Old-school dimmer switches were rheostats, and they got hot.
Pretty much all dimmers now are TRIAC-based, which is a semiconductor that turns on partway through the AC wave, then turns off at the next zero-crossing, repeat. It chops the waveform so the light only gets power for a fraction of the time.
An incandescent bulb works largely the same with either type. (You may hear the filament "sing" on a TRIAC dimmer since the fast-rising waveform edge has a lot of harmonic content, but this is usually very faint.)
LED bulbs are non-dimmable by default. The typical job of a power supply is to ignore variations in the source and deliver uniform power to the load, and that's just what they do, driving the emitters at a constant brightness regardless of what the dimmer does, until it's letting through so little power that the poor thing just shuts off. Or flickers madly.
Dimmable LED bulbs are actually super tricky, because the power supply has to measure the distortions in the incoming waveform, interpret that as a dimming command, and use that to control the output to the emitters. Any jitter in the measurement sampling means the resulting brightness will bounce around. Any jitter in the waveform, which an incandescent might've ignored as long as the area-under-the-curve was equal, might be picked up by the LED power supply and misinterpreted as a changing dimming level.
It all sucks and we should abandon it immediately. LEDs should be driven with DC. But there's an awful lot of installed fixtures to keep us from that utopia.
> LEDs should be driven with DC. But there's an awful lot of installed fixtures to keep us from that utopia.
Every LED luminaire or lamp already has a DC inverter inside of it.
Also, you can get (158) 28w 2x4 LED fixtures on a single 277V 20A circuit with #12 wire, DC lighting branch is never going to happen. For reference, that will light about 12,000 square feet of space assuming 9’ AFF for ceiling height.
Not 100% sure of the details but I think it’s more like a digital pwm system.
I had dimmers installed when we rewired our house and I thought they were rubbish. Replaced those with Varilight v-pro and they were noisy. Discovered that I could switch between leading edge and trailing edge modes (or something like that) and the sound went away. Love them now.
If a dimmer switch were a resistor, it wouldn't work at all with LED lights (the AC->DC converters in them don't just lower the current they provide when the AC input gets lower).
In incandescent lights, a variable resistor would burn a lot of power unnecessarily, so instead they use phase cut dimming (with a triac switch) where the dimmer cuts out a variable portion of the AC cycle. That way you reduce the effective duty cycle of the power without burning the energy. This works well for incandescents because the filament glow scales nicely with the with the power being delivered to the bulb. It works poorly with (some) LED bulbs because the turn on/off time is slow relative to the power cycle, and the LED brightness itself doesn't just scale nicely with the current from the rectifier.
A dimmer is either a "phase-cut" device, either (commonly) forward-phase/leading-edge or (less commonly) reverse-phase/trailing-edge. At all times, it's either on or off, and it cycles between on and off once per half wave, so it produces 120 pulses per second. A good-quality light fixture will smooth out that waveform and produce approximately constant output.
The worst choice is a no-neutral-required forward-phase dimmer. Neutral-required forward-phase dimmers are usually better. Reverse-phase dimmers can be excellent for LEDs (but disastrous for magnetic transformers) and always require neutrals. Some dimmers can operate in both modes.
"The ZEN77 Dimmer is recommended for 3-way and 4-way installations since you won't need to rewire your other switches in the set-up, you can simply replace the main switch with direct connection to power with the ZEN77 dimmer.
This model can control up to 100 Watts of LED bulbs but we don't recommend using it in installations with chandeliers or large groups of lights over 6 bulbs.
Version 1.0 and 2.0 of the ZEN77 (700 series Z-Wave chip) were MOSFET dimmers so if your bulbs work better with trailing-edge (or reverse-phase) drivers, those versions of the model worked best.
Version 3.0 of the ZEN77 (700 series Z-Wave chip) is now a TRIAC dimmer so if your bulbs work better with leading-edge drivers, this model will work better.
The 800 series version of the ZEN77 is also a TRIAC dimmer.
Why We Changed to all TRIAC: We found that newer LED bulbs dimmed better with TRIAC dimmers, and considering limited availability for MOSFETS, we decided to transfer the ZEN77 model to TRIAC as well."
A dimmer switch for an LED light is different from a dimmer switch for a non-LED light. If you try to use an LED light with a “normal” dimmer, it won’t work well.
