Diode-Capacitor Circuit Voltage Decay

A charged capacitor initially discharges very quickly through a forward-biased diode.  However the diode current’s nearly exponential dependence on voltage results in a drastic reduction in the rate of discharge.  Figure 1 clearly shows the transition in decay rate.  The voltage decay is a logarithmic function of time (for 5 decades) as shown in Figure 2. 

Figures from: Hellen, E.H. 2003. Verifying the diode-capacitor circuit voltage decay. Am. J. Phys. 71 797-800.

Figure 1

Measured voltage decay for a 0.1 microf capacitor through a 1N4148 diode.  Initial voltage is 0.62 V.

Figure 2

Measured voltage decay (small closed symbols) vs log(t) and predicted decay (large open symbols) using the exact solution, Eq. 4. Capacitor values from left to right, 0.001, 0.01, and 0.1 microf. The logarithmic decay portion is: