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 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.
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.
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