Convert ATX PSU to Bench Supply

The hardware of the PC gets the power from the Computer power supply units (PSU) through a number of cables with connectors by turning the incoming 110V or 220VAC (alternating current) into several DC (direct current) output voltages. 

ATX (Advanced Technology eXtended) is actually a  motherboard and power supply configuration specification that was developed by Intel.

Bench Supply gives DC (direct current) voltage for powering on a device like a circuit board or any other electronics.

In today’s article, we will be going to discuss how you can convert ATX PSU to Bench supply. Also you can Learn Basic Knowledge to Repair Power

introduction to ATX PSU

ATX Power Supply is an update of AT Power Supply types. There are 3  main voltage outputs that are generated by the ATX  power supply. They are as follows:-

  1.  +3.3 V
  2.  +5 V
  3.  +12 V
  4.  −12 V and +5 VSB (standby)

Connectors of ATX PSU

ATX Power Supply is an update of AT Power Supply types. There are 3  main voltage outputs that are generated by the ATX  power supply. They are as follows:-

  1.  +3.3 V
  2.  +5 V
  3.  +12 V
  4.  −12 V and +5 VSB (standby)

We already discussed at the beginning that the PSU supplies the power to the PC hardware through a number of cables with connectors. So here, let’s discuss the connector.

There are two types of connectors that are mainly used by ATX PSU . they are as follows:-

1) 20-pin Molex connector

20-pin Molex is a power supply connector  which is originally used for powering the motherboard that has +12VDC and +5VDC voltages. This older 20-pin variety is Version 1.

2) 24-pin Molex connector

24-pin Molex is a power supply connector that is the new version (version 2) of the original 20-pin version. 20-pin Molex is replaced by 24-pin Molex as it provides additional wattage ground lines. The diagram below will show the pinout diagrams of the two power connectors:-

main power connector (version 1 & 2)
Source:- http://www.computerupgradesrepairs.co.uk

Precautions to take before converting ATX PSU to Bench Supply

Before you start converting ATX PSU to bench supply, keep the following points in mind :-

  1. You should make sure that the  PSU is unplugged from the mains supply and also unconnected for several minutes before you start the conversion.
  2. You should also check properly that the metal box of the PSU is correctly earthed or grounded.

You must take care of these points, as you are working with AC voltage and a little bit of carelessness might result in something very hazardous.

Safety mechanisms of standard PC PSU

There are two safety mechanisms that are used by the standard PC PSU for preventing it from being switched “ON” without the motherboard attached. The 2 mechanisms are as follows:-

  1. A “Power-ON” zero voltage signal is given to the PSU to start up.
  2. To correctly regulate the +5V output voltage  the PSU needs to have some sort of load attached(at least 5W) .

I will explain to you how to convert an ATX PSU to a bench supply using two of the connectors. they are:-

  • 20-pin Molex connector
  • 2.)24-pin Molex connector

Convert 20-pin Molex ATX Connector to Bench Supply

convert-20-pin-Molex-ATX-connector-to-bench-supply

What are the Steps to convert 20-pin Molex ATX Connector to Bench Supply?

We will be discussing the steps to convert a 20-pin Molex ATX Connector to Bench Supply.  They are as follows:-

Step 1:-

You have to keep the 20-pin Molex connector attached or cut it off completely and group the individual wires together and also remember to keep the same colors together like keep yellow to yellow, blacks to blacks, etc.

Step 2:-

Now you have to keep the pin 14 – Green (Power-ON) with one of the common black wires (ground) in this way the motherboard will tell the power supply to turn “ON”.

 

The power supply will turn-ON when pin 14 will be connected to the ground for some time or permanently through a switch.

Step 3 :-

Now you need to give a small load on the +5V (red wires) output for tricking the PSU .

This trick will make the PSU think it is attached to the motherboard and so it will keep the power supply in  “ON” mode. 

How to do this?

You have to connect a resistor of 10 Ohms or less, with a standard power rating of 5W to 10W across the +5V output with just one set of the red and black wires, (pins 3 and 4 ).

You can relate this if you remember the Ohms Law with the equation:-

P = I2 × R or P = V2 / R

Where  

  • P = power developed in the resistor in watts(W),
  •  I = current passing through the resistor in amps (A), 
  • R = resistance of the resistor in ohms (ohm) 
  • V = voltage across the resistor in volts (V)

The voltage will be +5V and the power required will be 5W or above making it quite hot.

Test the Power Supply

Step 4:-  

Once you are done, you will end up something like this :-

20 pin molex connector

Now you have to turn the switch “ON” after plugging the PSU into the wall socket where you will find only two voltages present at the connector at the back of the power supply. They are as follows:-

  1. pin 14
  2. pin 9

These two of the voltages are basically used for the motherboard’s power control buttons.

You should now connect the following wires In the main power cables, they are as follows :-

1.)  Three red wires (+5V) all connected together 

2.)  Three black wires (0V) connected together as 

  1. Three orange wires for giving a +3.3V output     to power up some smaller devices or microcontroller boards.

Step 5 :-

If you end up doing everything correctly then the PSU should switch “ON with Bench power Supply.

Test the output voltages

Step 6 :-

You can use a multimeter for checking the Bench power Supply or connect a 12V bulb into the different sockets.

You can get a number of voltage combination as an output by the PSU , Those are as follows :-

  •  24v (+12, -12),
  • 17v (+5, -12), 
  • 12v (+12, 0),
  •  10v (+5, -5),
  • 7v (+12, +5), 
  • 5v (+5, 0) 

What are the Steps to convert 24-pin Molex ATX Connector to Bench Supply?

atx connector

So you already know that 24-pin Molex ATX Connector replaced the 20-pin Molex ATX Connector and it holds four additional pins. They are as follows:-

  1. pin numbered 11 which is +12v (yellow)
  2. pin numbered 12 which is +3.3v (orange)
  3. pin numbered 23 which is +5v (red)
  4. pin numbered 24 which is ground (black) 

Can we convert 24-pin Molex ATX Connector to Bench Supply?

Yes but this new version has become more tricky to convert as they use a ‘soft’ power switch function and applying a small load as you applied in the case of a 20-pin molex ATX connector will not work till you give a heavy load .

Although the output voltages will be the same as we discussed in the 20-pin Molex ATX connector.

Advantage of converting ATX PSU to Bench Supply

  1. You will get an amazing cheap bench power supply .
  2. You will gain a great knowledge practically about power supply unit.
  3. You are kind of recycling rather than buying a new one so it is good for the planet.

The disadvantage of converting ATX PSU to Bench Supply

  1. the cooling fan can get a little noisy
  2. Fresh air is needed to keep it cool.

PRECAUTIONS!!!

DO NOT forget to unplug the ATX PSU from the main power supply before you start the conversion process because if you forget to do that it may risk your life.

So BE CAREFULL !!

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