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cduma

Sun Jul 03 2011, 03:59PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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I am trying to use an atx power supply to power a subwoofer and the PSU is only 400 watts. I need to add a capacitor but, Im not sure if it would be ok to just add the cap to the output and not worry about surge current. The cap I have in mind is 76,000uF at 15V and it wouldnt be a permant solution, its just to test to make sure everything works properly before spending money on a big capacitor. Another problem is that it sounds like I am getting less than 5watts of audio output before the voltage dips. Could this be because I am not using anything other than the 12V?

*Can I add a cap and not worry about surge current?
*Do I need to add a load to the other outputs on the PSU?

cduma

Sun Jul 03 2011, 07:04PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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I decided to add the caps and it still only outputs a couple of watts before turning off. A bit of improvement but, not much

big5824

Sun Jul 03 2011, 07:49PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

You generally need to put a small load on the 5V rail, mine is sized to draw about 200mA if i remember correctly. Also charging a capacitor that size without a charging resistor will most likely trigger the short protection circuitry.

cduma

Sun Jul 03 2011, 08:15PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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So far power has been cycled at least 10 times with no problems for the caps. Ill try adding a 5V load and see what happens

cduma

Sun Jul 03 2011, 08:44PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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I added a 2ohm resistor to the 5V line and it helped a little bit. Following this pattern I should add a resistor to the 3.3V line too

Arcstarter

Sun Jul 03 2011, 09:22PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
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Well, what did you do with the green wire to turn it on? It should be grounded with most supplies rather than held high, just in case that is what you've done. Or maybe you've done nothing with the green wire. Both cases sound plausible, because that is what i have always observed my supplies doing when the green is either floating or held high.

The 3.3v rail does not need a load, only the 5v rail, at least with my ATX supplies. I used a 500ma load on my 5v rail (10 ohm resistor) and it works well, so if yours is dissipating too much, you could use higher resistance.

Also, im not sure why you need a capacitor, the supply has enough filter capacitance for its rated load. This is only on the order of hundreds or thousands of uf, because of the fact that high frequency has lower ripple per capacitance than low frequency. Inrush current will almost surely be a problem for a capacitor the size you are talking about. ATX PSUs have an overcurrent shutoff, and it shuts off very fast once overcurrent is detected.

cduma

Mon Jul 04 2011, 12:21PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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The green wire is held low.

Maybe the reason I am not getting very much power is the same reason the caps dont seem to be bothering it. Is there anything to this link?

Conundrum

Mon Jul 04 2011, 12:25PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
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Most if not all SMPS's NEED at least a 10% load on all rails before they will work at all.
Even if they do run the 5V and 12V at least needs to be loaded, or they can abruptly fail and potentially fry anything connected to them, I learned this the hard way.

Couple of medium sized CPU fans and power resistors ought to do it, plus you can use the extra rails to power other stuff.

As for preventing tripping, my suggestion is to use a constant current with a relay to short across it when the voltage hits a threshold, this method is used on car audio systems.

This works, I use an LM317T as the CC as it does not need a ground connection to work unlike many other regulators.

-A

cduma

Tue Jul 05 2011, 11:50AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Can you offer a schematic for how to use the LM317?

Mattski

Wed Jul 06 2011, 06:06AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Conundrum wrote ...

As for preventing tripping, my suggestion is to use a constant current with a relay to short across it when the voltage hits a threshold, this method is used on car audio systems.

This works, I use an LM317T as the CC as it does not need a ground connection to work unlike many other regulators.

-A

I'm somewhat confused here, are you suggesting to use a LM317 constant current supply to charge a filter capacitor from which the audio amplifier is powered? And to short circuit the output of the LM317 once the capacitor is charged to a certain value? This technique would work but it would involve a lot of power dissipation, especially considering the 400 watt amplifier cduma is using. The LM317 wouldn't be able to handle that power but there is probably a way to hook it up with a pass transistor to handle the main power dissipation.

Putting a large inductor between the ATX and the capacitor might help avoid the short circuit protection of the ATX, it will act something like a constant current supply, pumping a (somewhat) continuous current into the capacitor even while capacitor voltage drops due to a load current spike. One caveat would be voltage spike at turn-on. A series NTC thermistor might fix that, or some type of freewheeling diode circuit which often goes with inductors.

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