[Need help] ATX Power Supply OCP/UVP disabling

[ipullstuffapart]

Hi guys! I'm new to the forum here, but am indeed a fellow Eevblog viewer
Hoping you could help me out with a bit of a pickle I'm having with a project of mine.

I am attempting to run a car amplifier off of an ATX power supply.
As a note : Yes the power supply is capable of running the amp

So as I've found, trying to turn the amplifier on causes a rather horrific undervoltage lockout on the PSU as the amp turns on. This is a huge problem.
I have read on the RepRap forums that people have been able to disable the undervoltage protection on their power supplies to stop them from... well, turning off under load when used for 3D printing. This person was using an EST. 7502C (datasheet) and disabled it by shorting pin 4 to ground and breaking the trace to pin 4 on the board.

I have checked on my PSU and it is running a CP494 (datasheet), however as my knowledge of EE is hazy at best, I was wondering if I could get a bit of helpful advice on how to disable the UVP on my supply as I can't get my amp to run.

Thanks!
Sean

[Whales]

As a note : Yes the power supply is capable of running the amp

By what margin?  Keep in mind many PSU manufacturers hideously overstate the ratings of the PSU.  Even honest labels are arbitrary to some degree: some manufacturers give a stable max with a small amount of headroom, others give a theoretical max (where the unit starts screaming for its life). 

I'm only vaguely familiar with the topology of audio amplifiers, so I assume the inrush current to charge its primary capacitors is what's triggering your OCP.  Perhaps looking at inrush current limiting may be of help.  Mind you, if the PSU can't survive the capacitor charging, I severely doubt it will be able to supply enough power for the amp's max output -- YMMV, and new PSUs are relatively cheap.

[ipullstuffapart]

As a note : Yes the power supply is capable of running the amp

By what margin?  Keep in mind many PSU manufacturers hideously overstate the ratings of the PSU. 

Specifically speaking, there is very little headroom, the amp pretty much peaks what the power supply can handle (and it is a cheap supply so most likely overrated output).
> What doesn't help is that the amp, though rated as a 300w monoblock by Rockford Fosgate actually puts out just over 400w at 5%THD, so with head loss and everything accounted for, we're looking at about 35+ amps at 12v

However, it is not as if I will be having this amp screaming as it is going to be powering a sub for my computer, meaning for any decent SQ with my speakers, it'll be running at about 1/5th capacity, it's just the sudden charge load when it's turned on that gives it a peak to trip it.

To be honest however, I just want to do a workaround for the supply, as car amps can handle massive voltage changes when they create a bit of load (eg when it turns on, I'm not too worried about a stupid 1% or so drop in voltage that the UVP is tripping for, and if the power supply blows it's not a huge issue for me (I have about 10 of them ripped from a bunch of ex-govt computers) so replacement is easy.

[Kjelt]

You have to look at the buildup of your powersupply, if the 12V is a single rail or multi rail. For instance  if the PS is 300W rated that usually does not mean it can deliver 12V and 25Amps but for instance 2 times 12V 10 amps and the rest in 3,3V and 5Volts.
Then next step is to startup the PS with a small load and after it has start up add the caramp with a resistor in series to limit the current to 10Amps or so, if that all works bypass the resistor and play the music see if it still holds out. That should give enough information where it goes wrong.

[tszaboo]

Well, you can limit the inrush current wit a lightbulb. If you can find a big enough one...

[ipullstuffapart]

The PSU is indeed a single 12 volt rail, so no real worries there. I did try it with a few hard drives and DVD drives plugged into it and started the amp, same result. I guess a long read of the data sheets may uncover how to disable the UVP.... The idea is to have a $0 budget