The diacs, because of their symmetrical bidirectional switching characteristics, are widely used as triggering devices in triac phase control circuits employed for lamp dimmer, heat control, universal motor speed control etc.
Although a triac may be fired into the conducting state by a simple resistive triggering circuit, but triggering devices are typically placed in series with the gates of SCRs and triacs as they give reliable and fast triggering. Diac is the most popular triggering device for the triac. This is illustrated in the following applications.
1. Triac Lamp Dimmer Circuit.
The circuit for a triac controlled by an R-C phase-shift network and a diac is given in figure. This circuit is an example of a simple lamp dimmer. The triac conduction angle is adjusted by adjusting the potentiometer R. The longer the triac conducts, the brighter the lamp will be. The diac acts like an open-circuit until the voltage across the capacitor exceeds its breakover or switching voltage (and the triac’s required gate trigger voltage).
2. Heat Control Circuit.
A typical diac-triac circuit used for smooth control of ac power to a heater is shown in figure. The capacitor C1 in series with choke L across the triac slows-up the voltage rise across the device during off-state. The resistor R4 across the diac ensures smooth control at all positions of potentiometer R2. The triac conduction angle is adjusted by adjusting the potentiometer R2. The longer the triac conducts, the larger the output will be from the heater. Thus a smooth control of the heat output from the heater is obtained.