Atxpower supply

[rabdalilo]

hi friends I'm new, I'm looking someone who advises me to build a lab power from a micro ATX power
why I chose the model: ATX-400W RELISYS.
thank you to everyone.
rabdalilo

MOD EDIT Please use English when posting on this forum... Forum rules apply.

 

[MrAl]

Hi,

Most of these power supplies use a switching regulator controller chip. They usually have under and over voltage functions as well as the nominal voltage regulation function. The voltage regulation affects all of the outputs including +5v an +12v outputs. Lowering one means lowering both of those.

To make the +12v output adjustable you'd have to find out what chip the power supply uses and find the voltage set resistors and change one or more, using a pot somewhere also so you can adjust the output. You might get 2v to 12v output or maybe less like 1v to 12v but it's hard to say for sure what the low end would go down to.

But you'd also have to find the under voltage set resistors and vary them too to disable the under voltage function, otherwise the power supply will shut off. You also need a certain minimum load for the output or it will shut off.

So it's not the simplest thing to do. If you build your own adjustable output stage however then you have more control and you dont have to go hunting for what part they use and where the set resistors are and all that. So you would basically build a stage that simply regulates the +12v output line down to whatever you want like 1v, 5v, etc., using a pot.
The output stage can be linear if you dont need too much output power, but for higher power outputs you would probably want to use something like a buck circuit,
The linear stages are really simple to build however, so if you dont care too much about power you can certainly do that.

 

[ClydeCrashKop]

This is a good article.
https://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

 

[dr pepper]

If you want fixed voltages it straight forward.
I have tried this as an adjustable supply and it was tricky, data for later psu controllers is unavailable, there are voltage monitors for each rail on most controllers, if the voltages are not within limits the controller shuts down, you have to fool the voltage monitors to keep them happy and mess with the feedback for the controller.

 

[Ramussons]

You cannot have Independently adjustable outputs from an ATX PSU.

The problem of adjusting voltages lies in the Switching Transformer. A single transformer has the Secondary windings in a particular ratio to derive the main feed voltages +5 V and +12 V. In the ATX power units, Regulation is based on the 5 V line. Other lines have protection only for Under / Over voltage and current limit. ( This is why, in most ATX units, the unit will shut down on a "over" 5 V status if there is no Minimum Load on the 5 V line (about 1 Amp).

The 12 V line can, sometimes, be boosted to 13.8 V ( to make a 12 V car battery charger) by reducing the 5 V feedback. This will also increase the 5 V line proportionately.

Ramesh

 

[dr pepper]

Sorry rams your wrong this time.
I have such a thing.
It only has one output and definately does have adjustable voltage and current.

 

[MrAl]

Hi dr pepper,

Isnt he talking about power supplies with more than one output?
Like +12, +5, -12, etc.

For these supplies they usually use only one transformer so the outputs are all dependent on the PWM of the primary of the transformer, which can only be one set duty cycle.

There is also only one feedback for the chip used for voltage, which they might sense on the +5v line, but sometimes they add the +12 and +5 or other voltages to make up the feedback and that way both outputs get regulated to some degree, although no where near perfect.
The test is to load one line more than the other and see what happens.

There is a second feedback on most chips used in these supplies so they may use that one for over current feedback to protect the switch transistors.

It's amazing in this day and age they cant do it better though. Maybe some very expensive supplies do use more than one feedback and more than one transformer and chip. That would be able to hold those voltages to within a very small percentage of the required level.

 

[dr pepper]

Yes that correct at least on ones I've messed with, The supply I modded had feedback for 5 and 12 rails, and independant overcurrent.
Putting a pot in the feedback line and feeding the voltage monitors with a resistor string did the trick.

 

[Ramussons]

Sorry rams your wrong this time.
I have such a thing.
It only has one output and definately does have adjustable voltage and current.

Please read my first line.

Ramesh

 

[dr pepper]

Oops apologies, you meant more than one output, without serious mods your quite right.