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.

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    \$\begingroup\$ Depends. Are you in the same room with the transistor? \$\endgroup\$ – Scott Seidman Mar 20 '16 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 '16 at 16:30
  • \$\begingroup\$ What capacitor and what transistor? \$\endgroup\$ – Peter Smith Mar 20 '16 at 16:53
  • \$\begingroup\$ Electrolytic 50V rated. BC547 \$\endgroup\$ – Akash Mar 20 '16 at 16:54
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    \$\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\$ – Peter Smith Mar 20 '16 at 17:35

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 '16 at 19:48
  • \$\begingroup\$ What does the datasheet say? \$\endgroup\$ – Transistor Mar 20 '16 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 '16 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 '16 at 20:36
  • \$\begingroup\$ @Akash: Rapid discharge shouldn't be any worse than a rapid charge. Protect the transistor. \$\endgroup\$ – Transistor Mar 20 '16 at 20:39

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