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Old 12th March 2013, 02:03 PM   #1
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Default PC PSU To Power Car Amplifier

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/Huntk...er-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
Is my Powersupply giving enough... Power?

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.
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Old 13th March 2013, 03:39 AM   #2
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Does your PSU have any pots on its pc board? If so, these are most likely output voltage adjust and current-limit adjust. Start by removing all parts in the +5V, -5V (if any), +3.3V, and the -12V sections- resistors, transistors, capacitors, everything. This will leave you with just the +12V section(s) Now, as the voltage adjust pot is mostly for the +5V section, it also does affect the +12V section. Measuring the +12V output with a DMM, slowly adjust the voltage until it reaches anywhere from +13.8V to +14.2V. Your Amps and other audio electronics will run much happier than from straight 12V.

Hope this helps! And keep those questions coming- they're all good ones, and it sounds like you're off to a good start. Don't let any negative-sounding advide dissuade you from completing it. Please keep us posted of your progress. Pictures are always encouraged, too.
Best of luck,

Steve
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Old 13th March 2013, 09:43 AM   #3
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Thanks for the reply, but I don't understand you entirely. What do you mean by pots?

And how do I know what parts are for the each section?
If I am able to remove all these parts how would I adjust the Voltage?
It sound like this much modification leave a lot of room for error as I am not that experienced.

Sam
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Old 13th March 2013, 11:13 AM   #4
FoMoCo is offline FoMoCo  United States
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Exclamation Don't Do It!

Quote:
Originally Posted by N-Channel View Post
Does your PSU have any pots on its pc board? If so, these are most likely output voltage adjust and current-limit adjust. Start by removing all parts in the +5V, -5V (if any), +3.3V, and the -12V sections- resistors, transistors, capacitors, everything. This will leave you with just the +12V section(s) Now, as the voltage adjust pot is mostly for the +5V section, it also does affect the +12V section. Measuring the +12V output with a DMM, slowly adjust the voltage until it reaches anywhere from +13.8V to +14.2V. Your Amps and other audio electronics will run much happier than from straight 12V.
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.
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Old 13th March 2013, 11:16 AM   #5
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+1 Just don't do it.

Far better to purchase a suitable PSU. SMPS supplies can be bought for pennies these days.
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Old 13th March 2013, 11:29 AM   #6
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Well thats not my problem exactly.

The main problem at the moment is that I used my digital multi-meter and tested the +12 rail and it was running at about 10A. I also tested it under load with a car amplifier running a sub woofer at different volumes but it still said 10A. The only thing that changed was that the voltage dropped at the volume got higher for 12V to about 10V.

Sam
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Old 13th March 2013, 11:30 AM   #7
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What would you consider a suitable PSU? and one that won't eat my wallet

Sam
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Old 13th March 2013, 04:02 PM   #8
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“So far with the PSU I have opened it up and trimmed all the unessential wires and heat shrinked them.”

“ I put a 10A fuse in the main power line into the amplifier.”

“The only thing that changed was that the voltage dropped at the volume got higher for 12V to about 10V.”

The computer power supply has many wires in parallel to the power connector to carry the high currents. Cutting all of them off except one wire will cause voltage drops.

A 10 amp fuse is a resistor. If you measure across the fuse at full power you will see how much voltage it is dropping.

And lastly, 12 volt car amplifiers have there wattage ratings measured with 14.7 or 14.9vdc input at the amplifier terminals.
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Old 13th March 2013, 05:27 PM   #9
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by IXLoiero95XI View Post
Well thats not my problem exactly.

The main problem at the moment is that I used my digital multi-meter and tested the +12 rail and it was running at about 10A. I also tested it under load with a car amplifier running a sub woofer at different volumes but it still said 10A. The only thing that changed was that the voltage dropped at the volume got higher for 12V to about 10V.

Sam
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.
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Old 13th March 2013, 09:08 PM   #10
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“So far with the PSU I have opened it up and trimmed all the unessential wires and heat shrinked them.”

By that I mean that I have trimmed all of the wires except the yellow and black and green wire. I kept all the yellow because I knew only one would not be able to handle the current and black.
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