The control circuit in Figure 1a allows you to manually adjust the power delivered to a load. By changing the setting of potentiometer R3, you change the phase angle at which the thermistor (Q3) fires (Figure 1b), thereby altering the load current’s duty cycle. The adjustment range is about 0 to 180°. Q3’s off time is linear with R3, but of course the resulting load power is not linear with R3.
The full-wave diode bridge delivers pulsed-dc voltage to the load, making the circuit suitable for dc-control applications such as dimming. (The circuit can handle ac power if you substitute a triac for Q3 and make slight modifications.)
IC1 is a low-power—1 mW—timer configured as a mono stable mono vibrator. Zener diode D1 and filter capacitor C1, activated by pulses from the voltage divider R1/R2, form a dc supply for the timer. Q1 turns on and applies a negative-going trigger to the timer (pin 2) each time Q1’s base voltage approaches 0V. In response, the timer issues a positive pulse that turns on Q2 and turns off Q1, removing load power for an interval equal to 1.1R3C2.
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