Safely discharging large value capacitor without blowing it up

Question

This is related to my previous question. I have this circuit in which I discharge the capacitor by turning ON M2. The ON resistance of the MOSFET I have in mind (the picture shows the wrong MOSFET in an earlier design) is 0.0035 ohm. I've placed a 1 ohm resistor in the discharge path to limit the current. The 40 mF capacitor (4 X 10 mF) will have a voltage of 0.6V when fully charged. So I'd have 0.5979 A current during discharge. Is this safe? Will my capacitor spark/blow up? Will its lifetime decerease if this is a repeated operation? What would be a safer option for discharge? I could increase the resistance, but I'd like to know exactly how much current is safe.

I need the charge\discharge process to be controlled by an MCU, so any manual discharge methods\removal of capacitor is not an option.

How fast and often does this discharge need to happen? – Matt Young Dec 27 '13 at 17:38 — Matt Young, Dec 27 '13 at 17:38
@MattYoung 750ms, but may be relaxed if it is unavoidable. – Analon Dec 27 '13 at 17:40 — Analon, Dec 27 '13 at 17:40

score 1 · Accepted Answer · answered Dec 27 '13 at 18:20

The capacitor should be the least of your concerns here. Let's think about this for a minute.

Using your MOSFET's on resistance and the \$1\Omega\$ to discharge, your time constant will be:

\$\tau = RC = 40mF \times 1.0035\Omega = 40.14ms \$

Using the 5 time constants to charge/discharge rule, your discharge time will be:

\$5 \tau =40.14ms\times5=200.7ms\$

(Note: You don't give a datasheet for the caps, so the ESR is unknown, and if it's high relative to the resistor and MOSFET on resistance, all of these numbers are wrong.)

Using your peak discharge current:

\$P_{R} = I^2R = (0.5979A)^2\times1\Omega = 357mW\$

\$P_{Q} = I^2R=(0.5878A)^2\times0.0035\Omega=1.25mW\$

As you can see, over 99% of the stored energy is dissipated as heat in the resistor. You should make it a half Watt resistor. You could do the calculus and find the actual average power, and find a quarter Watt would work just fine.

As long as the discharge current falls within your MOSFET's \$I_D\$ rating, this is a perfectly safe way to discharge that capacitor bank. You need to look closely at the datasheet and make sure they are low ESR caps, and check out their thermal characteristics to be completely sure you won't hurt anything. Given their size, intuition says you'll be fine.

Safely discharging large value capacitor without blowing it up

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