How to Build a Power Supply for Electronics Hobby




Introduction: How to Build a Power Supply for Electronics Hobby

This power supply can be used by electronics hobbyists.

PC Power Supply with Cord
3/8" MDF Wood
On/Off Switch From Old Computer
1 Red LED
1 Green LED
6 Banana Jack Sockets
Heat Shrink Tubing
Plexiglass (Lexan)
Wood Screws

Drill Motor
Drimmel with metal cutting wheel
Wire Cutters
Soldering Iron and Solder
Color Laser Printer
Table Saw
Hot glue gun and glue

Step 1: Dissamble Old Power Supply

1. Find a PC power supply.  It should be in working condition.

2. Remove the metal casing, don't cut or unsolder any of the wires.  CAUTION:  do not open the case when plugged in unless you know what you're doing.  The capacitors may continue to have a high voltage charge even when unplugged... so don't touch them and/or discharge them before doing so.

3. Make a note of which color wires go together.... Naturally all like color wires will be tied together.  But during inspection if you see two different colored wires connected to the same pin on a connector then this is important and you should make note of it.  These type of connections are used to sense voltage levels in the power supply.  So don't mix this up.  If you do there are guides online that will show which color wires go together.  Its just easier to make a note and keep them the same from the start.

3. Cut the connectors off the wires.  Leave the wires as long as possible.

4. Group wires of like color together, and use heat shrink tubing to keep them together.  Strip the insulation from the ends of each wire.

The color code for the wires is: Red = +5V, Black = Ground (0V) , White = -5V terminal, Yellow = +12V terminal, Blue = -12V terminal, Orange = +3.3V terminal, Purple = +5V Standby (for standby LED and 180 to 220ohm current limit resistor, black on ground side of LED), Gray = power is on (for power LED and 180 to 220ohm current limit resistor), black on ground side of LED), and Green = Turn DC on (power switch, black on ground side of SPST switch).  Brown = senses +3.3V and should be kept connected with orange wires,  Pink = senses +5V and should be kept with red wires.

NOTE:  Some power supplies have other voltage sense wires too.... these are usually thinner than the main wires and are either the same color (but thinner) or brown or pink.  These should remain connected as they are or the power supply will not turn on.  Just make a note of how all the wires are connected to the connector pins prior to cutting off the connectors and don't change this wiring.

5.  When PC power supplies are not connected to a load they sometimes will not turn on (or remain on)... therefore you will need a load resistor connected between red and black wires to provide a load big enough to keep the power supply on with nothing is connected to the jacks.  This load should be about 10 ohm 10 watt resistor between DC ground and the +5V rail (red). Don't forget to heat sink this resistor.

Step 2: Build Enclosure

Build a wood MDF enclosure.  This basically is the same size as the metal case that was just removed... except there is more room inside for making wire connections to banana plugs and on/off switches.

Step 3: Dimensions of Enclosure

I have attached an AutoCAD DWG and PDF files contain part sizes.

The dimensions contained in these drawing show 4 decimal places.... NO you don't have to build this to those tolerances!  I just forgot to format the numbers correctly.



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    34 Discussions

    One thing you should mention is that even with the power cord removed there are still dangerously high voltages in the PS.

    There are usually very large, high voltage capacitors used for filtering and smoothing in these PS's.

    These capacitors can hold a charge for a very long time so just poking around in a PS could be dangerous.

    It's best that you know what you are doing first, at the very least you should discharge the capacitors (use a tool, NOT YOUR FINGERS!). I've seen these caps take chunks of screwdrivers off before when shorted, you don't want to mess around with those voltages.

    12 replies

    Maybe you should discharge capacitors with something other than a screwdriver? I'm looking at this schematic which is a typical SMPS and I'm not seeing any capacitors in it that look particularly dangerous to me.

    The really big caps in it are actually rated 16 Volts. They would still put out a heck of a wallop into a screwdriver though. Luckily I have fingers made out of flesh and bone, not metal, so I'd be safe from them. The ones on the high voltage side all have bleeder paths to discharge them. Engineers aren't stupid you know?

    I'm wondering if the PSUs you've tinkered with weren't broken before you even got to them? Something tells me they surely were after you were done!

    pfred2, Look again very carefully at that schematic.. Specifically C5 & C6, upper-left, just right of the 4 diodes used as a bridge rectifier. Those are your high-voltage capacitors that you need to worry about. In most supplies, they are rated to 200V, and roughly 1300uF. In a good 200W supply, they're about the size of a sub-C size NiCad. Note also, to the right of them, are a pair of bleeder resistors. These generally bleed off the voltage in about 10-15 minutes. (provided they, themselves haven't been compromised).. But, note also they're being fed from the line voltage (input is at the far-left).. Didn't see if there was a voltage jumper, or not.. so assume you could have 220V potential, if operating outside the USA where there's 115V.

    I've converted 2 supplies from another source for the hack.. Most of the newer ATX style supplies can carry 2x +12V (gotta love PCI-Express), and _NO_ -5V. (well, mostly it was just for RS-232c level handling.) the two supplies I have, range from 300W (an old office tower I scrapped. terminals are binding post/banana jack connections, had a 33-Ohm/5W resistor built-in on the internal circuit board, across the +5V), the other is a converted 160W Dell (w/o the -5V and good thing too.. Why do they use that for the pwr supply fan status? Yes, I've fried a few before)..

    I use mine for various Arduino projects, powering external hard drives to my laptop. (pushing 2x 1TB drives, with a pair of IDE-or-SATA-to-USB adapters.) They're major overkill on wattage for the projects, but still fun (and quiet) to use.

    Not sure if he uses a load resistor across the +5V, but everyone seems to think 10-Ohm, 10-Watt Sandblock.. The Dell i converted uses a 33Ohm-5W sandblock, and it stays cool, while maintaining the switching circuit activity.

    In a nutshell, Yes, those Big capacitors do exist, and they are the equivalent of lightning bolt waiting to find the fastest path to GND. (and UN-supervised hands)

    And... Looking at the schematic pfred2 listed, Yep.. C5 & C6, on the input side, read the parts list, lower-left.. both are either 220uF or 330uF, _200V_ !!!!!!!!

    Yes, the 16v caps are across the +12V, -12V, the +3.3V, & the +5V, and maybe the -5V (If provided), but the input side, before the switching circuitry, are rated 200-Volt. and if I remember the classes from the local power company, anything above 42.4V, is hazardous. (source: )

    sticking a 9V battery to your tongue, over-stimulates the tastebud nerves, which gives the kick, but you CAN put your hands on both terminals of a 12V car battery (as long as the regulator is working in the alternator, You shouldn't feel anything. (which general potential is 15V +/-) But 42.4V is the danger point, because it will exceed normal skin resistance.

    *sigh* Not worth arguing... Considering the capacitors were identified, and listed on the schematic you provided..

    I remember those. Look closer they both have bleeder resistors right across them!

    Haha, yeah, probably should have discharged with other than a screwdriver, but when you are trying to build 100 units a day you cut corners. These were older PSU's, but I just took a look at my 700W PSU and there are some pretty big caps in there. Nothing 16V, that's for sure. Just saying, PSU's are not something to play with and disclaimers should be made.

    I mean, if you feel it's safe to just go poking around without foreknowledge or warning, by all means go for it. I come from a safety oriented background and it's always better to err on the side of caution. But then again, I haven't been shocked (knock wood) in 15 years (3 phase shipboard power is a killer).

    I never would have thought I'd get called out on a safety warning (and I'm not playing chicken little here, I have experience).

    Personally I can't think of much I'd rather play with than electronics. Lately the more power the merrier too. The large filter capacitors in an SMPS are all going to be on the low voltage regulated side. Being as the highest output voltage differential in a PC PSU is 12 there isn't good reason for those capacitors to be rated much higher than that. They are still high uF and store a large charge, so through a screwdriver they will make a large spark. So will a well charged car battery though.

    And I certainly wouldn't put my fingers across the terminals of a car battery! :)

    Remember, it's not the voltage that kills you. Although it takes more voltage to push the same amount of amperage (current) through more resistance, it's the actual flow of electrons that cause your heart and and other electro-chemical processes to go haywire. They taught us in BEE (like a million years ago) that it takes less that 100ma to kill you - even less if it goes right across your heart. I don't know if that was the instructors trying to scare us but I took it to heart since a 9V battery is capable of supplying 100ma and modern Li-Poly batteries are capable of supplying tens of times more. I'm not sure about lead acid but they must have some serious amperage rating because they turn a starter motor (probably at least 20+ amps).

    They do make tools specifically for discharging though and they are not all used on CRT's (do they even make those anymore these days - I wonder if the Navy has switched to LCD flat screen radar and sonar monitors yet? - I worked on AQS13F which is the airborne sonar system in H-60s - it was still CRT in the late 90s).

    One final note, don't you usually want to put at least double the rated voltage on caps? I know it's a bit of a pedantic point, but some dusty neuron is firing and saying that you should always double your voltage rating on capacitors. So, if you expect 12 volts, you should have 25V rated caps. I know it's not very pertinent to the original post, but I wanted to make that observation.

    Post Script : I've seen your power instructables and I am especially impressed with the 300W linear power supply. Well done.

    The current it takes to kill us is small but our resistance is fairly high. So it takes a fair amount of voltage in order to get us to draw dangerous current. It varies but the minimum value is somewhere around 28-34 Volts. Anything less than that and you'd have to poke the wires through your skin.

    It is sound engineering practice to de-rate components 100% but it is also economic suicide to do it with low margin electronics. Plus if your PC's PSU ever hit 16 volts the PSU manufacturer might as well try to burn the evidence anyways. Everything else in your PC's case is going to be toast. Those caps blowing might in fact be your last line of defense.

    Glad you liked my linear supply. Today I took a picture of the AC side of it for the first time. Bit tough to make out what is going on in there but here it is anyways:

    It is such a mess because the transformer has a lot of secondary windings coming out of it. I did something with every one of them too.

    The outputs fill up 20 terminals outside the unit.

    A shot of the power supply in action:

    If You've ever seen the HV test leads. (link: )most have a fairly L-O-N-G spiral etched carbon film resistor in the tip (red area of example.) Not sure what the resistance rating, but Imagine that might be plenty enough to drain the stored voltage quicker. I've been kicked a few times too..(Mental note: when trying the CVS/Pure Digital one-time camera hack, and you have the camera disassembled, Beware the flash capacitor!!!!) I even had a partially disassembled ATX supply, which the bleeder resistors were causing to fail, sitting on my lap, and felt some of the stored voltage on the input side bleed through a sturdy pair of Levis!

    As I mentioned in a reply to pfred2, I never understood why everyone needs to make it a 10-Ohm resistor? 33-Ohm, 5W seems to make just a good a load to maintain the supply.. Also less power being burned off in heat, by the higher resistance.. I can't remember the maker of the supply I 1st converted, but it had a 33-ohm 5-Watt carbon-film (yes, Film! not wire. You would expect a carbon film resistor to burn up faster, but it doesn't hurt it.) soldered right to the board, across the +5V. (it was also well ventilated, but ran fairly cool.) when I did the 160W supply, I simply crimped a 33-Ohm 5W sandblock to one free +5V wire & a GND wire, and clamped it to the cooling ports, but it stays cool to the touch.

    Looking at a stack of old Pre-ATX supplies (the ones with the physical AC On/Off switches), I wonder if the same load would maintain them?

    nice project,

    one thing you forgot is the 10ohms 10watts bleeder resistor that has to be connected on the 5V high ampere rails, most pc power supply needs a load to stay on, if no load is detected the psu will go to standby mode and shut down.

    anyway you have a nice panel design.

    you use these connections on a low voltage application, does'nt it damage the application because higher amperage power supply.

    Hi everyone i'am new to instructable I'm looking to build a power supply out of a computer power supply i need 12v 4amp but the power supply says 12v 12amp for a amplifier please help me thank You

    Very nice! Although I didn't make mine in a separate case like yours, I still very much liked the LOOK. So, I thought I would show you how your front panel inspired mine.

    Thank you! ;-)

    1 reply

    Personally, I would just as soon leave the original PS enclosure in place and put it in an external enclosure, lining up the fan intake and power plug holes with the new enclosure (don't forget to give the new enclosure a way to exhaust the hot air too!) and draw the output wires to my banana jacks or terminal lugs. The original shell keeps the general user out of the capacitors and much improves RF shielding (their original purpose). Why take it out?