Computer Power Supply to Car Setup

[Patthehat033 10+ year member]

Okay,

So today we had to take my brothers subwoofers out of his car to get my hockey bag in. Well anyways while they were out I got the idea to wire up his subs to my computer.

We have a bunch of power supplies downstairs so I just grabbed one of those. Anyways I hooked up all the proper wires and the amp only turns on for about 5 seconds with the protective light and then turns off. This is with a 400 watt majestic amp. When I used a 100 watt amp, that we have laying around from awhile ago, the amp turned right on, but unfortunately doesn't have a low pass crossover so that amp didn't do much.

I don't have the positive and negative backwards, so I am confused. Do I actually need a full 400 watt Power supply to just get the amp to turn on?

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[Ronny 10+ year member]

Throw a multimeter on there and see what the voltage dips down to while you are playing. Chances are that the PS can't support the current needs of the amplifier, so it kick into protection.

 

[Ronny 10+ year member]

To support mobile audio stuff in a home application, you really do need a PS that can deliver some current.

 

[Patthehat033 10+ year member]

Yeah thats what its looking like. I haven't got to play a song with the 400 watt amp, it will only stayed powered for about 3 seconds. Then it shuts off the power supply and the amp. Do you think I would need a PSU that could produce 400 watts? Mine only produces like 150w.

 

[r0llinlacs 10+ year member]

Did the amp work in the car? It should definitely turn on and stay on. I ran a roughly 375wrms pioneer amp on a 350wrms computer power supply, rarely ever shut off and that was only on full load.

Try a different power supply, I know some of them can be weird. I tried running a deck off a different (older) and less powerful (250 watt) power supply and it did not like it one bit. It would shut off when I turned it up. Eventually it would only turn on for a split second, then shut off, even without the deck on it. There's no way it was overloading it, as a deck only draws 10amps maximum.

 

[Patthehat033 10+ year member]

No it did not shut off in the car it ran fine in the car all the time, we just had to take it out for hockey, so I decided to try it in my room off a power supply. I noticed that the car amp has 2 25 amp fuses meaning it needs at least 50 Amps of current, from what I have heard. The computer power supply is only putting out 20 amps from what it says. Also this amp is 400 rms, and the power supply is only 145w rms.

When I plug in the power supply the amp turns on for about 3 seconds with the red protection light on, and then the amp and power supply turn off. Until I unplug the power supply for a few seconds. Any ideas?

 

[r0llinlacs 10+ year member]

And the other amp worked fine? Are you sure you didn't short out the power wire on the amp when you were taking it out?

I don't know, I'd try a different power supply. You wouldn't get much from that 20 amps anyways. I don't know where you got that rating from, but usually each rail has it's own maximum current. Was the 20 amps on the +12v rail? Sometimes that rating is for combined power of all rails, 5 and 12 volt.

Take all of your yellows and bridge them all together, and do the same with the grounds and see if that doesn't help. My trusty old power supply (28amps on 12v rail) will power any amp up to at least turn it on. I bridged all yellows together and did the same with grounds and extended them with 12gauge speaker wire. Like i said in my previous post, I powered a 760(max) watt pioneer (380rms) and pushed it to the max on an old school JL W-4 12 and it pushed it ok. Definitely not even close to the full potential of the amp, but it was still loud. It put quite a big load on the power supply, the voltage drop was bad.

 

[Patthehat033 10+ year member]

Sorry, I didn't realize the different voltage rails. Okay so the 12V rail only puts out 2.5Amps on the 110W PSU. And the 12V rail on the 145W one only puts out 4.2Amps. Haha so that is probably not enough to power much. I have tried both power supplies, they work with the 100 watt amp but not with the 400 watt one, probably because the 100W is cheap and probably doesn't even have a safety other than a fuse.

Do you think if I wire up 3 power supplies in series, using the 5v rails, equaling about 15V at say 40+ amps would work? Since car amps can run at about 15 amps.

 

[All_Logix 10+ year member]

ig you do that you run the risk of the current being way to heavy for the thin rail wires...and smoky fire might occure...you could just parrallel them all onto a large gauge wire which will give you high current and maintain 12v...

 

[Patthehat033 10+ year member]

I did that but there was not enough amperage. What if I have 3 PSU's. And use the 3.3V rail, and 2 5V rails in series. Equally 13.3 volts, the small wires would all be connected from the psu's, to a bigger 10 gauge wire. What do you think?

Edit: Sorry I was confused, you mean parallel the 5v rails? If I do that how would I maintain 12V volts wouldn't I only be maintaining 5 volts with high current?

 

[All_Logix 10+ year member]

Yeah I was referring the PU with 12v rails, most of the ATX ones use 12v now. Let me ask when you paralleled were those 5v, because if you put 5v on the amp that is why they went into protect mode

 

[Patthehat033 10+ year member]

Keep in mind these are old power supplies. But no when I paralleled them they were at the 12V rail that barely puts out 6 amps with both Psu's put together. I was saying that maybe if I put the 5v rails together which have more amperage, they would be able to turn on the amp and keep it going. But I would need to get the 5v rails up to 12V or higher. I can try 2 Psu's at 10 volts, and see if it turns on first. What do you think?

Edit: So I voltimetered it, and I turn it on, and then it powers up to 11.33 volts exactly and shuts off every time.

 

[keep_hope_alive Premium Member]

stop trying to use computer power supplies - they will only give you issues and will not be sufficient for your needs. in general, don't parallel power supplies - they don't like that either (unless you have properly rated diodes isolating them (and dealing with the voltage drop across the diode).

for the price of a good power supply capable of higher current at 12.5-13.8VDC you could buy a plate sub amp that will do more than you'll need. Even this one is outstanding (i've tested it with a variety of car subs and it does great). it also includes crossovers, turn-on, etc.

Dayton Audio SA240 240W Subwoofer Amplifier 300-804

they also have 500W and 1000W versions if you think you need more.

this will do more than your "400W" amp could simply because of the difficulty in maintaining the DC supply voltage/current necessary to make rated power.

or look to ebay for 13.8V 25A power supplies. but every time, the sub plate amp wins. they are also great to have around for testing subs.

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i'm glad someone picked up on the different voltages out of the PC power supply and the limited current available at +12VDC.

 

[All_Logix 10+ year member]

Question Keep_Hope_Alive... Do you know this issue from power supplies by experience?....I never considered the current attempting to complete the circuit through an additional power supply?

 

[Kangaroux 5,000+ posts]

Computer PSU's are nice to test out amps but shouldn't be used to run setups. Most of the PSU's I've had only put out like low 11's for a voltage so the amp heats up a ton (not good!)