Silicon Diodes and Temperature Compensation

Temperature compensation counteracts the natural tendency of a pendulum to speed up or slow down as its shaft shortens or lengthens with temperature change. Temperature compensation in the electronically-driven pendulum takes advantage of two properties.

  • Forward voltage drop across a silicon diode varies inversely with junction temperature. As junction temperature increases, forward voltage drop decreases. At low currents (<100µA) junction temperature is mostly a function of ambient temperature.

  • A property of free-swinging pendulums is that, as swing amplitude decreases, swing period will decrease slightly, which is the same as saying the pendulum's rate, within a limited range, varies inversely with swing amplitude - a pendulum clock tends to run faster as pendulum swing amplitude decreases, and vice versa.

Since the diodes are connected across the storage capacitor, ambient temperature affects the charge on the capacitor. As temperature rises, voltage on the capacitor falls. This results in slightly lower amplitude pulses being applied to the pendulum, slightly lowering it's swing amplitude. This causes the pendulum to speed up. As temperature falls, the opposite effect is seen. Higher voltage across the capacitor increases swing amplitude, causing the pendulum to slow down. Changes in ambient temperature thus result in zero net change (more or less) in the pendulum's rate.

Δtemp - Δcomp = Δrate ≅ 0

Electronic Pendulum Driver