of dirt on the board. http://members.optusnet.com.au/sylviae/cbCrop.jpg Sylvia. Unless it was x-posted to s.e.d. I won't see it. It's also -10C here, with not much call for AC at the moment. I clean my central AC heat exchanger each fall - I assume that discrete units might benefit from similar treatment. I think you mentioned that the unit was 10 years old; can't have been too unsuitable an installation, if you're only having trouble with it now. Funny how people religiously clean their computer cpu heatsinks and fans, but expect AC units to clean themselves. Glad to hear you've had success reviving the unit.......fusible or flame-resistant resistors are not hard to get, with sources like DigiKey, on-line. By the way, what kind of scope were you using, that had Huntron-like branch stimulation capabilities? RL |
#36
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Understanding a split-mode power supply.
legg wrote:
By the way, what kind of scope were you using, that had Huntron-like branch stimulation capabilities? It was a low end Hameg 20Mhz that I bought in France about 25 years ago. Sylvia. |
#37
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Understanding a split-mode power supply.
"Sylvia Else" wrote in message u... Ok, as I mentioned elsewhere in this thread, I managed to get the A/C working again by replacing Q1 and the 8.2 ohm resistor. Actually, I'd misread it, and when I looked more carefully, it realised it was 6.2 ohm, as is the other one on the board with the same markings (i.e., a not blown one). Nice standard value that. So now I have to replace the resistor with a fusible, but sourcing a 6.2 ohm fusible is problematic. The situation is not made any easier because if the switching transistor fails by shorting out, then after it blows the fusible will have 340V across it. From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt 340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3 the power dissipation, and will handle twice as much. Therefore the 4.7 ohm will go open circuit before the 1.5 ohm could get hot enough to be a problem. Does this stand up? Sylvia. Are you absolutely sure that it is definitely 6.2 ohms ? That is a *very* odd value, particularly for a fusible type ... FWIW, I really don't think that the circuit would give a damn if you replaced it with a 6.8 ohm, which is a standard value. Did you actually measure the one that's ok with an accurate low ohms meter, and get a reading of 6.2 ? Seems to me that a grey band and a blue band might easily be misread one for the other, with some of the banding paints I've seen used over the years. I find that it is often very easy to misread red for orange or brown, especially if the resistor runs warm in normal use. Otherwise, if it definitely is 6.2 ohms, and you really want to replace it with exactly that value, your reasoning with making such a value in the way that you suggest, would be quite valid. Arfa |
#38
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Arfa Daily wrote:
"Sylvia Else" wrote in message u... Ok, as I mentioned elsewhere in this thread, I managed to get the A/C working again by replacing Q1 and the 8.2 ohm resistor. Actually, I'd misread it, and when I looked more carefully, it realised it was 6.2 ohm, as is the other one on the board with the same markings (i.e., a not blown one). Nice standard value that. So now I have to replace the resistor with a fusible, but sourcing a 6.2 ohm fusible is problematic. The situation is not made any easier because if the switching transistor fails by shorting out, then after it blows the fusible will have 340V across it. From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt 340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3 the power dissipation, and will handle twice as much. Therefore the 4.7 ohm will go open circuit before the 1.5 ohm could get hot enough to be a problem. Does this stand up? Sylvia. Are you absolutely sure that it is definitely 6.2 ohms ? That is a *very* odd value, particularly for a fusible type ... FWIW, I really don't think that the circuit would give a damn if you replaced it with a 6.8 ohm, which is a standard value. Did you actually measure the one that's ok with an accurate low ohms meter, and get a reading of 6.2 ? Seems to me that a grey band and a blue band might easily be misread one for the other, with some of the banding paints I've seen used over the years. I find that it is often very easy to misread red for orange or brown, especially if the resistor runs warm in normal use. Blue Red Gold Gold = 6.2 ohms, 5%. It is in the E24 series, but I've certainly never seen one before. When I made up an equivalent, my meter gave the same reading (allowing for tolerance) for the equivalent as it does for the identical resistor on the board. So, yes, I'm pretty sure. As for whether I could substitute a 6.8, maybe I could, though I'd have trouble getting one that has a 350 volt rating. The only supplier I know of that purports to have them doesn't give the rating. The suppliers that give ratings don't carry that value. If I understood the circuit better, I'd be more comfortable about changing the value. The mere fact that an unusual value has been used gives me pause - maybe it's the value that's required there. Even if another value worked, I'd not know the ramifications. Sylvia. |
#39
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
"Sylvia Else" As for whether I could substitute a 6.8, maybe I could, though I'd have trouble getting one that has a 350 volt rating. ** Huh ?????????? No need for that whatsoever. In operation, the resistor is not subjected to more than a few volts. ..... Phil |
#40
Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
Phil Allison wrote:
"Sylvia Else" As for whether I could substitute a 6.8, maybe I could, though I'd have trouble getting one that has a 350 volt rating. ** Huh ?????????? No need for that whatsoever. In operation, the resistor is not subjected to more than a few volts. As I commented earlier, if the switching transistor fails by shorting out, then the resistor will blow. After it's blown it'll have 340V across it. There's not much point in having a fuisible there if it remains conductive through insulation break down after it has fused. Sylvia. |
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