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.
- 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
- 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.
Things you will need!
- A multimeter unit (MMU)
- A rocker or a pole switch
- Binding posts
- 2x Red and Green LED's + 220ohm Resistors
- 10W 22ohms (for 12v rails) , 10w 5ohms (for 5v rails)
- Terminal Rings (For binding posts)
- Shrinking Sleeves
Remove the PSU lid and separate all the wires by colors and cut them to a decent length
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)
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.
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)
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.
Things you will need
- 500k, 200k Potentiometer + knobs
- LED Voltmeter Ammeter Module
- Binding Posts
- Shrinking sleeves
- An enclosure box /project box
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.
Wire the Voltmeter Ammeter Module according to the wiring diagram given by the manufacturer. should be something like this. Make sure to
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.
Well thats it thats all it is there for a variable lab bench PSU....
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
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