Triac lighting and heating controller uses few parts

The triac lighting-control circuit in Figure 1 is small and inexpensive because load and housekeeping power come directly from the line voltage, thereby eliminating bulky, expensive supplies. The CLZD010 closed-loop controller maintains constant light intensity by automatically adjusting the timing of the triac's firing until the feedback signal and setpoint command are equal. The 5V supply is a charge pump that energizes C₁ on the negative swing of the line voltage and then transfers charge to C₂ on the positive swing. Zener diode D₂, minus the forward drop of rectifier D₁, sets the 5V. Triac Q₁ is a latching switch that conducts in either direction until you remove gate drive and load current drops below its holding threshold, which occurs at the zero-crossing point of the line voltage.

The circuit pulses the triac gate for 100 µsec to turn on the load for the remainder of each 60-Hz half-cycle, so higher power accrues by turning on earlier in the half-cycle. R₃, at the timing pin of the controller chip, detects line phase. Controller pins CS3 to CS0 set the closed-loop configuration for an application. You can easily modify the lighting-control circuit for thermal control (Figure 2). Closed-loop timing is 134 sec for optimized temperature response, using controller pins CS3 to CS0. The circuit initially drives the heater at high power levels until the temperature nears its final value and then reduces the power to avoid overshoot. The CLZD010 controller is available from Flextek Electronics (www.flex-tek.com).

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