About dimmer switch and light bulb as 2 circuit elements,wouldn't power consumption of pair remain constant?

Question

you could make the light dimmer by reducing current to it, but to do so mustn't you increase the resistance of the circuit with the dimmer? Thus, regardless of how bright the bulb is, same power is consumed as the resistance of the switch is raised, right? Or is this not how it works?

Answer 1

WRONG

If you have a 100 ohm light and a zero ohm "switch" and 100 volts applied you have 1 amp and 100 Watts.

If you have a 100 ohm light and a 100 ohm "switch" you have 200 ohms so I = E / R 100 / 200 = 1/2 Amp P=I * E = 1/2 * 100 = 50 Watts TOTAL.  One half to the light and one half to the resistor

But dimmers are usually fast on/off digital switches.  When ON they have about zero resistance and when OFF they have about infinite resistance.  In each case the amount of power consumed by the "switch" is about zero.  The amount of light is controlled by the amount of time they are ON and OFF.  They switch thousands of time per second and it is the asymmetry in the switching time ON and OFF that controls the brightness of the light.

Answer 2

Ken

I agree on your assessment of the first person to answer.

And he himself is WRONG on a few things.

Essentially a standard dimmer used in residential or light commercial applications is a basic device that has a 4 layer semiconductor like a Triac that is in series with the load.

This Triac has a ''gate'' lead that is supplied with a simple circuit that varies the phase angle of the original AC waveform to permit conduction of this Triac, from very little, to all, of the AC waveform in your lighting wiring.

This style dimmer uses a k**b or slider that turns a variable resistor, with a few peripheral components, to do this phase angle change.

A 60 hertz AC power being common in the USA will have this Triac conducting 120 times per second.

The AC waveform goes both positive and negative from the ''zero'' voltage line on a graph.

Each ''swing'' of voltage above or below zero will do ''work'' ( light a lamp) and the Triac conducts, per the amount of phase angle difference, to pass a portion of the these voltage swings.

The waveform coming out of this type of dimmer has more or less of the AC sine wave ''chopped'' off. They can cause noise or buzzing problems in audio equipment because of the harmonics generated with the sharp corners ( edge ) on the waveform when conduction starts.

The amount of power loss as heat in these type dimmers is not real high.

Stage and theatre lighting in the ''old'' days used large rheostats in series with the lamp loads for dimming.

Yes - a lot of loss there in heat and therefore power wasted.

Modern dimming in theatres and live stages use the same principle of this Triac or dual SCRs.

And the gates are still controlled by circuits that permit more or less conduction per each 1/2 cycle of AC.

There was confusion on the ''many thousands of times per second'' as being the form of remote dimming control, as in a digital light control console that uses a data stream to the dimmers.

This data stream contains information such as console dimming slider position and digital addresses for every unique dimming channel.

The dimmers receiving these data streams sort that out and still use comparators to do the ''gate'' control function.

Still only 120 times per second in the load feeder to any given lamp or lamps. ( 120 half cycles equal 60 full cycles ) ( 60 Hertz )

I could go way deep on this but hope this helps