# Cap discharge question

#### erice1984

Joined Jun 9, 2007
16
what can I do to increase the discharge time of a capacitor

12vdc -> 100k Ohm -> 22uF -> gnd

in 3.5sec cap charges to 9.7v
but takes 10sec to discharge to 800mV

Anyway to make the cap discharge faster than charging or almost at same rate

Thanks

#### hgmjr

Joined Jan 28, 2005
9,027
One way to do it is put a diode in series with a smaller valued resistor and then put this diode/resistor combination in parallel with the 100K resistor so that the diode is reverse biased on charge and forward biased on discharge. That way when in the charge mode the diode is reverse biased and so only the 100K resistor is determining the rate of charge. Now when the discharge mode is selected the diode becomes forward biased and now the second resistor is in parallel with the 100K and so the discharge rate is speeded up.

hgmjr

#### erice1984

Joined Jun 9, 2007
16
good call, thanks!

#### hgmjr

Joined Jan 28, 2005
9,027
Erice1984,

I accidentally reversed the sense of forward-bias and reverse-bias in my recommendation so I corrected it in my earlier reply. I just wanted to let you know.

hgmjr

#### erice1984

Joined Jun 9, 2007
16
yeah, i understood the idea, i was having a huge brain fart on how to do that.

#### hgmjr

Joined Jan 28, 2005
9,027
So does that mean that you sorted it all out in the end?

hgmjr

#### erice1984

Joined Jun 9, 2007
16
yes i did, thank you very much

#### RiG615

Joined Nov 13, 2008
50
Is there a formula to determine the discharge rate??
and how much would the LED affect the discharge time?
how can you make charge/discharge the same time??