How to make a Variable Lab-Bench PSU using an ATX PSU

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PraveenAlexis
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How to make a Variable Lab-Bench PSU using an ATX PSU

Post by PraveenAlexis » Fri Aug 10, 2018 6:42 am

Disclaimer: ATX power supplies have high capacitance capacitors which will be charged even after the mains voltage is cut off, do not touch them or make sure they are discharged with a resistor before you continue. Do not try to replicate this project if you don't know what you're doing because mains voltage will be fatal.Do at your own risk!

First things first lets take a look at an ATX power supply (Computer Power supply); what it does , why we are using this and what does each colored wires represent.

What is an ATX Power Supply? And why we are using one of these for the project?

ATX PSU is a switch mode power supply which means it has switching regulators that used to convert AC to DC efficiently minimizing wasted energy unlike Linear power supply units. Also SMPS's have useful features such as Over Current Protection , Short Circuit Protection and voltage feedback sensing that you don't need to implement your self.

Wire Diagram

Image
  • Power good signal is a 5v signal that the ATX PSU supplies after the internal test is successful meaning all the Voltages are stabilized, this usually takes less than 0.5 secs.
  • To power on the PSU short the Power on to a ground (You can connect a rocker switch here to switch on and off the PSU)
  • 3.3v sense lead is used as a feedback sensor to detect the voltage drop across the leads to stabilize the PSU to stay in the 12v, 5v, 3.3v range all the time no matter the load (of course it depends on the max Amps of the PSU) connect this to 3.3v lead to make the PSU more stable.
  • Stand by outputs a 5v when the PSU has mains voltage, but not turned on
TIPs
  • ATX PSU's negative lead and the ground is the same therefore you cannot connect two PSU in series, which will short circuit one of the PSUs. you can disconnect the ground terminal of one PSU to achieve this (this will increase the risk of potential electric shock)
  • You cannot connect them in parallel because one of the PSU's will take most of the load, therefore it is inefficient.
  • Some PSU's contain 12v1, 12v2 and 12v3 leads they are all the same, if one of them is 12v Sense connect them to a 12 lead.
  • Some PSU's will not switch on unless there is a load on any of the output leads, there fore make sure you use atleast a 10w 22ohm power resistor to check them whether they work.
  • Some PSU's have 3.3v sense as an orange wire, don't be alarmed its the same.
Alright enough with the boring information, lets start to make this happen!

Things you will need!
  • A multimeter unit (MMU)
    Image
  • A rocker or a pole switch
    Image
  • Binding posts
    Image
  • 2x Red and Green LED's + 220ohm Resistors
  • 10W 22ohms (for 12v rails) , 10w 5ohms (for 5v rails)
    Image
Tip: Aluminium Clad resistors are expensive but has a good thermal solution (i recommend ceramic resistors because they are fairly cheap plus they can be found anywhere)
  • Terminal Rings (For binding posts)
    Image
  • Shrinking Sleeves
Wiring diagram for a DC bench supply

Image

Step 1
Remove the PSU lid and separate all the wires by colors and cut them to a decent length
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Step 2
Design a template or eye ball the places where you want the LED holes and Binding post holes should go, then drill them carefully (i recommend to move upwards to the size from the smallest drill bit for a neat finish and utilize the bigger hole to place the main switch)
Image

Step 3
Crimp the Terminal rings and screw them to the Binding post as seen on the below picture. secure the exposed metals with shrinking sleeves to avoid short circuits.
Use the 220ohm resistors in series of the LED's to limit the current through them.
Image

Step 4
Selecting a "Dummy Load" resistor
  • If your PSU has most of its power on the 5v rail, you should install the dummy resistor here (10w 5ohms)
  • if your PSU has most of the power in the 12v rail you should install the dummy resistor here (10w 22ohms)
Why Do I Need a Dummy Load Resistor?
SMPS's Usually dont turn on unless there is a load at the end, because in PCs there's always enough load to function a PSU (it is done by the Motherboard). therefor we must install a Dummy Load to fool it to think that the PSU has connected to a pc.

Well that's it you just made yourself a cheap lab bench PSU that can handle 12v, 5v, and 3.3v without any hassle

Thats no enough is it?
Now lets see how can we improve this project to convert this PSU into a variable bench PSU
Variable lab bench PSUs contain Constant Voltage and Constant Current features, therefore we need a Buck-boost converter that can handle atleast 10Amps at peak with CV, CC features.

i Found that "LTC3780 Buck-boost converter" has all the features needed and it has a feedback mechanism to check the voltage drop across the terminals. therefor im gonna use this module that i bought from ebay for about $10.
Image

Things you will need
  • 500k, 200k Potentiometer + knobs
Tip: you can use multi turn pots, although they are hard to find with 500k and 200k values.
  • LED Voltmeter Ammeter Module
    Image
  • Binding Posts
  • Shrinking sleeves
  • An enclosure box /project box
Step 1
Install the project box to the ATX PSU casing, take the lid of the project box and drill the holes for the binding post,LED's and potentiometers
Also cutout two squre holes for the Voltmeter anmmeter Module and the rocker switch.
Image

Step 2
Wire the Voltmeter Ammeter Module according to the wiring diagram given by the manufacturer. should be something like this. Make sure to
Image

Step 3
Connect all the Terminals and make sure to input 12v to the LTC3780 module because it is the most efficient input voltage as stated in the datasheet.
Image

Well thats it thats all it is there for a variable lab bench PSU....
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Ask any questions, i will try my best to help (if i know ofc) or i will provide enough material to figure things out
Also make sure to post your PSU's pictures if you ever create one


Reference:
https://en.wikipedia.org/wiki/ATX
https://www.electronics-tutorials.ws/bl ... upply.html

Last edited by PraveenAlexis on Sat Aug 11, 2018 12:33 am, edited 1 time in total.
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Neo
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Re: How to make a Variable Lab-Bench PSU using an ATX PSU

Post by Neo » Fri Aug 10, 2018 6:15 pm

This is truly one of the best articles I have ever seen on the internet. Very nice and keep up the good work !
PraveenAlexis
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Re: How to make a Variable Lab-Bench PSU using an ATX PSU

Post by PraveenAlexis » Sat Aug 11, 2018 12:54 am

Neo wrote:
Fri Aug 10, 2018 6:15 pm
This is truly one of the best articles I have ever seen on the internet. Very nice and keep up the good work !
I have few projects in mind that i need to get back :D hopefully
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