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.
.
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.
| Here is another example of a simple ramp circuit and regulated 10VDC power supply. | |
| 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. |
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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. Click here to download the schematic in PDF format.
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