Simple light dimmer examples.

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Light dimmer

AC phase control
In order to vary the brightness of a lamp the triac is turned on for only a portion of the AC cycle. By controlling when the triac is fired during each AC half cycle the average power to the lamp can be varied, and thus the brightness.
For a microprocessor this is a trivial task.Click here details from my article in Nuts and Volts magazine.
For analog circuits it takes just a handful of op amps and a few other parts.
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The dimmer circuits below consist of 4 main parts.

The power supply. A simple 78XX based regulator. The capacitator value associated with the 7812 regulator are not critical. Any values close will be fine. A part of the power supply for a dimmer is to provide a pulse centered around the AC zero crossing point.

The ramp generator. This is a ramp, in sync, with the AC zero crossing point. In the Dimmer schematic below, 1/2 of an LM358( U2b) and a couple of resistors create a pulse that goes low right around the zero crossing point. To create a ramp the pulse is fed to the other 1/2 LM358(U2a) with a .01 uf polyester Temp Stable cap in the feedback loop.
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Here is another example of a simple ramp circuit and regulated 10VDC power supply.

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Both of the examples produce a linear ramp with a 120 Hz frequency and a voltage going from 0 - 10 VDC. The only problem is that light bulbs do not respond linearly to the AC wave form. For a proper dim curve the ramp should take on a sort of S curve, usually called square law dimming curve or something similar. Linear dimmers are usually characterised by having limited range control of the fader. most of the lights response concentrated near the center of the fader. Dimmers with a proper curve respond to the full range of the fader. Of course a dimmer with a nice dim curve cost more due to the more complex circuit and usefullness.

On the top is a typical linear ramp that is produced by the circuits described on this page. In this case the ramp goes from 0 to 10 VDC. The dotted lines represent the zero crossing point of the AC line.

The bottom ramp is similar to the ramp found in "square law" dimming curve analog dimmers. These are a bit more complicated to produce but have a big effect on dimmer response to the fader position. Better digital dimmers will offer several dim curves because the curve in produced in software. It is not a problen for a DMX decoder to produce a dim curve reflected in the 0 - 10 volt output.

The comparator. The LM324 op amp is used to compare the control voltage to the instantanious voltage of the ramp. If the control voltage is higher the optoisolator is turned on.

The TRIAC driver. On the AC side of the optoisolator is a few components required to properly trigger the TRIAC gate.

This is a simple dimmer circuit that can be controlled with 0 - 10 VDC.
The schematic does not show the torroidal chokes necessary the prevent dimmer "buzz".
Oops, there was a mistake on the schematic. This is the corrected version. Sorry for any inconvience.

C lick here to download the schematic in PDF format.