Hello,

I am trying to find a suitable pc power supply in order to perform the variable voltage mod to the reference circuit. The only supply I have available right now is a Dell HP-P2507F3P and I have looked everywhere but cannot find a schematic for it. It seems more complex than some of the ones I've seen that were modified, so I'm not even sure it would be an easy mod. So.... I'm either trying to find out if this one can be modified and, if so, how... or try to find a cheap power supply that can be modified. I've been trying to search eBay and general searches. For the cheaper ones I keep running across on there, I then can't find any schematic.

Can anyone help? I have a small project I'm working on where a supply like this would be idea. I have some transformer based supplies on hand, but I was really hoping for a pc variable supply as it's much lighter and offers more current.

Thanks in advance,
Bill

 

I took the chance and opened up this supply in the hopes of maybe finding enough internal parts to match up with a generic schematic diagram. All I know so far is that this is a Mosfet type with 4 NEC2561 optoisolators, dual 431az regulators, and uc3843 and hp1001 IC's. One of the 431's has a variable resistor connected to the anode; the other 431 does not. I'd really like to try modifying this for variable voltage output, but I'm still not sure where to begin and I don't want to fry the supply.

One site that has sort of helped a little (but still not enough that I want to try) is here:
http://danyk.wz.cz/at_atx_en.html

Scrolling down to the "Forward converter supply (single MOSFET) mod" seems to be the type of supply I have except I have 4 optos instead of 3 and dual 431's.

Still looking for any help/ suggestions.

Thanks!
Bill

 

Why not just buy a bench supply?

I just ordered this one:

http://www.circuitspecialists.com/bench-power-supply-csi3303s.html

But they have other cheaper models. At those prices, it is hard to beat.

 

Thanks, but no thanks. I already have 2 or 3 units like those, but I need something for this project that is much lighter and more compact- the PC type with variable voltage would be ideal but only if I can find one that can be modified or if I'm able to get instructions to modify this Dell one I have.

What puzzles me is the total lack of schematic info on the newer supplies like the Dell I have. And it seems only certain ones of the older models are available and certainly not close to a majority.

 

Computer power supplies are designed to put out fixed voltages. This is very likely the wrong type of power supply to modify for a variable supply.
You did not say what voltage range and current you are aiming for.

 

True, but as I said there is a modification possible for certain PC supplies that allows variable voltage. It is not supposed to be difficult. There was a thread in this forum about the process within the last year, but not specific enough to the Dell I have. Voltage range 3-12 V DC @ 10 amps. If no other choice, I'll buy one of the modifiable units and go from there but venders must know of the capability and keep the cost higher. I think this Dell I have could be done if I could find a schematic.

 

The price of a unit is dictated by quantity sold more than anything else. PC power supply quantities sold are many times more than variable bench supplies.

 

True, but as I said there is a modification possible for certain PC supplies that allows variable voltage. It is not supposed to be difficult. There was a thread in this forum about the process within the last year, but not specific enough to the Dell I have. Voltage range 3-12 V DC @ 10 amps. If no other choice, I'll buy one of the modifiable units and go from there but venders must know of the capability and keep the cost higher. I think this Dell I have could be done if I could find a schematic.

You won't be able to get full current through that full range.

On modern power supplies, Each output, 3.3V, 5V, and 12V are discrete switching supplies built for a max current at that voltage. Most high power (750W+) have 2 to 4 "rails" for each of those voltage outputs to provide the required current. You could probably vary each of those outputs by a volt before losing efficiency in the SMPS inductors/transformer due to differing on the power being drawn. Trying to modify a single voltage to do that swing would likely result in low current at both ends of the range. The new power supplies are also very tightly controlled and monitored through PC Supply specific ICs and may simply shut the power down if the feedback isn't exactly correct.

When building a supply that is to provide a minimum of 2A and a maximum of 50A or more, that doesn't leave much room for flexibility in voltage while maintaining the rated current.

 

You won't be able to get full current through that full range.

On modern power supplies, Each output, 3.3V, 5V, and 12V are discrete switching supplies built for a max current at that voltage. Most high power (750W+) have 2 to 4 "rails" for each of those voltage outputs to provide the required current. You could probably vary each of those outputs by a volt before losing efficiency in the SMPS inductors/transformer due to differing on the power being drawn....

Correct, I believe this to be the case. I had a successful mod yesterday, but I left each voltage section intact rather than forcing one voltage throughout the entire range . There will be a variance of +/-30 percent around each voltage. Well, not +30% because I don't want to cause undue stress of the components, so a maximum of around 14V.

Trying to modify a single voltage to do that swing would likely result in low current at both ends of the range. The new power supplies are also very tightly controlled and monitored through PC Supply specific ICs and may simply shut the power down if the feedback isn't exactly correct.

Bingo, and that's why I decided to leave each voltage intact. I don't know yet what the maximum currents/ voltages will be at the new ranges, but I plan on testing today. I do know that when I accidentally ramped up to 16 V yesterday, the supply shut down but I won't need beyond 13-14 V anyway.