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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
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modding a pc power supply
I have a salvaged pc power supply I use as an adjustable bench supply.
But it won't go above about 14.9 volts. Apparently it has an overvoltage protection circuit. The board has a TL494 (house-numbered equivalent), and the only other chip on the board is a dual comparator. The power supply is a Fortron/Source FS-250S40G. I have another supply that also hits a ceiling when I adjust the voltage, a Falcon Technology PE-250X with a 494 chip and a 14-pin chip labeled DBL 339. Is there some way I can disable the overvoltage protection on these power supplies? |
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wrote in message oups.com... I have a salvaged pc power supply I use as an adjustable bench supply. But it won't go above about 14.9 volts. Apparently it has an overvoltage protection circuit. The board has a TL494 (house-numbered equivalent), and the only other chip on the board is a dual comparator. The power supply is a Fortron/Source FS-250S40G. I have another supply that also hits a ceiling when I adjust the voltage, a Falcon Technology PE-250X with a 494 chip and a 14-pin chip labeled DBL 339. Is there some way I can disable the overvoltage protection on these power supplies? Look up a datasheet for the controller chip and see how they do the protection, it shouldn't be too hard to trace the circuit and disable it. |
#3
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Is there some way I can disable the overvoltage protection on these power supplies? Look up a datasheet for the controller chip and see how they do the protection, it shouldn't be too hard to trace the circuit and disable it. If it was that easy, I wouldn't ask. Those chips don't have a shutdown pin or anything real obvious like that. I've spent a lot of time looking at the datasheet and the circuit board. I was looking for a germane response from somebody with hands-on knowledge of the issue. |
#4
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) writes: Is there some way I can disable the overvoltage protection on these power supplies? Look up a datasheet for the controller chip and see how they do the protection, it shouldn't be too hard to trace the circuit and disable it. If it was that easy, I wouldn't ask. Those chips don't have a shutdown pin or anything real obvious like that. I've spent a lot of time looking at the datasheet and the circuit board. I was looking for a germane response from somebody with hands-on knowledge of the issue. But what makes you think there is a voltage limiter in there? The design may simply be such that it won't output more than a bit above the intended output. After all, it's not a general purpose power supply, it's intended for a very specific purpose with very specific voltages. At the very least, again this is a good reason to look at the datasheet, there may be resistors in there to limit the range of the variable that you are presumably playing with, so someone fiddling with it cannot output too much voltage. Or there may be something else in the design that limits the maximum output voltage. So you have to fiddle with the design, and maybe find that it can't be done at all. Michael |
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At the very least, again this is a good reason to look at the datasheet, there may be resistors in there Nope, no resistors in the data sheet. |
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"Michael Black" wrote in message
... ) writes: Is there some way I can disable the overvoltage protection on these power supplies? Look up a datasheet for the controller chip and see how they do the protection, it shouldn't be too hard to trace the circuit and disable it. If it was that easy, I wouldn't ask. Those chips don't have a shutdown pin or anything real obvious like that. I've spent a lot of time looking at the datasheet and the circuit board. I was looking for a germane response from somebody with hands-on knowledge of the issue. But what makes you think there is a voltage limiter in there? The design may simply be such that it won't output more than a bit above the intended output. After all, it's not a general purpose power supply, it's intended for a very specific purpose with very specific voltages. At the very least, again this is a good reason to look at the datasheet, there may be resistors in there to limit the range of the variable that you are presumably playing with, so someone fiddling with it cannot output too much voltage. Or there may be something else in the design that limits the maximum output voltage. So you have to fiddle with the design, and maybe find that it can't be done at all. Michael Michael is correct. THese power supplies are designed very tightly in order to get good efficiency . If they were designed to be capable of generating significantly higher voltages, the efficiency would drop significantly. The switching transformer in particular is designed for exact voltages, and without modification of the secondaries, increased voltage probably isn't possible. Look at the article at http://www.pavouk.comp.cz/hw/en_atxps.html for a typical schematic and theory. Cheers!!!! -- Dave M MasonDG44 at comcast dot net (Just subsitute the appropriate characters in the address) Never take a laxative and a sleeping pill at the same time!! |
#8
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wrote in message oups.com... wrote: I have a salvaged pc power supply I use as an adjustable bench supply. But it won't go above about 14.9 volts. I didn't make it clear: the power supply shuts down. Turns itself off. Obviously, a pc power supply is not a linear power supply that one can adjust to any desired voltage. But there is a way around it. Suggestion: To increase the voltage, adjust it below the 14.9 Volts shut-off and then add your own windings in series with the 14.9 Volts windings to get a separate output voltage. Then use a switch to pre-select the amount of windings and that in turn will determine your preset outputs. |
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"DaveM" wrote in message ... "Michael Black" wrote in message ... ) writes: Is there some way I can disable the overvoltage protection on these power supplies? Look up a datasheet for the controller chip and see how they do the protection, it shouldn't be too hard to trace the circuit and disable it. If it was that easy, I wouldn't ask. Those chips don't have a shutdown pin or anything real obvious like that. I've spent a lot of time looking at the datasheet and the circuit board. I was looking for a germane response from somebody with hands-on knowledge of the issue. But what makes you think there is a voltage limiter in there? The design may simply be such that it won't output more than a bit above the intended output. After all, it's not a general purpose power supply, it's intended for a very specific purpose with very specific voltages. At the very least, again this is a good reason to look at the datasheet, there may be resistors in there to limit the range of the variable that you are presumably playing with, so someone fiddling with it cannot output too much voltage. Or there may be something else in the design that limits the maximum output voltage. So you have to fiddle with the design, and maybe find that it can't be done at all. Michael Michael is correct. THese power supplies are designed very tightly in order to get good efficiency . If they were designed to be capable of generating significantly higher voltages, the efficiency would drop significantly. The switching transformer in particular is designed for exact voltages, and without modification of the secondaries, increased voltage probably isn't possible. Look at the article at http://www.pavouk.comp.cz/hw/en_atxps.html for a typical schematic and theory. Silicon Chip had an article a while back where they did just that and were getting 200V+ out to drive a vacuum tube device. With such high frequencies you don't need many turns at all on the secondary, an extra turn or two will probably push it's capability easily past 14v. |
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Is there some way I can disable the overvoltage protection on these power supplies? Michael Look at the article at http://www.pavouk.comp.cz/hw/en_atxps.html for a typical schematic and theory. Thank you for the link. |
#11
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On 25 Sep 2005 15:11:35 -0700, put finger to
keyboard and composed: I have a salvaged pc power supply I use as an adjustable bench supply. But it won't go above about 14.9 volts. Apparently it has an overvoltage protection circuit. IME, the overvoltage detection threshold is determined by a zener diode. The board has a TL494 (house-numbered equivalent), and the only other chip on the board is a dual comparator. The power supply is a Fortron/Source FS-250S40G. I have another supply that also hits a ceiling when I adjust the voltage, a Falcon Technology PE-250X with a 494 chip and a 14-pin chip labeled DBL 339. That's probably an LM339 quad comparator. Is there some way I can disable the overvoltage protection on these power supplies? This should help you: http://www.pavouk.comp.cz/hw/en_atxps.html See the OV circuit at the LHS. Note the typo - one of the +5V inputs is actually +12V. -- Franc Zabkar Please remove one 'i' from my address when replying by email. |
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