converting a computer powersupply

[rasterburn]

Hello, I am working on a small project and I will be needing to go to the local electronics shop to pickup some parts for it, but I thought i might ask on here first which I will need.

I am converting an ATX computer powersupply to run as a bench power supply, I want to be able to utilize all the available rails supplied by the power supply. the color coding and the pinout of the wires leading off the PSU have already been marked and labeled in accordance to their voltage output but I will need to pickup resistors for the rails to provide the minimum operating load. I understand that not all computer power supplies require this load but its better to be safe then sorry

So the psu I will be using is rated at 250w
+12v = 14A
+5v = 25A
+3.3v = 18A
-12v = 0.8A
+5vsb = 2A

so far the only thing i have really done is cut the ends off the wires and wire in the power switch but I am not sure what resistors I need to purchase to provide the required load for +12v, +5v, & +3.3v.

I read on another site that 1A for the +12v, 0.3A for the +5v and 0.5A for the +3.3v was sufficient for proper operation, does anyone have any confirmation of this? or does someone know a more exact number?

 

[ericgibbs]

hi,
You only need one power resistor, look at this link.
How to Convert a Computer ATX Power Supply to a Lab Power Supply - wikiHow

 

[ghostman11]

i use ATX power supplies for my projects alot. most seem to work fine without a load on them. having said that some do indeed refuse to work without it. in my other thread on here about ADC the project is for my son who is building a bench supply out of a ATX psu, he is adding a small cpu fan for the extra caseing it will be housed in. at that adds enough load to the psu to start it and does something useful!
i have also used a led indicator to provide a load, sometimes this is enough sometimes not. anyway just a few ideas that might help you.
p.s if its a old supply then done try and draw anywhere near the maxium A ratings from the psu i have made more than a few smoke by doing that!

 

[ericgibbs]

i use ATX power supplies for my projects alot. most seem to work fine without a load on them. having said that some do indeed refuse to work without it. in my other thread on here about ADC the project is for my son who is building a bench supply out of a ATX psu, he is adding a small cpu fan for the extra caseing it will be housed in. at that adds enough load to the psu to start it and does something useful!
i have also used a led indicator to provide a load, sometimes this is enough sometimes not. anyway just a few ideas that might help you.
p.s if its a old supply then done try and draw anywhere near the maxium A ratings from the psu i have made more than a few smoke by doing that!

hi,
If I ever converted a ATX to a bench supply I would buy a high power White led, use that [with a series resistor] as a trailing mini bench inspection light.

Would help us wrinkles to see the fine print on the components.

 

[ghostman11]

hi,
If I ever converted a ATX to a bench supply I would buy a high power White led, use that [with a series resistor] as a trailing mini bench inspection light.

Would help us wrinkles to see the fine print on the components.

LMAO Eric it would take more than a bright light to help my eyes! some of theese components need a electron microscope just to get them out the packet! but i like the idea of an inspection lamp!!! i realy hate just using a resistor for a load it seems a waste of money just to produce heat. maybe i am just being tight

 

[rasterburn]

ok so if i only need 1 resistor, does it matter where i place it? I have read that it should be placed across the +5v. (I assume due to the nature of the psu) but can i bridge it across the +3.3v or use a useful load across the 12v? I am not an overly big fan of wasted power nor am I sure I need that much current but i do plan to place an indicator LED across the gray wire to let me know when the power supply is on (I assume that wont be enough load)

 

[ericgibbs]

ok so if i only need 1 resistor, does it matter where i place it? I have read that it should be placed across the +5v. (I assume due to the nature of the psu) but can i bridge it across the +3.3v or use a useful load across the 12v? I am not an overly big fan of wasted power nor am I sure I need that much current but i do plan to place an indicator LED across the gray wire to let me know when the power supply is on (I assume that wont be enough load)

The conversion states a 10R power resistor on a RED wire +5V to 0V for it to work successfully, so I would do just that.

 

[rasterburn]

ok so I have my dad looking around in his storage for the resistor (I am pretty sure he has a couple laying around, I know if I connect the +12v and the -12v through my test meter i get 24v but if i feed through a 24v load I trip out my psu, how would I go about setting up a 24v feed (I want it variable through a pot. and a meter) and I would like to be able to draw from it when needed without tripping out the PSU?

so for the most part i would like to have:

3.3v
+/-5v (white wire is not available so no -5v on this PSU)
+/-12v
and a variable 0 - 24v

forgot to mention, I also have the +5vsb which is rated for 2A (10w) aside from the +5v rated at 25A, would placing the 10ohm 10w resistor across the +5vsb rail have the same effect as placing it across the regular +5v rail? I am just thinking if it has the same effect i would prefer to utilize that instead. also I do understand that the +5vsb is the standby feed so i am not overly sure it would have the same effect due to the nature of the "soft power on" function of the PSU but with a physical toggle switch that is of course changed to a "hard power on"... just looking for a little insight on that rail

 

[ericgibbs]

hi,
Check the current rating of the -12V supply from the ATX.

I dont know how you plan to use all of the output supplies at the same time, but if you link the +12v and -12v together to make a 24V supply, be a aware that you cannot use say the -12V as ground reference if you want to use the +5V for other parts of your project.
The 0V's for all the voltage outputs are connected together.

Using a variable pot is not a good way to produce a variable output voltage, especially if intend to draw 'heavy' currents into varying loads.

Ref the +5Vsb I dont know, try it.

 

[MrAl]

Hi,

Just in case someone hasnt mentioned it already, the -12v output does not put out very much current so it probably wont make a good bench supply. Maybe two power supplies in series would work using the two +12v outputs, but i've never tried this. I guess i could set up a test though, but if anyone has tried this please let us know. That would give +24v at maybe 10 amps or more depending on the rating of the two power supplies.

There is a problem though if you want to use this as a bench supply, and that is that bench supplies usually have a voltage adjustment. With these PC power supplies there is little adjustment if any so you'd have to either hack the power supply(s) or add an external voltage regulator circuit.
The easiest way is to use an external voltage regulator circuit, but it will have to be able to handle the full current you intend to use at the voltage you intend to use too.
The harder way is to hack the power supply(s) by digging into the board itself and looking for the regulator chip and looking up the spec's on the web. This will tell you what has to change in order to use a pot with the chip to make the output variable. There may be other things to change too if the power supply has built in over and/or under voltage detection circuits, and you'd have to change some resistors there or else short something out depending on the circuit. This of course means you either need a schematic or you need to trace out the circuit carefully looking at the front and back of the board. This isnt too easy but it does mean you'll know how to change the circuit eventually. It helps to take pictures of the front and back and examine them on the computer. I've done this a number of times. I used a line scanner though to scan the back side of the PC board. It isnt easy but it works.

 

[SABorn]

I have dick around with many ATX power supplies as power sources, and as Eric has sated a resistive load is needed to fire the psu up.

One cheap and nasty thing i found that works best is to plug in a 3.5" floppy drive, and forget the resistors.
You will get perfect V-out and for a hell of a lot less stuffing around with resistors.
Its a bit bulky and not so pretty, but works well.
Considering the price of a 3.5" floppy drive its likely cheaper than the resistors.

Pete.

 

[rasterburn]

well, a 3 1/2" floppy drive would cost me nothing, think i have one sitting in my closet, and your right, it would be enough load and your correct it would be ugly as hell....

@eric:

The current rating of the -12v rail is only 0.8A in comparison to the 14A of the +12v rail when you say "reference" or "ground reference" what do you mean? I am not up to date with jargon, I have checked each rail and they all output the correct voltage -1v the 5vsb seems to be always live, suitable for a standby light

@MrAl:

a dual power supply unit may be a valid option in the enclosure my friend is going to build for me (will resemble a wooden version of the "APC Game Manager") I will be utilizing both "bays" as removable inserts, each insert could contain a psu, power outlets, or just a small toolbox the back would contain 2 power outlets as well (i cant imagine needing to plug much more then 2 items in at a time) I think 2 individual power supply units should crank out more then an excessive amount of juice for projects, I will have to have a look at the circuit board when i pull it from its current case, unfortunately i dont have an oscilloscope kicking around yet and i cant exactly afford one, oh well

 

[rasterburn]

ok so i have forgotten about this thread, but i am back, been a bit busy, it seems that my PSU works fine without a load across the 5v rail, this is a good thing, i havent put it through much testing but things do look pretty stable from my line of sight of course... though I have noticed that there is a +/-0.5v difference between having the psu fan plugged in and not having it plugged in, I have also noticed that the PSU fan though it is rated at 12V 0.3A operates at 5V off the psu main board. The PSU has a standby light on the back of it, this light is always on if all things are good (of course it blinks once when the PSU gets turned on) if something is seriously wrong (such as no load on the 5v) power will still feed out the rails but the light will not be on, of course another a different issue with the PSU will cause the Light on the back to rapidly blink, at a fairly slow rate, being off for say 80% and on for 20% of the time.

The variable rail, i could do off a completely separate transformer (16-0-16) that wont be a problem, not that i really need the -12v rail much anyways.

All I have to do is locate a proper sized enclosure for everything to sit in, anyone know of a good supplier for abs project boxes?