# discharging large capacitors

S

#### stoj

Jan 1, 1970
0
hi all
i have a home made power supply with a pair of 50v 4700uf capacitors
(charged to around 45V depending to the transformer) and i was wonder what
is the best to discharge them due to the fact that i have been zapped by
them a few times?

Q

#### Quark Ng

Jan 1, 1970
0

Try this...

Have a momentary pushbutton switch and a 4Kohm 1/2 Watt resistor
connected in series.

Then connect one end of that circuit to ground and the other end to
the positive side of your capacitor.

When you want to discharge the capacitor, just press the button for 5
seconds.

Note the time constant is tau = 1/RC = 1/(4K * 4700u) = 0.05319
seconds.
And 5 times tau = 0.265957 seconds.

So you're really safe already just pressing the button for 1/4 of a
second. But push it for 5 to be REALLY safe.

Remember, 1/2 Watt is important. P = V^2/R = (45V)^2/(4Kohm) =
0.506Watts. But since your V goes down quickly over time, it's ok to
use 1/2 Watt.

-Quark

G

#### GPG

Jan 1, 1970
0
Note the time constant is tau = 1/RC = 1/(4K * 4700u) = 0.05319
seconds.
Time constant = R*C= 18.8 sec for 4K.
Wiring 10K .5W permanently across cap will reduce 45V to about
12V after a minute. Vc = Vo*e^-T/R*C

Q

#### Quark Ng

Jan 1, 1970
0
Time constant = R*C= 18.8 sec for 4K.
Wiring 10K .5W permanently across cap will reduce 45V to about
12V after a minute. Vc = Vo*e^-T/R*C

OOPS, you are indeed correct. What on earth was I thinking of...
And this here has just proven why engineering companies need at least
two engineers to work on anything... one to check in case the other
engineer didn't get enough coffee.

-Quark

D

#### Diverdan

Jan 1, 1970
0
OOPS, you are indeed correct. What on earth was I thinking of...
And this here has just proven why engineering companies need at least
two engineers to work on anything... one to check in case the other
engineer didn't get enough coffee.

-Quark

Hi to all you tech's, I am just a cabinet maker and want to remove an
old Zenith TV out of a floor console. I am a little fearful of the HV
in the CRT. What can I do to discharge the HV before attempting to
remove the unit from the cabinet.
Can someone help???

J

#### JeffM

Jan 1, 1970
0
want to remove an old Zenith TV out of a floor console.
What can I do to discharge the HV ?
Diverdan

I would be more worried about fracturing the back/neck
of the picture tube and having it implode.

Having said that,
the way it is typically done is to hook a jumper lead to the chassis,
hook the other end of the jumper to the blade of a thin screwdriver,
slip it under the cup of the Second Anode lead where it contacts the CRT
and touch the conductor (clip) in the center.
Holding it there for 2 seconds is more than enough.

Lift the lip of the cup and disconnect the Second Anode lead.
Poke the shorting strap into the CRT's Second Anode (hole) once more
just to be sure.

J

#### Jerry Greenberg

Jan 1, 1970
0
Get yourself a 2.7 K / 2 Watt resistor and leave in in parallel with
the main cap. The supply will be safely discharged in less than 10
seconds after the power is shut off from it. The current draw will be
very little while the supply is operating. Most supplies are designed
with this type of bleeder resistor built in.

If you want to have less current pull and use a smaller resistor you
can go up to something like a 4.7 K / 1 Watt resistor. I would allow
about 20 seconds after shut down, to regard the supply to be safely
discharged.

Jerry Greenberg
http://www.zoom-one.com

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