(Doug McFadyen) wrote:
>I need to make a small mod to my audio AMP (changing the Phono/turntable 
>input so that it doesn't pre-amplify the input signal).
>There are four 50V 4,700uF caps in the final amplifer stage that like to 
>bite, long after the power is disconnected.
>Is the correct procedure for discharging a cap to just short it at the base 
>of its leads?  Or, should a resistor be used to "bleed" off the power slowly?
>If I should use a resistor, what value is good?  What wattage?

I do not recommend shorting the caps.  You probably have enough stored 
energy to melt and launch small bits of metal from the leads.  You might
hurt an eye or some other component.

I have used the following active discharge circuit for perminently
connected power supply caps.  I am only showing the barest of details.

   Plus DC power ----+----------------------+
                     |                      |
                     D1             e       |
                     |             |/-------+---------  load
                     +----+--------|  pnp   |
                     |    |       b|\       |
                     C1   R1        c|      C2  (big cap)
                     |    |          |      |
            ground --+----+----------+------+

The pnp is actually a darlington.

The time constant R1,C1 is short, so that on power up C1 is only 1 diode
drop below the dc power and the pnp never turns on.  On loss of power,
R1C1 time constant determines how fast the big cap discharges.

You might play with resisters and parallel diodes in the base circuit.
Don't forget the base current during discharge.

Opinions expressed herein are my own and may not represent those of my employer.

Date: 24 Jun 1996 14:22:36 GMT

Original Subject: Re: Help Discharging Capacitor Properly!

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