PC PSU To Power Car Amplifier

[toyer]

Hey I am brand new to the forum and this is my first post. I am a student studying Electronics at a senior school as one of my subjects so my knowledge is not very extensive with a lot of situations.

Here is a little information on my current situation.

I am currently working on a school project were I am making a stereo system some what like you would see in a car. I plan on having two 'towers' with two 6" 3-way speakers in them each. I also am going to have another piece to the system called the main hub. This hub will be a medium sized box that will contain all the electronics (power supplies, amplifiers and a fan or two). This hub will also contain a 10" sub woofer. I will seal the sub woofer off from the rest of the electronics so it doesn't shake them to pieces and I can ensure it has all its needed air volume.

On the electronics side of things. For the speakers, I will be powering them of two 50 watt amplifier kits I am building. These kits run off a 39V power rail (or something close to that) which will be supplied from Transformer circuit that will give me 240V AC to 39V DC. For the sub woofer I will be powering it off a old amplifier I have (Clarion APX 200.2). To power the amplifier I am using a 350 watt computer PSU ( http://www.skycomp.com.au/item/Huntkey-CP-350--350W-80mm-Fan-Power-Supply.aspx ) that have a 12V rail at 19A.

So far with the PSU I have opened it up and trimmed all the unessential wires and heat shrinked them. I also connected the green wire to the ground so it turns on. With all that done I wire up the PSU, amplifier and sub woofer.

And now to the problem ... almost. I was unsure how the power supply would do when the amplifier was at full power and according to the specifications the amplifier draws 28A at full power and produces 235 watts. So I didn't know if the amplifier would draw only up to 19A and go no louder or over draw the PSU and destroy it. And from that I put a 10A fuse in the main power line into the amplifier. The intention behind this was to see how far the volume meter would go and how loud 10A of power would be before it blew.

I used my multimeter and tested the amps which revealed about 9A to 10A both at no volume and full volume. At first the was unsure about it and thought it should drawing way more current than that. But then I tested the voltage and notice that as I turn the volume up the voltage drops to about 10V. Why is it doing that?

I played the sub woofer for a while changing the volume testing it often but got the same 9 to 10 amp reading.

I thought that as the volume increased the amp drew more current and increase the output, is that correct?

After about an hour of playing different song and testing the amp I unwired it all and tested just the PSU by itself. Testing the +12V rail and ground I got a reading of about 10A, why is the PSU not running at 19A like it says on the specifications?

I have also tried it with out the fuse in line. I still barely draws over 10A of power.

Some things I might note:
- It says Switching Power Supply on the PSU, is that a problem?
- If the you follow the link the description there says it have some protective circuitry
-I read this post
http://www.diyaudio.com/forums/class-d/137757-my-powersupply-giving-enough-power.html

And although it was of some help I don't understand the formula, the thing about the voltage and load; and the adding a capacitor in the voltage rail.

Any help would greatly appreciated and I apologise for the wall of text.

the pc psu is taking its main feedback trhought the 5 volt. so in order to get the 12 volt full load from the psu power supply you also have to load the 5 volt rail which u can not do easily. so when you draw so many currents from 12 volt rail its voltage will drop since the pc psu is not getting its feedback voltage from 12 volt.

you can change the feedback control loop from 5 volt to 12 volt but this time when you drawn too much current from 12 volts, control loop can stabilize your 12 volt but also it will also increase the other voltages. (5 volt and -12 volt). now its not stabilizing your 12 volt output since its getting its feedback from 5 volt and so your pc psu is save but 12 volt will decrease.


its a multioutput psu but its adjusting its output just taking only one reference from 5 volts. other are just the turn ratios on the transformer that is adjusting their outputs.

 

[macboy]

That's because the power supply isn't designed to do what you're asking. A multi-output power supply is not designed to deliver a ton of current into one output while the others sit idle.

Get yourself the correct power supply and save this one for something it'd be good for.

All modern PC power supplies are designed to deliver at least 75% of their total output power on the 12V line. Motherboards and video cards have their own on board switching supplies to convert the 12V into high current low voltage for the power thirsty CPUs/GPUs.

 

[N-Channel]

That's not how a PC power supply works. Most of them are flyback power supplies and removing all that stuff will not do much at all. Except break things.

If the feedback is tied to the 5 volt line, removing it causes an open feedback loop. Depending on the supply this could result in failure or shutdown (if it has overvoltage protection for the other voltage rails.)

Your suggestions don't address the primary cause of his problem. He has only 19A available and needs 28A. Modifying the power supply doesn't solve the problem. Even if he successfully defeats current limit, he'll have a trashed power supply when it fails.

@ FoMoCo:
Most computer PSUs are NOT flybacks. Most under, say, 150W may be, but if you read the OP's question correctly, he says he is using a 350W unit. Not too many PSUs at that power level use flyback topology. For computer PSU designers, half-bridge (HB) is the topo of choice here.

@ IXLoiero95XI: I'll get to my answer in a second, but I've got to defend my response from what appears to be a general lack-of-understanding of your question by those who responded. I realize this thread is over a year old, but I just saw it for the 1st time since answering your original question. Most of the posters here (KatieandDad, FoMoCo, and others) are missing the whole point: this is a DIY forum (that means Do It Yourself), which is what you clearly intend. Simply buying some never-heard-of company's off-the-shelf design is not in the spirit of DIY.

Now, on to the question: By "pots", I mean Potentiometers, or variable resistors on the pc board. If your power supply has one, by turning it, you can vary the output voltage up/down by a volt or two. This lets the manufacturer dial in each output to the exact value it should be.

HB PSUs can be configured to eliminate all sections except the +12V section, depending on how involved you want to get into it. You could re-wind the Xfmr to pour all its power into the +12V section, keeping in mind that most of the feedback from the output is taken from the +5V section (tightest regulated output).

To wind up this long-winded post, there are MANY threads in the Power Supplies forum here at DIY that address just this kind of thing: re-purposing a computer PSU for audio purposes. The two most common mods are: 1) Using only the +12V section to make a nice +13.8V bench-top supply for car amps & other 12V applications; and 2) changing the output to a bi-polar +/- output for powering an amplifier directly.

I did the latter a while ago, and had pretty good results. Anyway, I see this thread is still active, so, I hope you're still following it.

Good Luck,

Steve

 

[mlise]

Yep,
at 350 watts it's guaranteed to not be a flyback. I have seen a 350 watt flyback, so it is physically possible, but it was bigger than an ATX supply. Modern supplies generally are a single 12 volt supply with buck regulators to make the other rails. That's just the cheapest way to do it. Since the 12 V droops, it's an older supply with the regulation mostly coming from 5 V. Probably a 1 switch forward converter. Adding an automotive stiffening cap to the 12 V would be a no-no as it will totally screw up the control loop compensation of the ATX supply.
Fortunately, for the project at hand, car amps will tolerate pretty much anything, and have their own regulated supply to kick up the voltage. As the 12 V droops, the supply in the amp will compensate. The 10 A at idle seems a bit much and may be a measurement error, otherwise the amp would be burning hot all the time. The project will be great and will work until it's replaced with something better.

Cheers