Triggering an SCR with Positive Pulses
Low-sensitivity SCRs can be triggered with positive pulses from TTL or CMOS logic using the block shown in Figure 1. R1 and R2 are chosen according to the current necessary to trigger the SCR.
The values shown in the circuit are for TIC126 SCRs. Although the current necessary to trigger this circuit is very low, the voltage isn´t. The value of resistor R1 can also be increased to 1 M, and R2 and R3 to 10 k and 2.2 k, if the SCR is the TIC106. In this case, the circuit Will need less than 1 uA to trigger on.
Triggering SCRs with Negative Pulses
The circuit shown in Figure 2 is compatible with TTL and CMOS logic, as it needs only a few microamperes to trigger the SCR on. The input must be kept at the high logic level to maintain the SCR in the off state. When the input passes to the low logic level for an instant, the transistor is cut off, and the SCR triggers by the current flowing across resistor R3.
The circuit is very sensitive, and SCRs as the TIC126 and other can be used. Diodes can be added to transform this circuit into AND and OR gates.
To turn the circuit off, the input must be returned to the high logic level, and the circuit power can be cut pressing a momentary contact switch wired between the anode and the cathode of the SCR.
