PC psu conversion

[danrogers]

Im looking at converting an old ATX psu for use with a LiPo battery charger.
I'm testing it out at the moment, but it does not seem to be supplying 12v even with a load on the 5v line.

Its reading about 11.8 which is fine, but when I connect the charger and try to draw about 5-6A the voltage drops to below 10v which causes the charger to shutdown.

Now, I'm only pulling the current down one of the wires, but I don't think its a dual rail supply?

I could cut all the 12v wires and join them and see if that works but if it does not then its a total waste of the PSU.

By the way its a 300w unit, that says it should be able to supply 10A @12v

thanks

 

[Nigel Goodwin]

There's no regulation on the 12V rail, probably only on the 5V one - you should probably change it to regulate the 12V rail instead.

 

[Reloadron]

Try placing a load (10 Ohm 10 Watt) on the 3.3 volt rail. I am guessing it regulates off the 3.3 volt rail (Orange Wires on most).

Ron

 

[danrogers]

Nigel, could you please elaborate?

reload, that the value resistor that I was putting on the 5v, ill try again with it on the 3.3 thanks!

 

[alec_t]

I could cut all the 12v wires and join them and see if that works but if it does not then its a total waste of the PSU.

Can't you just use shorting links rather than cut the wires?

 

[Reloadron]

Something else to think about is that not all PSUs are created equal. Many use a 3.3 volt sense line. If the PSU is a newer having a 24 pin main motherboard connector pin 13 (pin 13 should be either orange or brown and if brown tie the brown to 3.3 volts). In the case of an older 20 pin main connector see if pin 11 is brown and if so tie it to 3.3 volts. So you may want to look for any brown wires on the main connector.

Ron

 

[danrogers]

well I've tried everything now lol, I put a load on the 3.3v and the 5v lines, and bridged all the 12v lines together and connected to the charger and it still cuts out

I think its a fussy charger but it works fine on a 4A switch mode laptop style psu

 

[Reloadron]

Interesting as a good PSU should have worked. I can't think of anything overlooked?

Ron

 

[danrogers]

I'm pretty stumped too. I have tested it on a 8A bench psu and the charger soaks it all up nicely. The charger states 10.5-18v so I just tried running it over the -12 & +5v lines to give me 17v but that cut the charger out too. I left my multimeter at a friends so could not see what the voltage pulled down too but that's wierd.

I trying with 2 atx psu's and they are both behaving exactly the same lol, they are pretty old mind you but surely they should be ok

 

[Nigel Goodwin]

Nigel, could you please elaborate?

Well I've just been googling, there are various ATX circuits available on the net, here's one example:

https://www.electro-tech-online.com/custompdfs/2011/09/atx-power-supply-pfc-schematic.pdf

They seem to be claiming regulation from BOTH 12V and 5V, but I'm certainly VERY dubious about how that could work.

I would suggest you need to remove the 5V regulation, and use only the 12V regulation - in that circuit removing R506/C502, and connecting R507 to the preset instead (and perhaps altering it's value) may be all that's required.

 

[danrogers]

Thanks Nigel, I'm a bit dubious to start trying to understand the circuits in the PSU's that I have really. Is there anything else I could try?

 

[Nigel Goodwin]

See if you can find a circuit for your exact PSU, or draw the relevant part out.

You could try increasing the load (substantially) on the 5V rail, see if that helps?.

 

[SABorn]

From my playing with them i found you need a load resistor on both the 5 volt rail and the 12 volt rail or on power up they wont regulate correctly.

A quick and dirty method i use that works the best, is to plug in an old 3.5 floppy drive into one of the output cables, then you should get perfect power.

I often use one for testing things like steppers etc where i need a reasonable bit of current and the old floppy drive is quick and all works fine, not the prettyest but it will let you see if the supply is faulty or not.

It could be a CD rom or hard drive instead of the floppy any will work.

Pete.