Turn old computer power supply into a bench top power supply?

boatsman

Joined Jan 17, 2008
187
Would such a power supply be powerful enough to run a 12vdc portable drill /impact screwdriver? I have several of these where the batteries (NiCad) are now unobtainable.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,852
@boatsman:

+12 v @ 18 amps. Should run it no problem.

Also, they sell individual NiCad batteries. You can open your battery case and physically replace the old batteries. Here's a link to some AA NiCads (if your old model takes AA):

http://www.parts-express.com/aa-nicd-cell-battery-with-tabs-1000mah--140-184

Or you can search the web for replacement batteries. Just open up one of your battery packs and know what you need before you order anything.

Besides, who wants a cordless drill with a cord on it?
 

ian field

Joined Oct 27, 2012
6,536
??? This is the P.S. output, it is isolated up until the final output which is earth grounded?
No need to touch any of the SMPS components.
Max.
The stock PSU has a 0V rail that is also tied to the safety earth.

If you want to float the secondary side - you have to meet double insulated specifications on the chopper transformer.
 

ian field

Joined Oct 27, 2012
6,536
The PS came out of a DELL, not too old. I looked all over the labeling to see some kind of identification but didn't see any. I don't know if it's ATX - or something else. It did have 24 pins. A couple wires I didn't use were the purple (+12v) and the grey (+5v). Think I read somewhere that the grey is for sensing a voltage. But since all is working as far as the meter goes - It remains unterminated and shrink-tubed.

I've seen some documents that suggest there's a negative 5 volt line, but this one didn't have that. It also had a couple yellow wires at +12v and a bunch of white wires +12v. the 12v is rated at 18 amps but I'm not going to be using it anywhere close to that.

.
The -5V rail was required for very early RAM chips - its omitted from most modern PSUs.

AFAICR: the -12V rail was only for even older RAM chips and the RS232 port, that rail is also omitted from some PSUs - although I've heard rumours that RS232 is coming back into fashion.
 

ian field

Joined Oct 27, 2012
6,536
The PS came out of a DELL, not too old. I looked all over the labeling to see some kind of identification but didn't see any. I don't know if it's ATX - or something else. It did have 24 pins. A couple wires I didn't use were the purple (+12v) and the grey (+5v). Think I read somewhere that the grey is for sensing a voltage. But since all is working as far as the meter goes - It remains unterminated and shrink-tubed.

I've seen some documents that suggest there's a negative 5 volt line, but this one didn't have that. It also had a couple yellow wires at +12v and a bunch of white wires +12v. the 12v is rated at 18 amps but I'm not going to be using it anywhere close to that.

.
Dell frequently use non-generic wire colours and pin assignments on the PSU.

IIRC: there's a pinout diagram for Dell PSUs on pinouts.ru
 

MrAl

Joined Jun 17, 2014
11,388
Would such a power supply be powerful enough to run a 12vdc portable drill /impact screwdriver? I have several of these where the batteries (NiCad) are now unobtainable.
Hi,

My drill draws 30 to 35 amps when stalled, so you'd need a really good 12v power supply to be able to handle that. They are out there, but the older ones usually dont have that much current ability on the 12v line because they used the 5v line more. The new motherboards hog the 12v line so the power supplies are following suite, so now you can get very high current 12v power supplies for PC's like 40 amps.

The typical PC power supply can be modified but you'd have to trace the circuit out because they are all a little different. Most use a single controller IC though, and a few other chips like comparators to check for over voltage and under voltage. The controller chip is almost always the same though, and that does all the regulation. By tracing the circuit you can find which resistors are responsible for regulating the voltages. Once you find them, you can make one a little variable in order to lower the output voltage. The catch here though is that the one chip regulates all the outputs including the 12v, 5v, and 3.3v, with the exception of the aux 5v line which may be regulated by a linear 5v regulator. So once you turn the 12v line down, the 5v and 3.3v lines will also follow. You cant really turn them up much either, maybe you can get the 12v line to 14v but it's hard to say, because the output caps might not be able to handle too much higher than that.

Another idea is to just run the supply at full output and dont worry about modifying it, then use external regulators to get the voltages you need. If you need 10v you can regulate the 12v line for example.
If you need a higher value, like 15v, you would have to use a boost regulator on the 12v output. Cant get full current that way but you can get pretty high if the 12v line has a high current rating already. Also makes it adjustable without having to dig into the circuit and figure out where everything is and what it does.
 

ian field

Joined Oct 27, 2012
6,536
Hi,

My drill draws 30 to 35 amps when stalled, so you'd need a really good 12v power supply to be able to handle that. They are out there, but the older ones usually dont have that much current ability on the 12v line because they used the 5v line more. The new motherboards hog the 12v line so the power supplies are following suite, so now you can get very high current 12v power supplies for PC's like 40 amps.

The typical PC power supply can be modified but you'd have to trace the circuit out because they are all a little different. Most use a single controller IC though, and a few other chips like comparators to check for over voltage and under voltage. The controller chip is almost always the same though, and that does all the regulation. By tracing the circuit you can find which resistors are responsible for regulating the voltages. Once you find them, you can make one a little variable in order to lower the output voltage. The catch here though is that the one chip regulates all the outputs including the 12v, 5v, and 3.3v, with the exception of the aux 5v line which may be regulated by a linear 5v regulator. So once you turn the 12v line down, the 5v and 3.3v lines will also follow. You cant really turn them up much either, maybe you can get the 12v line to 14v but it's hard to say, because the output caps might not be able to handle too much higher than that.

Another idea is to just run the supply at full output and dont worry about modifying it, then use external regulators to get the voltages you need. If you need 10v you can regulate the 12v line for example.
If you need a higher value, like 15v, you would have to use a boost regulator on the 12v output. .

With the open frame PSUs that were abundant on the surplus market; it was relatively simple to adjust for 15V from the 12V rail. You have to rewire the TL431 circuit so it senses (what was) the 12V rail. Usually you have to replace most of the electrolytics with ones that can handle the voltage. Many of these surplus PSU boards had a crowbar thyristor on the 5V rail - you can remove it, or adjust its trip voltage and use it to protect the (now) 15V rail.

Converting an ATX PSU is much more complex. They almost always have a voltage monitoring circuit - sometimes a quad comparator, sometimes a dedicated supervisory chip. As usual; most of the electrolytics will need upgrading (or removing) the regulation sensing circuit will need relocating and re calibrating to the voltage rail you intend to use.

It would be WAAAAY easier to use a couple of SLA batteries and rotate them between the tool and the charger.
 

MrAl

Joined Jun 17, 2014
11,388
With the open frame PSUs that were abundant on the surplus market; it was relatively simple to adjust for 15V from the 12V rail. You have to rewire the TL431 circuit so it senses (what was) the 12V rail. Usually you have to replace most of the electrolytics with ones that can handle the voltage. Many of these surplus PSU boards had a crowbar thyristor on the 5V rail - you can remove it, or adjust its trip voltage and use it to protect the (now) 15V rail.

Converting an ATX PSU is much more complex. They almost always have a voltage monitoring circuit - sometimes a quad comparator, sometimes a dedicated supervisory chip. As usual; most of the electrolytics will need upgrading (or removing) the regulation sensing circuit will need relocating and re calibrating to the voltage rail you intend to use.

It would be WAAAAY easier to use a couple of SLA batteries and rotate them between the tool and the charger.

Hi,

The chip in the ATX's that ive seen are similar to the SG3535 PWM regulator chip. So far every one i opened up so far had a chip similar to that one, and there are schematics on the web that use that type. That is the main regulator, and the comparators are used for under/over voltage sensing.
 

hp1729

Joined Nov 23, 2015
2,304
It's time to build a dual power supply for bench top experiments. I have attempted to use an old computer power supply in the past but have found that it requires some magical input to tell the power supply to turn on. How do I figure out which wire(s) need to be switched on? Or perhaps fed with some frequency to switch it on - or whatever the case may be.

And I'd like to turn it into a dual supply providing 12V+ to ground and 12V- to ground. I know there's a 5V circuit, and maybe that can be converted into a 5V± supply as well.

It's that or scrap a transformer out of an old stereo and build a power regulator circuit for that with the ± supply.

At present I own a 13.8 VDC, 9A supply but it's single ended, not ±. Any ideas?
To me there is a big concern working with such high current supplies. A short on the 5 V line will generate over 100 watts of smoke and flame before the power supply shuts down. Personally I opt for wall wart power supplies rated slightly higher than needed and current limited to just above expected current levels.
 

ian field

Joined Oct 27, 2012
6,536
Hi,

The chip in the ATX's that ive seen are similar to the SG3535 PWM regulator chip. So far every one i opened up so far had a chip similar to that one, and there are schematics on the web that use that type. That is the main regulator, and the comparators are used for under/over voltage sensing.
That's what I just said the comparators do.
 

MrAl

Joined Jun 17, 2014
11,388
That's what I just said the comparators do.

Hi,

Thanks much for your input, but sorry, i am not sure what your point is here.

Post #27:
Most use a single controller IC though, and a few other chips like comparators to check for over voltage and under voltage.
Post #28:
Converting an ATX PSU is much more complex. They almost always have a voltage monitoring circuit - sometimes a quad comparator, sometimes a dedicated supervisory chip.
Post #29:
That is the main regulator, and the comparators are used for under/over voltage sensing.
Post #31:
That's what I just said the comparators do.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
7,852
GEEZ! What's happening to my post? I'm done with my PS conversion. It works fine. My NEXT project is to build a board with fuses to protect the PS. I'm considering building current limiting circuitry and making it into a plug-in module that can be plugged into the PS and my projects can be plugged into the current limiters. In the even anything goes wrong the fuses should blow before the PS does. But that's a project for after my vacation.

Have a good one y'all.
 

ian field

Joined Oct 27, 2012
6,536
GEEZ! What's happening to my post? I'm done with my PS conversion. It works fine. My NEXT project is to build a board with fuses to protect the PS. I'm considering building current limiting circuitry and making it into a plug-in module that can be plugged into the PS and my projects can be plugged into the current limiters. In the even anything goes wrong the fuses should blow before the PS does. But that's a project for after my vacation.

Have a good one y'all.
Fuses aren't a bad idea - a PC PSU can supply a *LOT* of current.

If you haven't modified the internal circuitry, they usually incorporate overcurrent shutdown - the PSU should withstand moderate abuse. The fuses are mainly of benefit to anything you connect, and its associated wiring.
 
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