# DIY Lab Bench Power Supply

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## Introduction: DIY Lab Bench Power Supply

Everyone has those older or newer ATX power supplies laying around. Now you have three options. You can throw them in your garbage, salvage some good parts or build a DIY lab bench power supply. The parts are dirt cheap and this supply can deliver more amps than some modern variable ones. Let's build it.

## Step 1: Watch the Video!

The video gives you all explanation you need to build this project. But I will tell you the most important steps again, this way you can not screw this up.

## Step 3: Get the Right Resistor!

You may already noticed that we have to put a dummy load on the supply to keep it stable even when we only draw small amounts of current.

I recommend the dummy load should draw at least 0.5A.

Here is the calculation if you have most of your power on 5V/3,3V rail:

R=U/I=5V/0,5A=10Ω

P=U*I=5V*0,5A=2,5W

http://www.ebay.com/itm/10W-10-R-Ohm-Ceramic-Cemen...

Here is the calculation if you have most of your power on 12V rail:

R=U/I=12V/0,5A=24Ω

P=U*I=12V*0,5A=6W

## Step 4: Make the Correct Connections!

What does each wire mean and where does it connect to? You can find a schematic here which tells you how to connect the parts. Anyway here is my written version of this schematic:

Orange (3,3V) ---> 3,3V red binding post

Red (5V) ---> 5V red binding post

--------------> 10Ω 10W resistor

---------------> 220Ω resistor of 3mm green LED

White (-5V) ---> -5V red binding post

Yellow (12V) ---> 12V red binding post

Blue (-12V) ---> -12V red binding post

Brown (3,3V Sense) ---> 3,3V binding post

Purple (5V Standby) ---> 220Ω resistor of 3mm red led

Green (Power ON) ---> One side of the toggle switch

Black (Ground) ---> GND black binding post

-----------------------> Cathode of 3mm green LED

-----------------------> Cathode of 3mm red LED

-----------------------> 10Ω 10W resistor

-----------------------> other side of the toggle switch

Grey (Power Good) --> not connected

If you have most of your power on the 12V rail then you need to connect a 24Ω resistor to 12V instead of 10Ω to 5V.

## Step 5: Success!

Everything works! Now you can build even more awesome electronics projects with the help of this bench power supply!

Feel free to check out my Youtube channel for more awesome projects:

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• ### Big and Small Contest

Hi GreatScott!
I want to make one as well.
My Colourcodes are the same. Could you just tell me really quick, where you connected the GREY Wire to and Why?
Because you didn´t mention it in the video, but you can see it connected.

In your scematic you once connect it to an LED and Ground, but later in the listing it is not connected.

Best wishes

Found it out myself.
The video is wrong.
follow the scematic in step 4 and you´ll be good.

hi guys, followed this tutorial, and I connected two 10 ohms 5W in parallel, exactly as shown in the video. the resistors are getting so hot that I afraid it will burn the wire. my question is it normal for this to get so hot ? or I simply did something wrong

this is normal as the resistors can produce 5W of heat

hey, i was wondering: you say that you make a dummy load to keep it stable when drawing small ammounts of power. what exactly happens when you draw small ammounts of power without a dummy load?

This is my explaination for this:
What your buck/boost converter does, is oscillating the voltage towards your set voltage. The pulse of the PWM signal will charge the output with energy and with the feedback signal it will correct the voltage by regulating the duty cycle. Sometimes the PWM signal will delilver too much energy and the duty cycle must be lowered. Before that happens you must get rid of the excess energy. This is why you need to unload the voltage with a resistor, just as you would discharge a capacitor. As faster the energy will unload the faster you get the set voltage. On the other side you waste energy over the resistor and you cannot set this resistance too low as at some point the converter cannot compensate for its energy (depending on the energy the capacitor and inductor of the converter can store).
#noguarantee

I want to add the specification of ATX: https://en.wikipedia.org/wiki/ATX
For the grey wire it says 'A control signal that is low when other outputs have not yet reached, or are about to leave, correct voltages'. This is indicating that the power supply needs some time to find the right duty cycle to get the voltage. Maybe somebody with an oscilloscope could check how high the voltage goes when turning the power supply on and no resistor is attached. Btw this overvoltage will not kill any device as there could be an internal resistor which will be quite high and the voltage will not have much energy as the duty cycle will be very low so the higher voltage will not be sustained for a long period.

Sorry I dont get ist with the dummy load.

You say it should draw about 0.5 A.

For the 5 V rail thats 10 Ohms. Ok, done. 10 Ohms* 0.5 Amps is 5 Volt.

But then you take two 10 Ohm resistors and install them paralell, doubeling the Amps to 1.0 Amp. At the same time you raise the lost power from 2.5 to 5 Watts wich means 5 watts of the theoretical output go te the dummy load and are converted to heat.

To make things even more confusing the resistors are drawn serial in the schematic resulting in an combined 20 Ohm resistor, and drawing only 0.25 amps instead of the recomended 0.5 in the text.

So..... whats right? 0.5 amps, two resistors or one, serial, or paralell?

I am sorry if I didnt use the right terms, but I am a native german speaker, my english is far from technical standart

In parallel, the equivalent impedance (resistance) is given by this formula : Req = (sum of R^-1)^-1 .

Two 10 Ohms in parallel will be equivalent to a single 5 Ohms resistor.

Ok, I will try again.

Video says in the Overlay Text: 5V: 5Ohm 5Watt.

He uses two 10 Ohm in paralell, resulting in a combined 5 Ohm resistor.

That much is clear. Its also clear that

5V/5Ohm = 1 Amp

and

5V*1Amp = 5 Watt

absolutely clear as far as the Video goes.

BUT: In the instruction Text he says at least 0.5 Amps ( half as much as in the video). Correctly he gives the formula

5V/10Ohms = 0.5 Amps

and

5V*0.5Amps = 2.5 Watts (also half as much as in the Video)

To add some more confusion here comes the schematic with the Resistors in serial, instead of paralell. ( R3/4 47)

10 Ohms + 10 Ohms= 20 Ohms

5V/20Ohms = 0.25Amps

5V/0.25Amps = 1.25 Watts

Now we have 3 different Values for one Instruction. I would just like to know wich one of the 3 is right. If you tell me the Power Supply needs about 0.5 Amps of dummy load I can arrange that. But if you tell me it needs 0.25 and 0.5 and 1 Amp I get confused.

Second Problem is the Grey "Power good" Wire. The schematic connects it to the resistor of Led1. Text and video use a red wire on Led1. Text says Grey wire is not connected.

Again: 1 Problem, three solutions.

The schematics says 'R 3/4 4.7' which means there are two resistors in serial called R3 and R4. Both have 4.7 Ohms which results in about 10 Ohms. So one thing you misunderstood is that the resistors in the schematic are each 4.7 Ohm instead of 10 Ohm each.

Indeed the video says 5V -> 5 Ohms and the instruction says 5V -> 10 Ohms. More Ohms mean less waste of power and less heat while less Ohms, up to a degree, mean more stable voltage at low output current. Try with 0.5 A first (10 Ohms) and if you experience much noise in output voltage, raise the current. Don't forget to recalculate the power so you don't overload the resistors (or just use them in parallel) and don't go too high on that current (do not use more current on the resistor and your output then specified by the specs of your power supply. You probably don't need to draw more than 2 A on the resistor)

Hi GreatScott, i have few questions on this topic. I have relatively new power supply which has 3.3V(36 A) rail, 5V(30A) rail, 4x12V rails (18A each).

So two questions:

1) Shall i combine all 18 cables (+12V) to single binding post ? (18 cables are quite thick :D )

2) On which voltage shall i put the resistor ? on 3.3V (biggest current), on 5V or on 12V (as it is 4*18A=72A) ?? If it is on 12V shall i use same 24 Ohm on 6W ??)

Any help is appreciated :)

Thank you :)

Resistor in parallel on 12V. Still 24 Ohm and capable of >= 6W. Put all 12V wires together as you need the combined thickness to deliver 72 A. You pbly don't need 72 A, but better safe then sorry. Also make sure that everything you hook up to the wires can handle the used current.
#noguarantee

I have a question, I want to build this project with an "old" power supply I have laying around. I found a schematic online define the minimum load (included image). Does this mean I need to put a dummy load on every voltage line to be able to use it properly?

Hey i just wanna ask i can add another post for a 5volt with a 4amp or 5amp current? Because im gonna use that to charge 18650 batteries..

Is there a possibility to do that?

And also all of 3.3, 5 and 12 volts are all 16 amps, where should solder the brown wire?

My psu is Delta Electronics DPS-220U B-5 A if you wanna check it out..

I really need you held right now man..

Is it fine to use 2 resistors 10Ω 5w in paralel as shown in video????? It will be for 5v rail

Hi I build one but when turned it on the green led turns on for a second and turns off quickly and power supply does not turn on can someone help me thank you

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

the 'amps' or 'amperage' of a power supply rates the maximum current (or electricity) the power supply can deliver.
(Like how a firetruck can deliver more water than a garden hose).
Your computer supply can deliver 12amps wich is plenty for the amplifier to run off. Think of it as watering your garden with the firetruck, it's overkill, but it will do the job.