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Old 8th October 2006, 01:26 AM   #1
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Default Rail capacitor discharge circuit

I was searching for a rail capacitor discharge circuit to discharge all caps relatively quickley when the power has been shut off.

I know a very popular and simple method is to put two large resistors in parallel with the rail caps:

Does this significantly affect the power delivery capability of the caps considering they have a constant resistance to supply in addition to the amp itself?

I was thinking about using a switch mechanism that engages the resistors only when the power is turned off. Is there a simple circuit to do this?
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Old 8th October 2006, 03:04 AM   #2
anatech is offline anatech  Canada
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Hi rtarbell,
This topic has come up before in threads. All you really need to do is ensure some current draw. One idea would be to keep output bias current flowing and let the output stage do this for you. Complicated maybe.

Resistors have worked for many companies over time. Why mess with success? Just move them off the filter caps where they are normally found.

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Old 8th October 2006, 07:47 AM   #3
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The first thing you must decide is how long the discharge time has to be at the most. When you know that you can start to think about solutions.

What do you have?
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Old 9th October 2006, 06:00 AM   #4
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I am planning on about 22,000uF per rail, and I would like to discharge the capacitors in under a minute (hopefully).

This would require approximately a 550 ohm resistor, and at 80V, that means I would need a 12W resistor.

I was thinking of a simple circuit to only engage the discharge resistor on power-down, instead of having the resistor across the capacitor at all times.
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Old 9th October 2006, 07:47 AM   #5
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What about the idle current of the amp. How large is it?

Have you asked yourself why you need to shorten the discharge time?
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Old 9th October 2006, 08:05 AM   #6
lineup is offline lineup  Sweden
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hi


instead of discharging supply caps
many use
a good audio relay from amplifier output to loudspeaker

this relay will disconnect LSP
and no sound will be heard after SWITCH OFF

of course one other great benefit from using high quality output relay
is to use together with a safety circuit
that will save your Hi-Fi loudspeaker
in case something goes wrong with power supply and/or amplifier


lineup
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Old 9th October 2006, 11:41 PM   #7
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Thank you for all the replies!

About why I would like a relatively short discharge time,

One reason is definately for the turn-on thump if the amp is switched off, then on again in a short period of time.

The other reason is safety (my own safety) if I have to operate on the amp. At my workplace, we have some power supply modules that we have to troubleshoot at times, and they have very large capacitors (they are switching power supplies). The capacitors are small capacitance (470uF), but large voltage rating (400V).

One time, I had to work on one that had been sitting on the bench (unconnected and unused) for two days, and I found that the capacitors still had ~180V on them. There was nothing to discharge them through when the power supply was disconnected. Had I made the mistake of assuming that the unit was safe to operate on, I would have probably gotten zapped.

I was worried that this may happen in an audio amplifier if the circuit does not discharge itself. I would think that the circuit idling current would be enough, but I wanted to be on the safe side.
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Old 10th October 2006, 12:38 AM   #8
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Maybe you could use a couple(or a dual) normally closed relay in series with a power resistor (current limiting device) to bleed the charge from them a little faster, but would be active (or open) during operation when the amp is on. This circuitry should be quite simple.
Personally, I detest relays on the speaker output.
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Old 10th October 2006, 12:51 AM   #9
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If you are uncertain about caps being charged, just install a LED with an appropriately sized dropping resistor across them, then when you open it up, you have instant indication of potential hazard.
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