This blog post describes how a setup can be protected from overvoltage that occurs when switching off a coil. Fast changing currents induce transient voltages in an inductive load which can be expressed by the formula:
V = - L (dI/dt)
L is the inductance in Henry and dI/dt is the rate of change of current in Ampere per second.
The energy stored at the inductance at the moment of the interruption can be calculated with the following formula:
W(i) = 1/2*L*i^2
where W(i) is the energy in Joule and i is the instantaneous current in Ampere at the time of interruption.
Transient voltages can be much higher than DC bias voltages used by a developer.
Setup
As illustrative example, consider the circuit shown in Figure 1. The inductance in the setup is 10 µH, the resistance R1 is 50 Ohm and limits the current through the inductance. Such a setup can behave like a spike generator. When the switch is closed current is flowing through the inductance. In this configuration, high voltage transients can be measured when the switch is opened and the energy from the system is dissipated via the parasitic resistance R_parasitic.
