Introduction: A Simple Bench Variable Power Supply

Picture of A Simple Bench Variable Power Supply

As I have started messing around with electronics more I had struggled a bit with a whole lot of different adapters for testing. After doing some research I came to a conclusion that building one from an old PC power supply would be more than adequate, being how it gives 12, -12, 5 and 3.3v available (some older ones have -5v also but mine being newer didn't) making it great for testing most electronic projects.

Step 1:

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Tools and materials required:

  • old (or new) working PC power supply to repurpose (mine was 300W)
  • amplifier binding posts (got mine here cheap)
  • on/off switch
  • wood
  • screws
  • heat shrink tubing or electric tape
  • wood glue
  • hand saw
  • jigsaw
  • sandpaper
  • soldering iron and helping hands
  • drill and few bits
  • pliers

Step 2:

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First I cut the base for the bottom, then cut the holes in the back section for the fan, power plug and the indicator LED. I also made screw holes and screwed the panel to the back.

Step 3:

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I cut off all except the 24 pin wire cable, leaving about 5 cm of each wire before wrapping them in electric tape. This is made so in case later I decide that for some reason I want to use those wires again.

Step 4:

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Next I cut a piece of wood for the side, attached it to the bottom part and the back side. I also cut a smaller piece for the other side and attached it to bottom (not pictured here).

Step 5:

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Cut a piece for the front and drilled the holes for binding jacks and on/off switch. Space between the holes is 2.5cm (1 inch). I also drilled the pilot holes and countersinks for the screws.

After that I put all the binding posts in and tested if all the cables would fit in the front when the panel is mounted.

Step 6:

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I cut the wires to size and soldered them to the binding posts; isolated them with heat shrink tubing to make sure there would be no unwanted contacts. All the wires of the same color give out same voltage so some of them are bundled together. You don't have to do that, you can can just pull one of the appropriate color wires to the jack and cut off the rest. Green wire and ground is connected to the on/off switch; grounding the green wire will turn on the power. Colors have outputs as follows:

  • Yellow +12v
  • Blue -12v
  • Red +5v
  • White -5v (non existent on this PSU)
  • Orange +3.3v
  • Black Ground
  • Green Power ON
  • Gray Power on indicator (attach 330 ohm resistor and led to it and ground it if you want an indicator)
  • Purple Standby (not used)
  • Brown (older PSU) 3.3v check, must be connected to orange wires if you want 3.3v output

Step 7:

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I cut some more wood for the top, bearing in mind to keep the ventilation holes open. I checked all the connections and finally plugged in the power.

Step 8:

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After checking the voltages I wrote them down on the front panel. Hope you enjoy the project and good luck making your own.


tildesarecool (author)2017-09-13

I don't think this was asked already. Would it be improper and/or not work to daisy chain one ground to each thing that requires a ground? I mean if multiple 5v and one or two 12v power sources each require ground can they use a common ground?

Or do all them need their own separate ground wire for each rail? I'm kind of a beginner (sorry).

RobertC2 (author)2017-06-05

Many voltage combinations are possible. This was the face plate design for the one I made 2 yrs ago. At the top right of the photo you will find a list, and a very important notice about current rating. The current limit for combined voltages is the lowest of the two rails used.

RobertC2 (author)RobertC22017-06-06

oops... lets try that again...

SiDawg (author)2016-12-04

Nice one! I've been doing a bunch of electronics projects too and think ill go down this route. However (and this may be a bit of a n00b question) what's the difference between a -5V connection and just reversing the polarity of a 5V connection? i.e. if I put a multimeter red to red it would read 5V, if i put it on red to black it would read -5V... if voltage is "potential difference" then it's all relative isn't it? Or for that matter, folks suggesting combining +12v with -12 v... if you had two pairs of red/black +12V, could you not just "patch" from red to black and get +24V from the remaining terminals?

RobertC2 (author)SiDawg2017-06-05

SiDawg, I believe part of your question was left unanswered.

The last part about TWO (2) pairs of +/- red/black 12v wires...

I think you were asking about 2 separate rails of 12v wires... the colors you mentioned were confusing given that in this device the 12v rails are yellow. One is Yellow and the other is Yellow w/a Black stripe (2nd 12v rail only on modern PSUs).

The answer is no, you cannot use those two 12v rails together to make 24v because they share the same black wire. So, if you try what you said, it would simply short out one of the rails, in which case the rail is shut down and you will never get your 24v by using these rails in series.

Sadly, using +12v(yellow) and -12v(blue), typically, will give you only about one amp...

Theoretically, there is another way to get 24v at a reasonable current by combining those two 12v rails in parallel and feeding them into a boost converter. Because they are in parallel, they combine their current rating, and although some of that is lost in the step-up process of the boost converter, you are still getting reasonable current if the boost converter is rated for high amperage. Of course those are expensive...

Alternatively, a cheaper solution is to just buy a 24v supply and get on with your life. ;-)

darthwolf (author)SiDawg2016-12-05

The negative rails do not provide nearly the same amperage so you could end up overloading your PSU. Also, since there is only one -12v wire you wouldn't be able to combine it with others.

I think I understand what you mean but I don't see a way you could do that with the PSU.

zolv (author)2017-04-22

I think it's one of the best cover concept I ever seen. I plan to build something similar based on Your idea. Thank You for inspiration.

Dawsie (author)2017-01-05

thanks for this very intersecting it's a shame it's only for testing items if you could use it power them up that would be great but testinfis fine lol,I,have a few things around the place that I need to work it if they are worth keeping ?|

kdac (author)2016-12-07

Sorry if you've already gone over it somewhere, but what would a person use this for?

darthwolf (author)kdac2016-12-08

For powering on electronic components and circuits before you connect them to other stuff (and checking if they're faulty).

sgtphilipm (author)2016-12-06

love it great idea for recycling old computer parts into a workshop great tool

beardland1993 (author)2016-12-02

My psu has two yellow wires with a black stripe and well as the ones listed above. It is labeled as +12v2. What is the difference between this and the solid yellows labeled +12v1?

Eee1997 (author)beardland19932016-12-06

The 12V1 and 12V2 notation indicates that there are (2) 12v outputs. 12V1 differs from 12V2 in that they are derived from separated secondary winding of the isolating transformer. They are intended for different functions as specified in the end product and generally will have different current ratings. One may find some supplies with more than two. For example, 12Vn where n is any integer (1,2,3...).

Yonatan24 (author)beardland19932016-12-02

+1'ing this question too.

I thought it meant that the power supply contained two different 12V sources, but I was never able to find a second or third, in some.

What is that?

darthwolf (author)Yonatan242016-12-03

+12V2 is second rail that supplies processors voltage regulator circuit on motherboard. while +12V1 supplies the graphics card, fans and motors of HDD and CD/DVD. They each have an overload sensor to protect the various circuits; a short on those will immediately shut down the PSU.

beardland1993 (author)darthwolf2016-12-03

Thank you! I also have two solid black wires that are separate from the others. I can't see what they are labeled on the board. they should still be ground right?

kimseymd (author)beardland19932016-12-05

Could this be for the different current requirements? Different fuse ratings?

darthwolf (author)beardland19932016-12-03

All black solid wires are ground, yes :)

mjkirk12. (author)darthwolf2016-12-04

And with +12V1 and +12V2 each having it's own regulation circuit, when combined together they can "fight" each other. They may work OK for a while, but could cause failures over the long term, as one of the regulators may fail.

Eee1997 (author)2016-12-04

As an electronic engineer with 35 years experience in implementation and design of these power supplies (PS) in various end products, I would like to voice some important safety concerns with this design. First of all it is important to know the current and VA ratings of each DC output. They should never be exceeded. Anything over 240 VA (volt/amps) is considered enough energy to create an energy hazard and should not be accessible to the user. Even if it's only 5vdc. Any current over 8 amps for 60 sec has enough energy to present a fire hazard in the end product if there is a component failure. Many of these type PS exceed these limits and must be addressed in the end product design. Secondly, the current rating of each output should never be exceeded. This can be complex because many of these supplies may be at max total rating with only 2 of the outputs at rated load. In addition, the ventilation holes on the PS should not be blocked or restricted as with the front holes in your design. Overloading and over heating of the power transformer will degrade the protective insulation and may result in fire or shock hazard. Openings on the top if larger than 4 mm are not allowed by safety agencies such as UL, CSA, TUV unless they are situated within the end product where entry of objects are not possible. Although these PS may carry UL/CSA/TUV recognition they are required to be inspected a second time before an end product may be labeled with safety certification Marks. I recommend that you obtain the Conditions of Acceptibility from the manufacture of the PS so you know what you are dealing with.

Eee1997 (author)Eee19972016-12-04

please also note. Most of these power supplies will have 2 5vdc ratings and they are not relative to one another. Typically one of these outputs will have a much lower current rating and is derived from an independent secondary winding of the transformer. Therefore you do not want to combine these 2 outputs in series to get 10 vdc. The return paths may not be the same. One of these two outputs are noted as 5vsb or stand-by. This one will always be active when the front stand-by switch is on or off.

kimseymd (author)Eee19972016-12-05

I didn't see any mention of current limit or over voltage protection in your comment! These used to be an integral part of any power supply for our protection!

Eee1997 (author)kimseymd2016-12-05


You are correct there are current limit built into these power supplies per the specifications of there customers. The current limits however are in most cases much higher than you need or too high for safe accessibility of the end user. For example, many power supplies may have a current limit on a 5 vdc line in excess on 30 amps. This is not only a fire hazard but also an energy hazard. You will find these that this information is documented in the Conditions of Acceptability (COA's) for the specific power supply you are using. The COA's are not standard. Each specific model will have their own COA's. These type power supplies are for building-in high power computer products and are not considered safe until the Conditions of Acceptibility have been addresses in the end product. Overvoltage has been tested and safe during the power supply safety investigation and is not usually ap problem for the end product. However, there may be units that have low voltage output such as 5vdc and are not considered SELV. They may only be ELV. The difference here is ELV does not have reinforced insulation from ac mains. Again, the COA's will give you this information. Reference Standard for safety IEC/UL/CSA 60950 for more details. Thank you.

JohnC430 (author)Eee19972016-12-05

are the two +12V outputs separately regulated? are they separately regulated from the +5V? somehow i had the impression that there is only one output that is regulated (possibly the high current +5V output)and the rest are just followers except for the 3.3V output. can anyone please comment on my question? i understand power supplies but am not familiar with those in the pc's. also i thought they were all designed as flyback configuration. if so then should one not have at least a minimum load on one output?

Eee1997 (author)JohnC4302016-12-06

The type power supply used for this project is what called a switch mode power supply abrieviated (SMPS). The outputs of these supplies are usually very well regulated by means of a feedback loop from the secondary to the controlling IC located in the primary circuit. Regulation is achieved by pulse width modulation (PWM) of the primary side winding. In most cases the +/-12v, and 5V and others are regulated independently. I do not believe I have ever seen a follower configuration. But that does not mean you will never come across one. Sometimes the 5V are regulated down from the 12v winding, but I would not like to speculate here. It depends on the power supply manufacture and type you are using. To know for sure, one would have to take a look at the power supply schematic to determine how the outputs are regulated. Hope this helps.

Eee1997 (author)Eee19972016-12-05

Please also note: These power supplies are not intended for use as a stand alone power source. They are intended for building-in to an end product and must have a proper fire and electrical enclosure. The specifications such as V,A,Watts available energy of the outputs are dictated by the end product manufacture to the power supply manufacturer for their specific needs for custom product such as a computer. These power supplies may not be safe for your specific application. They require additional attention to potential hazards depending on your specific use and construction. Before tying outputs together in order to double voltage, I recommend that you have high level knowledge and expertice with these type power supplies, obtain schematic diagrams and have necessary skills to do so.

Eee1997 (author)Eee19972016-12-04

I would like to clarify what is meant by VA. It is very similar to wattage. It is the product of volts times amps. Each output has a volt, current and a VA rating. VA is an energy rating of the output. For example, 12vdc output capable of 20 amps is 240 VA.

DaltonL7 (author)2016-12-05

I'm surprised you used 3.3 volts because most diy bench power supplies only have a 12 and 5 volt ouput

darthwolf (author)DaltonL72016-12-05

That one I often use when testing the LEDs.

hubi (author)2016-12-02

nice work. But there is nothing variable at all. All voltages are fixed.

It is a cool powerfull supply.

darthwolf (author)hubi2016-12-02

Actually it is variable in the means that you can combine +12v and -12v to get 24v, +5v and -12v to get 17v DC and +12v and +5v to get 7v :)

wjcarpenter (author)darthwolf2016-12-04

That's not variable in the sense of what most people think of as a variable power supply. It's still a fixed number of preset combinations. You might as well say that your phone charger is a variable power supply: +5v when plugged in, 0v when unplugged.

Not a big deal, and doesn't affect the instructable, but we might as well have the right terminology in case of readers who are learning.

Yonatan24 (author)wjcarpenter2016-12-05

So... Adjustable fixed output power supply? :)

pdpprd (author)wjcarpenter2016-12-04

Agreed - not variable.

Yonatan24 (author)darthwolf2016-12-02


So if you connect +12V and ground to your component (both sides...) you get 12V, but if you connect +12V and -12V to your component, you get 24V?

darthwolf (author)Yonatan242016-12-03

That's correct. If you connect +12v and -12v rails you get 24v.

mjkirk12. (author)darthwolf2016-12-04

I think you need to be careful with the current load when combining outputs. Usually +12V is good for many amps of current, say 10 amps or more, but the -12V side is usually less than 1 amp - they are not balanced. So for +24 volts, you need to limit the load to < 1 amp, otherwise you risk damaging the -12V output. These are designed with the lowest cost components to meet the specifications.

Also some PC power supplies typically require a minimum current draw (100 milliamps?) on one of the rails (+12 or +5 V) to properly regulate the other outputs. If you neglect to have a load on the regulated voltage output, then it may not regulate other output voltages properly, or even shutdown as protective action.

mjkirk12. (author)mjkirk12.2016-12-04

For more info on base load, minimum load and balanced loading, see this:

kimseymd (author)mjkirk12.2016-12-05

Excellent information and I hope anyone going to attempt this is aware of your statement! Your also now using an output for ground either the +12 or -12 volts!

DavidL138 (author)hubi2016-12-04

it can easily become variable.

kimseymd (author)2016-12-05

I've been a PC technician all my life and never thought once to do this. What a great idea:) I've built my own power supply's with transformers, diodes and capacitors and never thought to do this:) again great idea!

MichaelT177 (author)2016-12-04

And what do you use it for? Whats the purpose of it? I would like to have seen pictures and a description of what you were using it for along with more pictures showing it being used.

Galabb (author)MichaelT1772016-12-04

@ MichaeIT 177 .... If you don't know, you don't need it. Look for basic electronics 101 tutorials instead. :)

kimseymd (author)Galabb2016-12-05

Most likely it wont because of current limit and over voltage built in any commercial power supply's.

emerson.john (author)MichaelT1772016-12-04

Whoa, MichaelT177! Why attack the author, who generously took the trouble to show us another interesting way to package old PC supplies. This repackaging is extremely common among home experimenters in need of bench power sources, and darthwolf has a good looking design. (Thanks to the author)

Chill out, emerson.john. That didn't sound like an attack to me. I wondered the same things.

darthwolf (author)MichaelT1772016-12-04

Hey there! It is used for testing of electronics before you connect them with others or put them in enclosures.

Lets say you make a LED USB powered light but you don't want to put it into your computer on the first run in case crossed wires or reversed the positive and negative. So if it malfunctions it's gonna damage the old cheap power supply instead of your 1000$ computer :)

zednought zeta (author)2016-12-04

So to make sure, will the color of the wires you mentioned here be universally applied to every brand? I'm thinking of trying this, but I'd rather not find out halfway through that my values are all wrong.


DavidL138 (author)zednought zeta2016-12-04

The wire colors shoud be universal. Some PC power supplies have just one main connector, others have two half-size. In addition, there should be one to three smaller connectors that supply 3.3 and 5 vdc.

pdpprd (author)2016-12-04

It's a good project although slight mis-named. I would like to see some kind of warning advice as these power supplies can have lethal voltages present, especially if sufficient time has not elapsed since their last use.

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