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Triac regulator


Sometimes a mains-powered device needs lower than full power. To adjust it, a triac regulator can be used. The power of heating elements, lightbulbs, or the rotational speed of commutator motors can then be easily adjusted by turning a knob.


The regulator is built as a compact unit for laboratory purposes and ad-hoc setups. The input power connector is the IEC 60320 socket, the output is a standard European power socket. An off-the-shelf KPZ12 box was used for the housing.

A third-party brush motor regulator was used as the base schematics. It was however rather poorly specified at some points, e.g. the type of the  triac was omitted; it was apparently different than the BT138 one used here, necessitating changes of values of some components.

Regulator circuitry

The regulator itself is a standard circuit for a triac switch, using a DIAC to make the switching more symmetrical for both positive and negative half-wave. The resistor-capacitor network provides timing; when the capacitor reaches breakdown voltage of the DIAC, the DIAC switches and dumps the energy from the capacitor into the triac's gate and opens it for the rest of the half-wave.

The moment the triac switches therefore depends on the value of the series resistor. The regulator potentiometer should work from zero to full scale. The series and parallel fixed resistors are therefore necessary to fine-tune the operation range. The 2.2 MΩ resistor was chosen to eliminate the dead space on the left side of the pot scale where the triac was not switching on at all (by reducing the resistance of the pot when on max), the series 1 kΩ resistor sets the full-scale limit (by addition to the pot resistance when it is on min). The exact values depend strongly on the triac and diac used.

The high-current path in the schematics is emphasized with thicker lines.

Snubber circuit and filtering

The 100 nF capacitors with one parallel resistor act as a snubber network, filtering out the transients (present especially when inductive load is being regulated) that could cause trouble like false triggering of the triac or excessive EMI.

The choke in series with the rest of the circuit serves to filter out the EMI from the triac switching. Its value turns out to not be mission-critical, at least for the low-hundred-watts motors and tens-of-watts lightbulbs used as test loads. Original schematics suggested 200 µH; such value was not readily available for high enough current, a choke from a reusable-parts bin was therefore chosen and turned out to be satisfying.


Two neon bulbs are used as indicators. One shows the power being present on the input connector, the other one is connected across the load and shows the relative power available by its brightness (and has full brightness across the scale when there is no load).

For rough measuring of the current flowing through the load, a low-cost ammeter was fashioned from an extra-cheap VU meter. Its full-deflection voltage was measured and a suitable shunt resistor was made from a length of resistive wire, with assistance of a milliohmmeter. To make it work with the AC current, a diode bridge was employed.



Inside view

Inside view

Ammeter detail

Triac board detail

Triac board detail

Both halves of the box

Outside view

Outside view

Outside view

Outside, side view

Outside, top view

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