modding a pc power supply

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

Nope, no resistors in the data sheet.

I didn't make it clear: the power supply shuts down. Turns itself off.

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

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!!

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.

supply,

order

generating

The

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.

Thank you for the link.

On 25 Sep 2005 15:11:35 -0700, snipped-for-privacy@billburg.com put finger to keyboard and composed:

IME, the overvoltage detection threshold is determined by a zener diode.

That's probably an LM339 quad comparator.

This should help you:

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.

I have done this recently. There was an 8 pin dual comparator chip, and one side was used to generate the 'power good' signal, the other side was used for the overvoltage shutdown. I think the chip you have with the 339 in the part number is probably a clone of the LM339, so download that datasheet. The overvoltage comparator had a diode from the output pin to the non-inverting input so that once triggered it would stay latched shut-off. It also had a diode from the +5V power output terminal to [many other things and] a resistor to the non-inverting input, which would cause the thing to trip when the diode turned on. I put a few more 1N4148 diodes in series with the original diode to raise the threshold as necessary, whilst still preserving some overvoltage protection.

Trace the circuit diagram of your supply, it is worth it, and it won't take long if you concentrate on the interesting part of the board. Use a waterproof overhead marker to draw in the components on the solder side of the board if this helps. Don't worry about the component values at first.

Good luck,

Chris

" snipped-for-privacy@billburg.com" bravely wrote to "All" (25 Sep 05 15:11:35) --- on the heady topic of "modding a pc power supply"

ke> From: snipped-for-privacy@billburg.com ke> Xref: core-easynews sci.electronics.repair:343101

ke> I have a salvaged pc power supply I use as an adjustable bench supply. ke> But it won't go above about 14.9 volts. Apparently it has an ke> overvoltage protection circuit. The board has a TL494 (house-numbered ke> equivalent), and the only other chip on the board is a dual ke> comparator. The power supply is a Fortron/Source FS-250S40G. ke> I have another supply that also hits a ceiling when I adjust the ke> voltage, a Falcon Technology PE-250X with a 494 chip and a 14-pin chip ke> labeled DBL 339.

ke> Is there some way I can disable the overvoltage protection on these ke> power supplies?

Sure look for a comparator IC something like a 339 or something and ground the input from the resistors sensing each supply voltage. Some better supplies have a crowbar detector sensing the 5 volt line and are designed to self-destruct if there is an overvoltage.

Beware however NOT to exceed the voltage rating of the filter capacitors as these will readily explode if used outside their specs.

Accordingly, the 12 volt supply might only be good for 16 volts tops but remember corresponginly less current as well. If rated 12V @ 8 amps then it can only supply 6 amps at 16 volts. Otherwise fizzzzt!

A*s*i*m*o*v

... The current limits placed are based on resistance