An electrolytic capacitor of 47uf/50V is charged through a 2M ohm resistor.When it charges to 13V, the circuit shuts down and the charge remains on the capacitor. To reset the shut-down state the power is switched off and then turned on. As the cap will take too long to discharge on its own or through a bleeding resistor, Ive used a NPN transistor that switches on for 5ms and grounds the cap. Is it safe to discharge the cap this way? The circuit seems to be working fine, no faults yet.

  • 1
    \$\begingroup\$ Depends. Are you in the same room with the transistor? \$\endgroup\$ Mar 20, 2016 at 16:26
  • \$\begingroup\$ The NPN transistor's base is connected to a 10K resistor followed by a 3.3uf cap. The base is also grounded through a 100K resistor. \$\endgroup\$
    – Akash
    Mar 20, 2016 at 16:30
  • \$\begingroup\$ What capacitor and what transistor? \$\endgroup\$ Mar 20, 2016 at 16:53
  • \$\begingroup\$ Electrolytic 50V rated. BC547 \$\endgroup\$
    – Akash
    Mar 20, 2016 at 16:54
  • 1
    \$\begingroup\$ I think I understand the circuit, but a diagram would be useful. There is a schematic editor tool available in the edit controls. \$\endgroup\$ Mar 20, 2016 at 17:35

1 Answer 1


You need to figure out the ESR (equivalent series resistance) of the capacitor and with that and the peak voltage you can work out worst case current. That in turn can be compared against the transistor's ratings.

If the peak current is too high then add some further series resistance to reduce it. It will be kinder to everything.

  • \$\begingroup\$ What about the capacitor? Is the capacitor safe coz the cycle will be repeated several times during its operation? \$\endgroup\$
    – Akash
    Mar 20, 2016 at 19:48
  • \$\begingroup\$ What does the datasheet say? \$\endgroup\$
    – Transistor
    Mar 20, 2016 at 19:49
  • \$\begingroup\$ Datasheet shows only temperature ratings and some other parameters. Have you ever discharged a capacitor abruptly in any of your previous application? What was your experience? The cycle will repeat once every time the circuit is hooked up. \$\endgroup\$
    – Akash
    Mar 20, 2016 at 19:55
  • \$\begingroup\$ I think your tiny BC547 will have more problems coping with the discharge than your capacitor. Even if the transistor goes into saturation its maximum pass-through current will not be a problem for the capacitor. To be safe, add a resistor and lengthen your discharge cycle a bit. \$\endgroup\$
    – JvO
    Mar 20, 2016 at 20:36
  • \$\begingroup\$ @Akash: Rapid discharge shouldn't be any worse than a rapid charge. Protect the transistor. \$\endgroup\$
    – Transistor
    Mar 20, 2016 at 20:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.