RC delay capacitor discharge

[snaoz]

Hi

This is a simple RC delay circuit. Only problem is what is the way to make the capacitor discharge when the power is removed ???. Using a resistor would be a very slow way to discharge the cap.

 

[thylacine1975]

A common trick is to connect a diode across (in parallel with) R such that it is reverse biased in normal operation. For example: if V is the positive supply in your schematic, connect the anode to Vc and the cathode to V.

When the supply voltage is removed, the diode becomes forward biased and the capacitor C rapidly discharges via the diode [as it attempts to drive the supply and power the rest of the circuit].

The traps to consider with this simple approach are:
* C only rapidly discharges down to the forward voltage of the diode (~0.6V for silicon, 0.3V for schottky)
* The reverse leakage current through the diode D can swamp the current through R - especially if R is big, the temperature is high or D is a schottky.

 

[snaoz]

A common trick is to connect a diode across (in parallel with) R such that it is reverse biased in normal operation. For example: if V is the positive supply in your schematic, connect the anode to Vc and the cathode to V.

When the supply voltage is removed, the diode becomes forward biased and the capacitor C rapidly discharges via the diode [as it attempts to drive the supply and power the rest of the circuit].

The traps to consider with this simple approach are:
* C only rapidly discharges down to the forward voltage of the diode (~0.6V for silicon, 0.3V for schottky)
* The reverse leakage current through the diode D can swamp the current through R - especially if R is big, the temperature is high or D is a schottky.

But i don't see how this will work if a switch was used between V and the power source supplying the V. The circuit would be open and the capacitor wouldn't be able to discharge. Please correct me if i am wrong.

 

[thylacine1975]

You're correct! It won't work

The arrangement I described is commonly seen on microprocessor reset circuits, power supply soft-start networks etc. If there's a switch imposed between V and the power supply you'll need something else entirely - perhaps a transistor across C, controlled by the same thing that's opening/closing the switch?