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-   -   Series Caps in Power supply (http://www.diyaudio.com/forums/solid-state/13617-series-caps-power-supply.html)

Jonathan M 11th April 2003 03:35 AM

Here's the info I have:
 
The power amp is Rod Elliotts P68 subwoofer amplifier. It delivers 300W or thereabouts into a 4 ohm load.

It is powered using a 300VA 40-0-40 Toroid to a standard 35A bridge rectifier.

The filter caps are NEW 12000uF 50V electros (Computer grade?) that are in series to give 6000uF 100V. This is per rail.

I want to add a resistor across each cap individually to balance the voltage across them.

Assuming the that 50k will be good, that's what I'll use. I don't see the point of using a smaller resistance (It'll draw more power) unless it is of particular advantage - please advise.

Thanks for the responses so far. I've mailed Rod as well to see his views on the matter.

Cheers,
Jonathan

Mad_K 11th April 2003 03:41 AM

The info you guys need can be found here:
http://www.evox-rifa.com/electrolyti...c_appguide.pdf
>page 4

;)

Jonathan M 11th April 2003 03:58 AM

Thanks Mad_k
 
Useful info.

Tells me why I haven't had any trouble yet (Worse case scenario with 20% tolerance is around 33.6V across a cap - they're rated for 50V)

Also tells me I should use 1000000/(0.015*12000) = 5k6 Ohm
resistors.

1W will easily be enough (Will only dissapate 0.15W in each resistor once everything is balanced.)

Cheers,
Jonathan

nw_avphile 11th April 2003 03:59 AM

Re: WRONG AGAIN.
 
Quote:

Originally posted by fdegrove
Where did you come from with figures?

No way to calculate any values from what's given so far here, I reckon?

Why don't you reread the thread instead of contradicting yourself over and over again in a single post?
Cheers,;)

Look at a cap datasheet fdegrove. Most manufactures give the following forumula for their DC leakage specification:

DC Leakage = .01CV

So, we take .01 * 12000E-6 * 28v and we get 3.4ma. Your method of 100k total would allow0.56ma to flow which isn't enough to balance the caps if one has say 1ma leakage and the other 3ma. My values may have been overkill, but I'd rather be safe than sorry. I don't know the age of these caps and leakage current goes up with age.

As for the power specification, do I need to explain ohms law?

And where did I contradict myself?

nw_avphile 11th April 2003 06:49 AM

Re: WRONG AGAIN.
 
Quote:

Originally posted by fdegrove
If no bleeder is present you could try a pair of 50K Rs, just make sure to use equal values and that they have ample wattage for the current you're drawing..

Quote:

Originally posted by fdegrove
Where did you come from with figures?

No way to calculate any values from what's given so far here, I reckon?

Well I reckon that P = V**2/R. So let's see... the voltage across each resistor will be 28 volts and they're 50k ohm resistors, so 28**2 = 784 divided by 50,000 = 0.016 watts which I rounded up to 0.02 watts.

So your advice to "make sure they have ample wattage" is really out of place as it's hard to buy a resistor that won't handle 0.02 watts. So please explain the "Wrong Again" part of your subject line?


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