2
\$\begingroup\$

This is probably a dumb question, but I am sort of confused. What direction does a cap discharge when a system is turned off? For example, decoupling caps that go to ground. When I shut a system down, does the current flow to ground or does it flow in the reverse direction toward the supply (since it is now 0). Would putting a resistor in series to ground have any effect?

\$\endgroup\$
  • \$\begingroup\$ Same as battery \$\endgroup\$ – GR Tech Apr 9 '15 at 3:20
  • 1
    \$\begingroup\$ When you turn the power supply off, the system voltage begins to decay towards ground. The charge stored in the capacitors goes towards the rest of the system (that is, to where the power supply is connected) and, essentially, keeps the system running for a very short time longer. But as the supply voltage decays, so does the voltage on the capacitors. \$\endgroup\$ – Dwayne Reid Apr 9 '15 at 4:42
2
\$\begingroup\$

For example, decoupling caps that go to ground.

Caps block DC, so V+ will want to return to ground through the circuit, as usual, taking the easiest path. Decoupling caps are not usually the issue. It is the bulk caps that act like batteries when the power is disconnected.

When I shut a system down, does the current flow to ground

Current only flows toward lower voltages. If voltage is trapped in the circuit, either because the switch physically disconnected V+, or because the power cord was physically disconnected, the device will continue trying to work, consuming the remaining power. Caps will eventually dissipate their charge if there's nowhere for it to go.

or does it flow in the reverse direction toward the supply (since it is now 0).

If a transformers input is switched, but the output is not, the V+ terminal may be the shortest path back to 0V. However, a rectifier between transformer and circuit negates that, leaving GND the only path back.

Would putting a resistor in series to ground have any effect?

A resistor in parallel is called a "bleeder" and is common when the circuit may inadvertently retain unwanted/dangerous voltages. They are basically "soft" shorts, in case the circuit's GND gets disconnected. The drawback of a bleeder is that it dissipates power whether the circuit is operating or being bled.

\$\endgroup\$
0
\$\begingroup\$

When Capacitor discharges current always flows in opposite direction. Current does not flow through the capacitor only chrages the plates (known as virtual current). vtingole

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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