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#1
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RFG70N60 Mosfet testing...
I am trying to test two RFG70N60 Mosfets, with my meter set on testing
diodes one Mosfet comes up with a reading of 0.001 the other 0.003, this seems kind of low. These are both out of a UPS that is having problems, does anyone know what the readings for the Mosfets should be? I emailed Fairchild Semiconductors but they never replied. Thanks, -Landon |
#2
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Landon,
Put your meter on ohms and measure between the drain and the source. These are the righthand two leads. You should get a high reading, either no reading or high megohms. If not, it's shorted, the usual failure mode. John Musselman "Landon" wrote in message ... I am trying to test two RFG70N60 Mosfets, with my meter set on testing diodes one Mosfet comes up with a reading of 0.001 the other 0.003, this seems kind of low. These are both out of a UPS that is having problems, does anyone know what the readings for the Mosfets should be? I emailed Fairchild Semiconductors but they never replied. |
#3
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Hi, thanks for replying.
I found a procedure online somewhere that basically said the following: ------------------------------------------------------------------------ -Set meter to check diodes. -Put positive lead on the GATE, put negative lead on the SOURCE (this will charge the gate, before doing this the DRAIN and SOURCE pins should give a HIGH reading). -Then move the positve lead to the DRAIN, it should give a LOW reading. -Touch the GATE and SOURCE pins with finger tip, the gate should discharge, then the DRAIN and SOURCE pins should again give a HIGH reading. ------------------------------------------------------------------------ This is where I am stuck, the Mosfets are acting the way the test says that they should but I am used to seeing diodes reading at 0.5xx to 0.7xx, these are reading at 0.001 and 0.003, I'm not sure if this is too low or right where it should be. -Landon John Musselman wrote: Landon, Put your meter on ohms and measure between the drain and the source. These are the righthand two leads. You should get a high reading, either no reading or high megohms. If not, it's shorted, the usual failure mode. John Musselman "Landon" wrote in message ... I am trying to test two RFG70N60 Mosfets, with my meter set on testing diodes one Mosfet comes up with a reading of 0.001 the other 0.003, this seems kind of low. These are both out of a UPS that is having problems, does anyone know what the readings for the Mosfets should be? I emailed Fairchild Semiconductors but they never replied. |
#4
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Landon,
That's a cute procedure and may even work. In the "diode test" position, your DVM is generating a small current and reading what voltage is generated. So a diode will read out the voltage across the junction (the "forward voltage" or Vf) with this current passing through. In a diode, this tends to be 0.5 to 0.7 for a silicon diode, and varies little with the current. In a MOSFET, there isn't a diode junction. It's just like a resistor, so a low reading is a low resistance. (A low voltage is generated by the test current). The low number is OK, if you have, indeed, charged the gate. You should get a high reading (or overload, or infinity) when the gate is discharged or shorted to ground. John "Landon" wrote in message ... Hi, thanks for replying. I found a procedure online somewhere that basically said the following: ------------------------------------------------------------------------ -Set meter to check diodes. -Put positive lead on the GATE, put negative lead on the SOURCE (this will charge the gate, before doing this the DRAIN and SOURCE pins should give a HIGH reading). -Then move the positve lead to the DRAIN, it should give a LOW reading. -Touch the GATE and SOURCE pins with finger tip, the gate should discharge, then the DRAIN and SOURCE pins should again give a HIGH reading. ------------------------------------------------------------------------ This is where I am stuck, the Mosfets are acting the way the test says that they should but I am used to seeing diodes reading at 0.5xx to 0.7xx, these are reading at 0.001 and 0.003, I'm not sure if this is too low or right where it should be. -Landon |
#5
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Thank you for the reply...
I believe that answers my question (that the 0.001 to 0.003 readings are normal), and yes when the gate is discharged it is giving an infinity reading. I guess I will keep troubleshooting this UPS, my next guess is that the LM324 that is driving the two MOSFETS isn't giving them the correct waveform or isn't being told to give them the correct waveform. Thanks again, -Landon John Musselman wrote: Landon, That's a cute procedure and may even work. In the "diode test" position, your DVM is generating a small current and reading what voltage is generated. So a diode will read out the voltage across the junction (the "forward voltage" or Vf) with this current passing through. In a diode, this tends to be 0.5 to 0.7 for a silicon diode, and varies little with the current. In a MOSFET, there isn't a diode junction. It's just like a resistor, so a low reading is a low resistance. (A low voltage is generated by the test current). The low number is OK, if you have, indeed, charged the gate. You should get a high reading (or overload, or infinity) when the gate is discharged or shorted to ground. John "Landon" wrote in message ... Hi, thanks for replying. I found a procedure online somewhere that basically said the following: ------------------------------------------------------------------------ -Set meter to check diodes. -Put positive lead on the GATE, put negative lead on the SOURCE (this will charge the gate, before doing this the DRAIN and SOURCE pins should give a HIGH reading). -Then move the positve lead to the DRAIN, it should give a LOW reading. -Touch the GATE and SOURCE pins with finger tip, the gate should discharge, then the DRAIN and SOURCE pins should again give a HIGH reading. ------------------------------------------------------------------------ This is where I am stuck, the Mosfets are acting the way the test says that they should but I am used to seeing diodes reading at 0.5xx to 0.7xx, these are reading at 0.001 and 0.003, I'm not sure if this is too low or right where it should be. -Landon |
#6
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What symptom is the unit showing?
Paul |
#7
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Everything works fine when it is plugged in, it stays at 120VAC, when it
looses power during a power outage (or if you just simply unplug it) it immedietly plummets to 106VAC and then keeps dropping until the battery goes dead. After plugging it back for a while the battery recharges and stays around 12.8VDC, even after being unplugged and removed from the case the battery still stays charged at just over 12 volts. The windings in the transformer appear to be fine (visually and when ohm tested), nothing is obviously burnt or smells burnt. The windings for the transformer have 12 volts going to the center tap and then each size of the center tap is turned off/on with the two mosfets to simulate AC voltage, they are in turn driven by an LM324N, both gates of the Mosfets have about 2.5 volts on them but I have no idea what the wave form looks like. -Landon loedown wrote: What symptom is the unit showing? Paul |
#8
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The SLA battery you are talking about is most likely going to be the
culprit, although it stays at 12V after you unplug it, you have to check the battery under load conditions. Very common for the battery to die and the explanation that it starts at 106VAC and goes down hill from there fairly rapidly points squarely at the battery, UPS draw reasonable current and the battery can't keep up. Mind you, I don't have the unit in front of me. Paul |
#9
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I agree with Paul. I think the battery needs to be replaced.
John |
#10
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On Mon, 18 Apr 2005 14:02:11 +1000, loedown wrote:
What symptom is the unit showing? Absense of context. -- Pig Bladder on a Stick |
#11
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Thanks for the reply, the battery going dead early would cause exactly
what it is doing, I been going down the path of the frequency wasn't correct to make the MOSFETs switch on and off fast enough to create the proper voltage coming out of the transformer. A couple of things came to mind... 1. The 10:1 ratio in the transformer would create 106 volts if the battery had suddenly dropped to 10.6 volts (normally this would be 120VAC from 12VDC), if the the battery voltage kept doing down so would the output voltage. 2. When I equated this to the last time my car would not start because the battery was trying to die it all made sense. When I get the thing re-assembled I'll monitor what the battery voltage is doing when the unit looses power. More later... loedown wrote: The SLA battery you are talking about is most likely going to be the culprit, although it stays at 12V after you unplug it, you have to check the battery under load conditions. Very common for the battery to die and the explanation that it starts at 106VAC and goes down hill from there fairly rapidly points squarely at the battery, UPS draw reasonable current and the battery can't keep up. Mind you, I don't have the unit in front of me. Paul |
#12
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Hi,
Well, I don't know what to think now. The battery is holding at 12.4 volts with an output voltage of 102VAC with no load on it at all. I let it recharge for a while, the output was holding at 120VAC with no load on it for over five minutes with the battery still above 12.5 volts. When I plugged my 140watt soldering gun into it and pulled the trigger it promptly shut itself off. 140watts at 120VAC only comes to 1.17amps which should not cause the UPS to shut down, when this UPS was still functioning normally it had a computer, monitor, 60watt lamp, and power supplies for an answering machine, computer speakers, and a PALM recharger all plugged into it and would still run long enough to shut Windows down. I waited a few minutes then turned it back on, the battery voltage is still over 12 volts but the output has dropped back down to around 100 volts. Any suggestions? Landon wrote: Thanks for the reply, the battery going dead early would cause exactly what it is doing, I been going down the path of the frequency wasn't correct to make the MOSFETs switch on and off fast enough to create the proper voltage coming out of the transformer. A couple of things came to mind... 1. The 10:1 ratio in the transformer would create 106 volts if the battery had suddenly dropped to 10.6 volts (normally this would be 120VAC from 12VDC), if the the battery voltage kept doing down so would the output voltage. 2. When I equated this to the last time my car would not start because the battery was trying to die it all made sense. When I get the thing re-assembled I'll monitor what the battery voltage is doing when the unit looses power. More later... loedown wrote: The SLA battery you are talking about is most likely going to be the culprit, although it stays at 12V after you unplug it, you have to check the battery under load conditions. Very common for the battery to die and the explanation that it starts at 106VAC and goes down hill from there fairly rapidly points squarely at the battery, UPS draw reasonable current and the battery can't keep up. Mind you, I don't have the unit in front of me. Paul |
#13
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Measure the battery's voltage when the unit is on load, not when it is out
of circuit, not when it is charging or at any other time, this is the only way you will see if it is truly the battery. Even if this isn't possible, get a 12V filament lamp and connect that to the battery out of circuit and monitor the voltage. Without doing these things, you are just compounding the issue and not determining whether the battery is at fault, or the unit itself. SLA charging voltage should be around 13.8 V Paul |
#14
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Hi again,
I let the battery charge all last night and all day until just a few minutes ago, it was charging at 13.8 volts. With a 25watt soldering iron plugged into it the lowest I could get the battery to drop was 12.2 volts. I plugged my 100/140watt soldering gun into it and put it on 100watts, within about a second the voltage plummeted at 10.something volts and then the unit shut itself down. I let it charge again up to 13 volts, as soon as I put it on 140watts the unit instantly shut down. On 100 watts the draw is slow enough to notice that the voltage plummets to 10.6 volts in about 3 seconds, then it shuts down. This is almost the identical test that is done on car batteries, they are supposed to stay above 10.5 volts for so many seconds, if they drop below 10.5 too soon that battery is considered bad and in need of replacement which is what I will be doing with this thing. Thanks for your help, I never would have suspected the battery. -Landon loedown wrote: Measure the battery's voltage when the unit is on load, not when it is out of circuit, not when it is charging or at any other time, this is the only way you will see if it is truly the battery. Even if this isn't possible, get a 12V filament lamp and connect that to the battery out of circuit and monitor the voltage. Without doing these things, you are just compounding the issue and not determining whether the battery is at fault, or the unit itself. SLA charging voltage should be around 13.8 V Paul |
#15
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On Thu, 21 Apr 2005 23:08:42 -0700, Landon top-posted:
I let the battery charge all last night and all day until just a few minutes ago, it was charging at 13.8 volts. With a 25watt soldering iron plugged into it the lowest I could get the battery to drop was 12.2 volts. I plugged my 100/140watt soldering gun into it and put it on 100watts, within about a second the voltage plummeted at 10.something volts and then the unit shut itself down. That's it, then. It's the battery. You need a new one. Sorry. Rich .... loedown wrote: Measure the battery's voltage when the unit is on load, not when it is out of circuit, not when it is charging or at any other time, this is the only way you will see if it is truly the battery. Even if this isn't possible, get a 12V filament lamp and connect that to the battery out of circuit and monitor the voltage. Without doing these things, you are just compounding the issue and not determining whether the battery is at fault, or the unit itself. SLA charging voltage should be around 13.8 V Paul |
#16
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Hi,
I found the website for the company that makes the battery, the life span for this battery "Vision CP1250" is 3-5 years, it lasted about 4 so I'm not surprized that it is trying to die. I found a local supplier that sells them for around $24.00(US). -Landon Rich Grise wrote: On Thu, 21 Apr 2005 23:08:42 -0700, Landon top-posted: I let the battery charge all last night and all day until just a few minutes ago, it was charging at 13.8 volts. With a 25watt soldering iron plugged into it the lowest I could get the battery to drop was 12.2 volts. I plugged my 100/140watt soldering gun into it and put it on 100watts, within about a second the voltage plummeted at 10.something volts and then the unit shut itself down. That's it, then. It's the battery. You need a new one. Sorry. Rich ... loedown wrote: Measure the battery's voltage when the unit is on load, not when it is out of circuit, not when it is charging or at any other time, this is the only way you will see if it is truly the battery. Even if this isn't possible, get a 12V filament lamp and connect that to the battery out of circuit and monitor the voltage. Without doing these things, you are just compounding the issue and not determining whether the battery is at fault, or the unit itself. SLA charging voltage should be around 13.8 V Paul |
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