Triac light Dimmer Active Reset Schematic

TRIAC Dimmer with Active Reset

This light dimmer control has active timing capacitor reset or AC line zero-crossing synchronization. The 13 additional components are garden variety and cost less than $2. Performance at the low end is exceptionally smooth and snap-free –better even than the TRIAC Dimmer Avoids Snap-ON version that uses a passive reset technique. This is a superset of the aforementioned control, so refer to that web page for additional information.

Schematic of the Light Dimmer Circuit

Triac light Dimmer Active Reset Schematic

Bill of materials

Dimmer BOM

Bill of materials file

The object of line-crossing reset

One of the shortcomings of the DIAC trigger diode circuit is that the timing capacitor (C1) does not automatically discharge to zero volts when it triggers. In fact, the residual voltage is an unknown variable that is a function of the DIAC turn-off properties, aging and perhaps temperature. DIAC turn-off properties are not tightly controlled or specified. Eliminating this residual voltage greatly stabilizes the timing circuit and improves the low end performance. The zero-crossing reset circuit senses when the line voltage is at zero and at that instant, it fully discharges the timing capacitor –all within about 200uS.

How it works

The way Q2 & Q4 are connected, they can only discharge the timing capacitor (C1). On the other hand, the timing capacitor charges only via the timing potentiometer circuit. To enable it to function in either polarity (opposing half-cycles), two circuits are required. The positive discharge circuit uses NPN transistors, while the negative discharge circuit uses PNP. The line synchronization signal comes through R6 to the bases of transistors Q1 & Q3. C4 attenuates any potential noise at this node. At zero volts, both Q1 & Q3 are turned off –at that point, they can no longer short out the base drive to Q2 & Q4 respectively. R-C network consisting of R7 & C3 provide an alternating power supply bias voltage that turns on either Q2 or Q4. C3 provides phase shift so that the voltage is near maximum at zero-crossing –the polarity is subsequently for each half-cycle. Fixed voltage sources could also do this, but it would add yet more components –this is a simple and effective solution, and worked immediately upon circuit completion –did not even have to diddle with circuit values for circuit optimization other than to add noise filter C4.


Since line voltage must be brought into the control via a 3rd lead, it is not suitable for two-wire applications without rewiring.


Oscillograph 1

Observe that at AC line zero-crossing, the timing capacitor discharges very quickly. Nothing else is happening here because the potentiometer is set at maximum resistance.

Oscillograph 2

In this oscillograph, observe that the DIAC is firing. Observe trace 4 that is monitoring Q1 & Q3 base inputs. Trace 3 is the phase shifted bias voltage signal –observe that the voltage is near maximum at zero crossings. Trace 2 is the DIAC output signal. Compare with the oscillographs in the previously published circuit.

Oscillograph 3

This shows operation at a low conduction angle.

Oscillograph 4

This shows operation at mid-range.

Light Dimmer Photos

Isolation transformer

Observe that I used an isolation transformer during the testing phase to prevent damage to test equipment and reduce shock hazard. Of course, in normal applications it is connected directly to the AC line.


Although somewhat complicated, performance is exceptional. At the very least, this makes a great experiment that deals with some issues of line voltage synchronization.

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