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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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Redesign, was low voltage drop transistors for power supply
OK, thanks to all who provided some guidance in the original post (see
'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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#2
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Redesign, was low voltage drop transistors for power supply
Albert wrote in message . ..
I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. Rerun the calculations using the output of your 60Hz transformer as a full-wave doubler. (uses only two diodes, but needs two capacitors). This should produce a no-load voltage less than the LTC1775's input rating of 36V, based on your previous measurements for full-wave no load. I think you'll find that raising the input voltage to the converter increases the potential continuous output rating, as the duty of the mosfets is closer to 50% - unless this was the product of the peak current limiting function of the controller alone. If your load demands higher current, it might not respond the same as it used to with the linear, which simply went out of regulation while producing the needed juice. RL |
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#3
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Redesign, was low voltage drop transistors for power supply
Albert wrote in message . ..
Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Of course I've not seen the unit but it strikes me that sorting those bugs out may be quicker than a complete rebuild - they dont sound like hard to fix issues. The main plus with that is you get out the full amount of oomph that your power TF is capable of giving. If its got a 25A meter on it, chances are it can give at least 20A, and probably 25A. Other plus is its less work. Regards, NT |
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#4
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Redesign, was low voltage drop transistors for power supply
Albert wrote: OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Yup. Abandon the switcher idea if this is to be used for a ham station. Put two decent pass transistors in there. If you are dead set on a redesign, use an LM317 with the pass transistors in collector wrap around configuration. If you don't want it adjustable, use a 7812 as if it was an LM317 (gnd leg is the adj leg), with the resistors chosen to give you 13.8. If it can't sustain 13.8 across the supply output at full load, use Watson's suggested boost circuit. When you have that working to your satisfaction, add a crowbar. The down-side of the above is not getting to play with the switcher design. The down-side of the switcher is noise. |
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#7
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Redesign, was low voltage drop transistors for power supply
On Sat, 01 May 2004 05:37:51 GMT, wrote:
If you don't want it adjustable, use a 7812 as if it was an LM317 (gnd leg is the adj leg) That's a bit ambiguous, the pinouts in the TO220 variants go thus: 78XX = output / gnd / input LM317 = input / output / adj The tab on the 78XX is Gnd while on the LM317T it is the output. Data sheets are on the web from NatSemi and others for verification. Peter -- Peter & Rita Forbes Engine pages for preservation info: http://www.oldengine.org/members/diesel |
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#8
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Redesign, was low voltage drop transistors for power supply
(R.Legg) wrote in message . com...
(N. Thornton) wrote in message . com... Albert wrote in message . .. Of course I've not seen the unit but it strikes me that sorting those bugs out may be quicker than a complete rebuild - they dont sound like hard to fix issues. This is called 'shooting the engineer', and sometimes it's a good idea. I assume the original owner really just wants it to do what it did, before the fuse went. If you can fit in some small improvements (like reducing 'chirp'), that don't cause a lot of delay, then it might be best just to get it off of your bench asap. shooting the engineer? reducing chirp on a PSU? PSU delay? Just call me puzzled  Regards, NT |
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#9
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Redesign, was low voltage drop transistors for power supply
Peter A Forbes wrote in message . ..
We've seen moving iron 'output amps' meters in the ac circuit to the bridge rectumfrier, thus giving a rather inflated view of the output current.... Don't forget that the available DC output current from a full wave bridge with smoothing cap is only about 0.6 of the AC current, although the DC Volts is 1.44 X (in theory) I thought about this as a suggestion but couldn't remember the relationship for average current in this position. Peak to average ratio is quite high here so anything measuring rms wouldn't give a good output DC current indication. Moving iron meters are pretty rare. Moving coil meters indicate average current, so the error wouldn't be large. RL |
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#10
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Redesign, was low voltage drop transistors for power supply
Peter A Forbes wrote in message . ..
We've seen moving iron 'output amps' meters in the ac circuit to the bridge rectumfrier, thus giving a rather inflated view of the output current.... Don't forget that the available DC output current from a full wave bridge with smoothing cap is only about 0.6 of the AC current, although the DC Volts is 1.44 X (in theory) I meant the error in an average-responding meter wouldn't be large after the rectifier. RL |
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#11
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Redesign, was low voltage drop transistors for power supply
wrote:
Albert wrote: OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Yup. Abandon the switcher idea if this is to be used for a ham station. Put two decent pass transistors in there. If you are dead set on a redesign, use an LM317 with the pass transistors in collector wrap around configuration. If you don't want it adjustable, use a 7812 as if it was an LM317 (gnd leg is the adj leg), with the resistors chosen to give you 13.8. If it can't sustain 13.8 across the supply output at full load, use Watson's suggested boost circuit. When you have that working to your satisfaction, add a crowbar. The down-side of the above is not getting to play with the switcher design. The down-side of the switcher is noise. His original problem was a very low overhead voltage out of the rectifier -- using a high drop-out regulator would be a _bad_ idea. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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#12
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Redesign, was low voltage drop transistors for power supply
Tim Wescott wrote: wrote: Albert wrote: OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Yup. Abandon the switcher idea if this is to be used for a ham station. Put two decent pass transistors in there. If you are dead set on a redesign, use an LM317 with the pass transistors in collector wrap around configuration. If you don't want it adjustable, use a 7812 as if it was an LM317 (gnd leg is the adj leg), with the resistors chosen to give you 13.8. If it can't sustain 13.8 across the supply output at full load, use Watson's suggested boost circuit. When you have that working to your satisfaction, add a crowbar. The down-side of the above is not getting to play with the switcher design. The down-side of the switcher is noise. His original problem was a very low overhead voltage out of the rectifier -- using a high drop-out regulator would be a _bad_ idea. No, the original problem per the poster was a burned out supply. The poster's description included the impossibility of a PNP in parallel with an NPN as the pass transistors, so someone was in there with a "golden screwdriver" before he got it, or his post was incorrect. Regarding headroom/low drop out: The LM317 needs a bit less than 2 volts headroom at 1 amp, per the datasheet. With the "Watson boost circuit" (if needed), he'll get about 1.9 volts extra or about 16.9 total, assuming his 15 volt measurement was accurate. That's 3.1 volts headroom. The 7812 datasheet shows a slightly lower headroom requirement. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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#13
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Redesign, was low voltage drop transistors for power supply
Albert,
I have looked at all the responses to this, and your other posting. Probably the simplest solution is to use the brute force solution of adding a 6V 1A transformer, as somebody suggested. Put a bridge rectifier and filter on this transformer to give you about 8VDC. Connect the negative side of this supply to your 17V. The positive side of this rectifier will now be your boosted B+ of about 25V. Run your output transistors off the 17V. Change them to 2N3771 or better. While you are at it, you may want to increase the 17 V capacitor size. Don't push the voltage rating. Run the regulator and driver off the 25V. If there is an emitter follower driver for the output transistors, run that off the 25V also. Your regulator voltage drop will be equal to the VCEsat of the 2N3771, which is 1.4V max, .5V typ. Gain will be 40. As a comparison, the 2N3055 will have a VCEsat of 3V and a gain of 15. The 2N5886 is marginally better than the 3771. I have a schematic to my Astron supply, and that is precisely what they have done. Tam "Albert" wrote in message ... OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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#14
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Redesign, was low voltage drop transistors for power supply
Albert wrote: OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Sounds like you don't have to change a thing- the 3055 is operated in "inverted" mode and this will give you the lowest series voltage possible: Please view in a fixed-width font such as Courier. Inpn ---- Iload 3055 ----- V+ --+------------ e c ------+-----+- Vregulated | \ / | | | ---- Ipnp | | | b ---- | | | 2955 | Rbn | | +----e c ----+----/\/\--+ | | \ / | / ---- | Rbp b error amp | / | / \ | /| \ | | /+|------------ / +------+------- | \ \-|--Vref | \| | | | --- /// Light loading supplied by PNP 2N2955- Rbn sized to cut-in NPN at heavy loading for boost- At MAX Iload NPN is inverse saturated- very low VCE Ie,npn=5A Ib,npn=5A - Inpn=Ic,npn=10A Ic,pnp=Ib,npn + Ipnp= 5A +10A= 15A @ Veb,npn =Low Pdiss Iload/MAX=Ic,npn + Ipnp= 20A Vbc,npn@5A - 12V Rbn= ---------------- 10A |
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#15
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Redesign, was low voltage drop transistors for power supply
Fred Bloggs wrote: Albert wrote: OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Sounds like you don't have to change a thing- the 3055 is operated in "inverted" mode and this will give you the lowest series voltage possible: Please view in a fixed-width font such as Courier. Inpn ---- Iload 3055 ----- V+ --+------------ e c ------+-----+- Vregulated | \ / | | | ---- Ipnp | | | b ---- | | | 2955 | Rbn | | +----e c ----+----/\/\--+ | | \ / | / ---- | Rbp b error amp | / | / \ | /| \ | | /+|------------ / +------+------- | \ \-|--Vref | \| | | | --- /// Light loading supplied by PNP 2N2955- Rbn sized to cut-in NPN at heavy loading for boost- At MAX Iload NPN is inverse saturated- very low VCE Ie,npn=5A Ib,npn=5A - Inpn=Ic,npn=10A Ic,pnp=Ib,npn + Ipnp= 5A +10A= 15A @ Veb,npn =Low Pdiss Iload/MAX=Ic,npn + Ipnp= 20A Vbc,npn@5A - 12V Rbn= ---------------- 10A Make that last equation Rbn= (Vbc,npn@5A)/10A ..... |
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#16
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Redesign, was low voltage drop transistors for power supply
"Tam/WB2TT" wrote in message ...
"Albert" wrote in message ... So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? I'm unclear what the problem is at this point, but if its a slight lack of voltage, some extra turns can sometimes be added on the transformer - just be sure to put them over the sec and not the pri! Also For us europeans we often used to use the old tweak of adusting the transformer mains tap from 240v to 220. But it can caue problems, even fires, so if you do run a transformer like that you need to at least ensure youve got proper overheat safety protection on it. If you use a separate small TF to add a volt or two, connect it direct to the main one, dont waste your V with 2 sets of rectifiers. Finally, there is one for-play-only scheme for minimising V drops, and thats a buzzing rectifier in parallel with your diode bridge. Basically it gives you a metal connection at the V peaks. Not for commercial use. Regards, NT |
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#17
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Redesign, was low voltage drop transistors for power supply
wrote:
Tim Wescott wrote: wrote: Albert wrote: OK, thanks to all who provided some guidance in the original post (see 'low voltage drop transistors for power supply' post. Clearly, this was a 12v supply and not a 13.8v supply. The actual output current ratings are not known, the only hint is that the original meter was a 25 amp meter. The transformer is much larger than 100 va and is actually larger than a 200 va unit (based on core size). When I tore down the rest of the assembly, I found more shaky stuff including sockets for the transistors that were soldered, several etch cuts and lifted run on the PCB, meter shunt was missing (probably discarded when the original pcb was replaced). In other words, it probably needs a complete redesign rather than using any of the original parts and pc boards. I ran LTC's power supply design software and it came up with a 13.8 volt output switching supply that will supply 14A with the existing power transformer. It uses the LTC1775 switching regulator, 2 X Si4410DY mosfets, an 11 uH inductor and a couple of 1N5818 schottky diodes. The supply itself (not including transformer losses and rectifier losses) provides 98 percent efficiency at 14A output. So, I'm considering building up this switching supply. I need suggestions for a schottkey bridge rectifier that will handle 20A or more. I find the diodes themselves are readily available, but I'd like to have a bridge assembly if possible. Any suggestions?? Yup. Abandon the switcher idea if this is to be used for a ham station. Put two decent pass transistors in there. If you are dead set on a redesign, use an LM317 with the pass transistors in collector wrap around configuration. If you don't want it adjustable, use a 7812 as if it was an LM317 (gnd leg is the adj leg), with the resistors chosen to give you 13.8. If it can't sustain 13.8 across the supply output at full load, use Watson's suggested boost circuit. When you have that working to your satisfaction, add a crowbar. The down-side of the above is not getting to play with the switcher design. The down-side of the switcher is noise. His original problem was a very low overhead voltage out of the rectifier -- using a high drop-out regulator would be a _bad_ idea. No, the original problem per the poster was a burned out supply. The poster's description included the impossibility of a PNP in parallel with an NPN as the pass transistors, so someone was in there with a "golden screwdriver" before he got it, or his post was incorrect. Regarding headroom/low drop out: The LM317 needs a bit less than 2 volts headroom at 1 amp, per the datasheet. With the "Watson boost circuit" (if needed), he'll get about 1.9 volts extra or about 16.9 total, assuming his 15 volt measurement was accurate. That's 3.1 volts headroom. The 7812 datasheet shows a slightly lower headroom requirement. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Burned out supply and low output from the rectifiers with a suspicion that the original supply was for 12V, which is probably why the original post was "low voltage drop transistors...". -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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#18
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Redesign, was low voltage drop transistors for power supply
Fred Bloggs wrote:
Sounds like you don't have to change a thing- the 3055 is operated in "inverted" mode and this will give you the lowest series voltage possible: Please view in a fixed-width font such as Courier. Inpn ---- Iload 3055 ----- V+ --+------------ e c ------+-----+- Vregulated | \ / | | | ---- Ipnp | | | b ---- | | | 2955 | Rbn | | +----e c ----+----/\/\--+ | | \ / | / ---- | Rbp b error amp | / | / \ | /| \ | | /+|------------ / +------+------- | \ \-|--Vref | \| | | | --- /// On the 2N3055, The "e" and "c" terminals should not be inverted? --- Ing. Remberto Gomez-Meda http://ingemeda.tripod.com/ INGE - Ingenieria Electronica. Puerto Vallarta, Jalisco, Mexico. |
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