Instructables

Another Benchtop Power Supply from PC Power Supply

Picture of Another Benchtop Power Supply from PC Power Supply
This instructable will show how I built my benchtop power supply from the power supply unit in an old computer. This is a very good project to do for a number of reasons:

- This thing is very useful for anyone who works with electronics. It supplies nice, clean DC power in a number of voltages with overload and short circuit protection built right in!
- It is a very easy project. Most of the work is already done for you inside the computer. It's just a matter connecting a few wires and you're done.
- It's very cheap. I got the old computer for free and the rest of the parts were under $10. A commercially built benchtop power supply like this could run you more than $150!
- It's somewhat environmentally friendly since your recycling old parts to make something new.

I should mention that this is not an original idea of mine. I learned everything I know about this project from other Instructables about power supplies (there are dozens). My project is unique only because of the enclosure I built for it. The guts are the same as any other one.

My particular unit is capable of suppling +12, +5, +3.3 VDC and -12, -5 VDC. These 5 rails along with the Ground rail can be mixed and matched to provide many different voltages eg. the voltage between the +12 and -12 rails is 24 volts). There is also a handy on/off switch in the front with lights that indicate how the unit is operating.

Since I don't have any electronics projects on the go yet, I am only able to demonstrate a simple relay circuit. Here you can see the relay powering different combinations of indicator lights based on the state of the pushbutton.
 
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maumorse9 months ago
I need 18.5 volts, only 3.7 Amperes. What can i do? Watts do not matter too?
EcoMotive (author)  maumorse9 months ago
Hi, you can get 18 volts by combining the +12V and -5V rails or by combining the -12V and +5V rails. The amount of current you can draw through the rails depends on the specific power supply you are using. Look at the sticker on the power supply that shows the maximum current it can supply at each voltage. Going over the maximum current will cause the power supply to overload and shut down.
The "power" used by an electrical load is measured in watts and is a product of the supplied voltage and the current. The voltage is whatever you supply to the load (ie. 12 volts, 120 volts or 240 volts). The device will draw whatever current it needs to in order to operate (ie. 3.7 amps). The power is the voltage multiplied by the current. In your case it is 18.5 volts multiplied by 3.7 amps equals 68 watts. Hope this helps. Thanks for your question.
The_Tom9 months ago
My grey wire doesn't give 5 volts. it doesn't give any voltage. what should i do?
EcoMotive (author)  The_Tom9 months ago
Hi, first of all make sure that the power supply unit is plugged in, the switch is on and the green wire is touching off of the metal case. The fan should start spinning to confirm that the unit is up and running. Now check the voltage with one multimeter probe on the grey wire and one on a black wire. Make sure it's set to "DCV".
If it still doesn't register a voltage then check to see if the other wires are supplying the correct voltages. If they are you can simply ignore the grey wire as I would assume that it simply disconnected from the circuit board. The only thing is that you can't use an LED to have a "power on" status light.
If the other wires are not producing a voltage then there must be a problem with the power supply. Check to make sure there are no shorts in the wires. Otherwise I think you'll have to get another unit.
Thank you for your question.
The_Tom EcoMotive9 months ago
Thanks for the reply, All other wires gave correct voltages but the grey reads zero.

The standby light is always on and the Power on only lights up when the terminals are active? just want to know which i'm losing.
EcoMotive (author)  The_Tom9 months ago
No Problem. The standby light should be on whenever the unit is plugged in, with the built-in switch in the "on" position and the green wire removed from the case. When the above conditions are met AND you touch the green wire off the case the "Power on" light (grey wire) would come on while the unit is supplying power. It should only light up when the terminals are active.
If I were you I would omit both of the status lights and just secure the green wire to the case and use the built-in switch to turn the unit on and off.
The_Tom EcoMotive9 months ago
Thank you so much for the help! i think it came out pretty nice!
1388737623028.jpg1388741522481.jpg
EcoMotive (author)  The_Tom9 months ago
Looks great! Glad I could help.
The_Tom EcoMotive9 months ago
Yea, i think ill ignore the lights. there's a permanent standby light near the fan and i don't have a built in switch so i'm adding one to the green wires. also my fans never turned on, and im almost finished with the housing for it.
turtledrake10 months ago
Hi there, great Instructable!
I don't understand what the "power ok" wire and LED do in this build. Could you elaborate for me?
Thanks!
EcoMotive (author)  turtledrake10 months ago
Hi, the "power ok" wire supplies 5V whenever the unit is up and running. I'm not exactly sure but I think it's meant as a signal to the motherboard of the computer that the power supply is running and ready to supply power. In that case the motherboard proceeds to boot up.
Since the signal is there anyways it does no harm to wire up an LED to it to use as an indicator light whenever the power is on. Think of it as a warning light that tells you when voltage is being supplied to the equipment that you hook up to the power supply. You can just leave it out if you wish it does not harm either way.
Thanks for your question.
dberry91 year ago
If a person was wanting to do something like this for a subwoofer amp what's the best way to get say a 12v 50amp service for the sub amp?
Wow man, 50 amps? That's a lot :) I get stuff from this place once in a while, but wow, they only have a 40 ampere amp (ha, i didnt want to say "amp" twice, lol)
http://www.parts-express.com/pe/showdetl.cfm?partnumber=120-558
That's a monster! I know this from trying to run car audio amps in the house - I had to use a deep cycle battery bank + charger rather than come up with the insane $$ to purchase the equivalent real-time amp/volt power supply. You just have to have enough battery to last you for how long you play your music and then a charger to charge the bank back up. Wow, so costly!
EcoMotive (author)  dberry91 year ago
Hi, sorry for the late reply but I noticed your question just now. 50 amps is WAY more current that any conventional electronic power supply that I know of could handle. For intermediate use you could draw that amount of current from one or more car batteries but you'll have to get a car battery charger also. I don't know of a car battery charger that could supply 50 amps either. It would work intermittently.
If the subwhoofer is going in a car then you draw the power from the car battery. You'll probably have to get a high performance battery and alternator to go along with the subwhoofer or you might end up damaging your car.
If that doesn't help I suggest contacting the manufacturer of your sub and seeing if they sell a power supply for it. Good luck and thanks for your question.
smichigan1 year ago
One of the most nicely written and clearly illustrated instructables I've read! I also appreciate how you give credit (several times) to others who've posted similar instructables in the past. I'm not an electronics buff, and I don't know what I'd use this for, but after reading this I'm tempted to build one of these!

Thank you!
mbtria1 year ago
I am not conversant with electronics so please forgive me if this is a wasteful question. Why did you chose to use studs instead of females for the power supply? To my untutored thought, it would make it easy to either shock self, or smoke a circuit?
EcoMotive (author)  mbtria1 year ago
Hi. You may indeed be better off using female connectors for your power supply rails. I chose to use "male" studs so that I could secure the bare end of a wire by wrapping it around the bolt stud a couple of times. The threads on the bolt help a lot with keeping it secure. When I'm testing a circuit I'd rather not mess around with banana jacks, just plain wires. As for electrocution the highest potential here is 24 volts, not really enough to accidently shock yourself. The power supply has a built in short circuit protection so if you accidently cross wires from two different studs the unit just shuts off. There's no such thing as a wasteful question. Thank you for asking it.
Except for a missing electrocution warning, it is cool!
A wooden enclosure wouldn't be the best idea with this project I wouldn't think. 1 spark = 1 fire. Get a surge and the caps and resistors go pop, things catch on fire, not a good thing. Like the design though. I think the easiest way of getting a good enclosure would be to gut a second psu and feed the cables from the front of the first one into the back of the second one then panel mount everything to the front of the second one. Then bolt the 2 of them together. Minimal construction required.
Hycro4 years ago
Wait...that doesn't look like the computers I had so many of, that looks just like the old PIII 550 I had...it was an awesome little rig, half the parts were missing when I got it, and I built it into a pretty reliable computer...
Hycro4 years ago
I had 10 or more computers just like that one...still got all the mobo's and cases from them...but half the power supplies have burnt out, and a couple didn't have a power supply when I got them...:P
carlos66ba4 years ago
Often switching power supplies need to have a load to operate at all.  Most AT/ATX benchtop power supply designs I've seen (and I built 2) employ a 5 -10 Ohm power resistor between the +5V and GND posts. 
EcoMotive (author)  carlos66ba4 years ago
Yes I have read about the 10 watt resistor used for the minimum load on these power supplies. However I cannot find any at the local electronics stores and I don't want to order one online unless I'm making a big order.
For now I'll just use a few lamps or something if I need to draw more power to get it to run stable.
i love the idea of bolts as banana jacks ! I made a Hot foam cutter using 3mm mdf (looks like what your using) and butt joints with hot glue (apart from i cut it out with a laser cutter)