No MJ, it's a zener diode. When forward biased, it only breaks down until a specific voltage is reached. Predominately used in voltage regulators.. .. .A thyristor or SCR only conducts when a voltage is applied to the gate. (it has three legs as opposed to the two legs of a zener). .. .The zener may be there for two reasons. One would be to stop the field from being excited in the wrong polarity due to residual magnitism left in the field coil. The other as previously stated would act as protection against spikes of excitation before the back emf is established.. . . . <small>[ 18 March 2002, 21:52: Message edited by: ECAM MSG ]</small>