Electronics
Always use analog multimeter to test this component
Select the x1 scale
Take the probes to measure the SCR
Put the black probe on anode and the red probe on cathode. To test the shot, close a contact between anode and gate by clicking the trigger. The multimeter should show a low resistance.
No reading should appear with inverted probes.
Putting the black probe on gate and the red probe on cathode, the reading should show a low resistance.
In other positions should mark infinite.
SCR measurement
Shoot
TRIAC in good condition
Burned TRIAC
TRIAC in short-circuit
Formulas
Phototransistor
A phototransistor is a bipolar transistor encased in a transparent case so that light can reach the base–collector junction. The phototransistor is similar a photodiode, having higher sensitivity to light, because electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain. Phototransistors also have significantly longer response times. They are used inside optical couplers, with an infrared light emitting diode.