Light Dimmers

I’ve personally known people, who believed out of the depths of their souls, that conventional light-dimmers are rheostats, which would mean that they have one active component, that being a load-bearing variable resistor, in series with the presumed lightbulb they were designed to regulate.

In reality, the type of light-dimmers that were mass-produced in the 1970s, were of the same design as those still mass-produced today. Their main component is a Triac, which is actually a pair of silicon-controlled rectifiers facing in opposite directions – so that the pair will be able to conduct in both directions, thereby satisfying the need to regulate an A/C power-supply – But the SCRs that make up a triac, have a common, shared gate.

I will described the device, as if a Triac was just the fancier, A/C equivalent of an SCR.

A silicon-controlled rectifier has as essential property, that it will fail to ‘turn on’ – i.e. become conductive between its two main electrodes – until the voltage applied to the gate – where a smaller amount of current will flow – exceeds a certain voltage. Then, an SCR tends to stay turned on, until the voltage across its main electrodes becomes zero again – or just very low – at which time it switches off.

A/C house current in Canada and the USA is supplied as a voltage-sine-wave with a frequency of 60 Hz, while in some other countries, a frequency of 50 Hz is used instead.

What a light-dimmer does, is to delay the turn-on point of this triac, within the 1/120 of a second of a half-wave of supplied A/C voltage, but 120 times per second. It will turn off the voltage supplied to the lightbulb, each time the A/C it is fed crosses zero. So different settings will produce active durations that vary between 100% and 0%, causing the lightbulb to glow with different levels of brightness.

The way this was done in the 1970s, and which remains the cheapest way to manufacture them, a light-dimmer possesses a smaller variable-resistor, aka a potentiometer, through which a much smaller amount of current will flow, than is meant to flow through the load, the wiper of which additionally has a capacitor connected to it. Obviously, the resistance and capacitance need to be calculated somewhat precisely, to take into account the A/C voltage as well as frequency. This wiper is additionally connected to the shared gate of the triac.

(Last Updated 08/12/2017, 15h20 : )

Continue reading Light Dimmers