Transistors, SCRs and TRIACs
SCRs or THYRISTORS
Silicon Controlled Rectifiers are equivalent to 2 transistors one PNP and one NPN with their collectors and bases connected to the other transistor's base and collector (swapped around).
If an on condition of +0.6v is now placed on the NPN's base, and there is a +ve supply/load on the PNP's emitter, current flows into the PNP's base turning it on, which in turn holds the NPN fully on.
Only the removal of the supply current, will turn off the SCR switch. With the SCR a -ve gate signal does nothing, not even turning off a turned on SCR, this is due to the gate connection only being on the edge of that layer and not affecting the currents flowing further in. There are turn off capable types made though.
False triggering is possible if the anode pulses +ve very quickly (dV/dT) due to stray capacitance the gate.
Unlike transistors, SCR and TRIACS peak current handling is very large, with a 1 Amp rated device able to withstand 40 Amp surges.
TRIACS
These are AC triggered switches, like SCR but have 2 additional junctions which are shorted together, it is a complex 3d device in practice.
It can be triggered to the on state with either +/-0.6V on the Gate with either +/- on the Anode'' although triggering sensitivity varies over the 4 states.
The Gate can only handle low power (eg 1/100,000 the anode switching power) so narrow pulse triggering is often used.
A diac (not the battery) is like a triac with internal 30v zeners anode to gate. So over 30V it goes short until the power is removed. With a suitable CR it provides simple AC phase shifted gate pulse trigger, used in mains light dimmers.
73 De John, G8MNY @ GB7CIP
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