Timing

Charging a capacitor takes time (p. 43). The time taken to charge a capacitor introduces a delay into the operation of a circuit. The sequence is: • Discharge the capacitor completely. • Let current flow into the capacitor through

a resistor. • Wait until the voltage across the capacitor

reaches a set level. • The delay is the time taken to charge to the

set level. The problem is to monitor the voltage across the capacitor. We need to do this without drawing currrent from the capacitor. The solution is to use an op amp which has highresistance inputs.