Anything you want to know about dimmers

Inductive Loads

Normal tungsten loads are considered essentially non-reactive however; the transformer used with low voltage lamps is highly inductive. Consequently, the current and voltage are not in phase; this causes problems where the dimmers zero-crossing circuit is triggered by the current, instead of when the voltage passes zero. The result of the reactive component presented by the transformer primary is erratic behavior of the control. The solution is to extend the pulse to last beyond the zero crossings of the current or issue a series of pulses instead of just one. In this way the dimmer will operate without flickering.

If inductive loads like low voltage transformers are to be used, then a more advanced design that pulses the triac gate is required. A pulse capacitor C3 is added to the circuit, this charges up almost simultaneously with C4 and functions as a reservoir. As soon as C4 discharges and the triac starts to conduct, C3 starts to recharge C4 via resistor R3. This causes another trigger pulse at the triac gate. This happens such that the triac gate is fed with bursts of three or four pulses so that the dimmer will operate smoothly.

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