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Author Topic: Safe way to discharge Capacitors.  (Read 5873 times)

Posts: 85

« on: September 28, 2007, 04:18:30 PM »

Can someone suggest a safe way to discharge capacitors? If there is a way.

Charlotte 2E0BSS

Posts: 5689

« Reply #1 on: September 28, 2007, 05:57:31 PM »

The answer is, "Through a resistance to ground."

However, more information is needed in order to calculate the resistance value.  

Voltage level and size of capacitor comes into play here.  

Rule of thumb, a ~470 - 2K, 2W resistor in series with an alligator clip lead can discharge most without the big spark.  Connect the other end to chassis ground, of course.  Higher the resistance the longer the discharge rate.  Use the voltmeter to check, or watch the voltage fall.  


Posts: 7440

« Reply #2 on: October 02, 2007, 07:18:32 AM »

Figure out how much energy is involved. E = 1/2 times C times V squared. The figure how quickly you want to discharge it -  lets say 5 seconds. Then, very roughly, aim for R times C = 1 , and you have 5 time constants. Then figure a wattage rating for the resistor of E squared over R, and a good average is to divide by about 4 because of the way the volts drop.

If you're talking of a supply above 500 or so volts, you need several resistors in series, especially when up at the 2 or 3 kV level. It all gets more critical as the volts go up. But for anything under about 300 volts, the previous advice holds - just use a fairly  meaty resistor.

Posts: 5689

« Reply #3 on: October 02, 2007, 10:40:42 AM »

I happen to know that Charlotte is experimenting around with basically QRP level stuff from her previous posts...

But it is always good to make points clear as you have, never know when someone going inside the linear amp for the first time may decide to apply a general rule of thumb suggestion where it shouldn't be done!

Thanks for adding that,


Posts: 15776

« Reply #4 on: October 02, 2007, 07:08:06 PM »

If you are dealing with 12V on the capacitors, then a 12V light bulb
connected across the capacitor will give you a quick visual indication
of when the capacitor is discharged.  If the output of your power supply
is 12V it may have 24 volts or so on the big filter caps, in which case
a bulb rated at 28V may be a good choice.  Put a wire with an alligator
clip (for ground) on one side and a wire running to some sort of probe
on the other.  

Or if you are using a resistor probe you can connect an LED + dropping
resistor across the power resistor to monitor the discharge:  the dropping
resistor should be calculated to draw something less than 20mA at the
maximum voltage to be discharged to keep the LED within limits.  The
LED brightness will indicate the state of capacitor charge.

If you are discharging capacitors with high voltage on them (such as in
a power supply for tube equipment) then you have to make sure the
handle on your discharge probe is well enough insulated for the voltages
you are touching.  A standard voltmeter probe is good up to 600V or
so, but above that you may want a longer handle to keep your fingers
away from the voltage, and/or heavier insulation around the wire.

When I built my first power supply one of the features was a shorting
probe permanently grounded to the chassis.  It used an old ball point
pen with a metal ink cartridge inside:  I just stuck a wire down the back
end into the metal tube and grounded the other end.  Since I neglected
to include a resistor to limit the discharge time it didn't take long before
the shock of repeated discharged blew the ball out of the tip of the pen,
but it still worked for its new purpose.  (It is best if the pen is empty,
though, and most modern pens use plastic ink casings.)

Posts: 5689

« Reply #5 on: October 02, 2007, 10:19:28 PM »

Why am I alive?  

I was a dumb kid in the 50s who drug every old TV and Radio home from the sidewalks around town, got them into my basement shack and if I couldn't repair them then stripped the parts and tried to build ham stuff from them.  Lots of CW transmitters with weird tube types designed for series string filament sets and stuff.  

And lots of shocks.  

Line Voltage, High Tension Plate Voltages, RF, -- hey, when the transmitter chassis is a cornbread pan turned upside down or a pine board and you are 9 years old.  

Great times, weldin' screwdrivers to those TV chassis...

Caps were for charging up and leavin' around for your sister.  Or the cat.  

Flybacks were for attaching to the basement doorknob and waiting for your friends to come by so you could yell, "come on in!"  


Posts: 2402

« Reply #6 on: October 30, 2007, 12:34:00 PM »

In college EE lab we just left them lying around.  Sooner or later somebody would pick it up.  When we heard the yell, we know the capacitor had been discharged.
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