Diode suppression of EMF Generated HV Spike
Transient-voltage-suppression diode
When an electromechanical relay is de-energized rapidly by a mecanical
switch or semiconductor, the collapsing magnetic field produces a
substantial voltage transient in its effort to disperse the stored energy
and oppose the sudden change of current flow.
A 12VDC relay, for example,may generate a voltage of 1,000 to 1,500 volts
during turn-off.
The protection diode connected across the relay coil ( inductor) is normally reverse biased,
as the voltage on its cathode, connected to the +V supply rail, will be more positive
than its anode on the collector of the transistor. At switch off however, a large voltage
spike of opposite polarity appears across the inductor, due to the collapsing magnetic field.
For the duration of this voltage spike, the collector of the transistor could be at a higher
potential than the supply, except that if this happens, the diode will become forward biased
and prevent the collector voltage rising any higher than the supply rail.