Question about PC PSU

Hi LPF,

I'm planning on building a bench PSU from a PC PSU in the near future, and since I don't have any PC PSUs lying around I went online looking for one. I found a secondhand Thermaltake Litepower 450W for $17. It has been barely used (the guy bought the wrong PSU) and in very good condition. My questions are:
1) Will I be able to convert this into a PSU?
2) According to THIS review it has rather low output currents(against the tutorials I saw, some of them claiming a PSU rated only 300W getting output currents up to 30A!), what will my PSU's current output capabilities be? I saw that with the -12V rail I can only give 0.5A output, let say I use the +5V rail and the -12V rail to give a 17 volt output (is that calculation even correct?) will I only be able to get a 0.5A output? Thank you for your help in advance! :thanks:

Good luck on tackling this project! I haven't had the guts to try something so complicated yet. I look forward to seeing how this project of yours goes.

Cube777 said:

Hi LPF,

I'm planning on building a bench PSU from a PC PSU in the near future, and since I don't have any PC PSUs lying around I went online looking for one. I found a secondhand Thermaltake Litepower 450W for $17. It has been barely used (the guy bought the wrong PSU) and in very good condition. My questions are:
1) Will I be able to convert this into a PSU?
2) According to THIS review it has rather low output currents(against the tutorials I saw, some of them claiming a PSU rated only 300W getting output currents up to 30A!), what will my PSU's current output capabilities be? I saw that with the -12V rail I can only give 0.5A output, let say I use the +5V rail and the -12V rail to give a 17 volt output (is that calculation even correct?) will I only be able to get a 0.5A output? Thank you for your help in advance! :thanks:

Click to expand...

Easily, but,

Not like that.

My advice is to use the +12V rail only (against common ground, of course) and construct a voltage (or current) regulator to be powered with that.

Normally PC PSUs are a bit overkill for bench lasers, too bulky and noisy, if you want you can pick up these smaller PSU units:
5 volt 5 amp universal 90-240VAC power transformer, AixiZ

Or in general anything of this standard,

Thanks Eud, but I'm not only going to use this just for lasers. I want to use this for all my projects and the higher voltages will come in handy. Thank you for your input! I just didn't want to buy a PSU and not be able to convert it to a bench supply. I see there are a 12V1 and a 12V2 rail, is there a difference or is just two separate rail, which I can use simultaneously? Can I connect the two in parallel to get an even higher current draw? Thanks for all your help!

I believe you can parallel them, that's what you normally do when you hook up more than one power adapter to your graphics card for example.

If that's the case then, then yeah, as long as you can have some nice proper power regulation, then it's OK. I'd advise getting some cool LED or LCD screens to have a readout of Volts and Amps and you should be good to go.

Don't spare money on potentiometers though. You'll want something really precise if you are going to power some delicate stuff with this.

due to the way a PC power supply works, You'd need a load on the 5V and/or 3.3V outputs before you can get a decent output on the auxiliary 12V outputs. The different 12V outputs are sometimes wired in parallel inside (just wired to different output connectors) or are totally independent outputs (but wired with common ground).

I'd also suggest Eud's advice for using a dedicated power supply. These doesn't cost much these days. You'll end up with a smaller power supply too as you'll have only the output that you need.

Well if the guy already has a PSU handy, might as well use it, no point in buying replacement PSU actually. Right?

In any way, yeah, regarding that 5V and 3.3 V load, you can use those to power the displays or some cute LED lights or whatever

Yeah, on the tutorial I'm going to follow there is already a load across the 3.3V and 5V output to prevent the PSU from failing, thank you for all your help guys! Also already have to digital voltage panel meters just for this purpose If I can, I'm going buy the PSU tonight so it should be here Monday or Tuesday, can't wait!

You need to draw at least 0.5 amps on the 5V rail to stabilize the voltages. A 10 ohm 5 watt resistor should do the trick.

Thanks Zeebit, that is exactly what the tutorial said. Sorry, I am a complete noob when it comes to PC parts. THIS review shows what max current I can pull from each voltage rail, does this mean I can pull all this current simultaneously for different applications at the same time? Or can I only pull current from one rail at a time? Thanks!

All at same time, obviously - this review clearly shows the absolute maximum output of that model: Declared 450W, they managed to pull 542W which is 120% performance load.

It'd be real bad if you could only have one component in your PC active in the same time, wouldn't it?

Cube777 said:

Thanks Zeebit, that is exactly what the tutorial said. Sorry, I am a complete noob when it comes to PC parts. THIS review shows what max current I can pull from each voltage rail, does this mean I can pull all this current simultaneously for different applications at the same time? Or can I only pull current from one rail at a time? Thanks!

Click to expand...

To really answer this question you have to open up the PSU. I recently just completed a project just like this.

http://laserpointerforums.com/f67/blue-ice-machine-adjustable-power-supply-mod-pc-power-supplies-82822.html

On mine I looked at the board, and all the yellow wires (12VDC) were connected in one large solder blob. Multiple yellow lines were used just to spread the current evenly, to keep the wires cool at higher currents.

I would recommend staying away from resistors to load down those lines. I used the 5VDC lines to power fans and auxiliary equipment, and had some fun with the 3.3VDC. I've also heard of someone using an automotive light bulb. It provides some backlighting, and it will stay very cool because you're loading it with 5V, instead of the full 12V it was designed for. Those resistors will get really warm inside the case and may cause failures later on.

As far as the -12VDC goes, I haven't seen many PSUs that have the headroom for much useful current there. I didn't utilize it because of that.

Thanks Spectre, I'll keep that in mind. I got just two more questions (sorry):
1) Say I put the two 12V rails in parallel, does the output just add up? Say 20A+20A= 40A output? There is much controversy on the web about this, can anyone clear this up?
2) On the label of the unit there is a min current that needs to pulled from each rail, do I need to pull that amount of current from each rail to keep the PSU stable, or just the 5V or 3.3V rail? Thank you for all your help guys!

It's difficult to know for sure if you absolutely need to load down everything, due to the slight differences in each PSU. I stuck with loading down the big ones, 12V, 5V, and 3.3V.

As far as the multiple rails go, I didn't want to steer you wrong so I did some measuring myself. The PSU I utilized had all the yellows clearly tied together. I have another one here that died for unknown reasons, and its are seemingly separated, from looking at the board. It has 4x separate 12VDC rails (according to the label) and is advertised as a 1000W PSU.

I took out a multimeter and ohm'd out each of the yellow lines. All are electrically tied together. For your PSU I would assume the same, but a quick check with a DMM would let you know for sure. If they are the same, paralleling them may add current capacity. Just be sure to provide some fault tolerance against overcurrent on your wires, and the resulting heat. If you combine them, make sure the all the wires are connected to a heavier gauge wire that can handle the max combined current. There are lots of spec sheets online that provide this information. Is the sum of each rail additive? In this case yes, but I would add the specification provided on the lablel, not the measured specifications from that website. This will ensure the original manufacturer's tolerances are still intact.

If they are separated, you may have to load each one down or utilize them independently, with a two way switch if you want the capability for both.

Did you check just the solid yellows or the yellow wires with the black stripe on them also? According to THIS site you can parallel them for a higher current output, but it is not just the sum of the outputs. But according to the label of this PSU HERE The current output just adds up.

I would think it adds up, since they are parallel. I wouldn't try to make 34A on the 12VDC rail a goal, but I would think delivering 25A or so wouldn't be a problem for that particular power supply.

To answer your question, yes, I measured the black stripe wires as well. All +12VDC rails on these supplies are common, according to the measurements.

On top of what the power supplies provide, you also have to consider the load of your regulating circuitry, or whatever the next stage is.