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Dimmers, Relay Packs, and Why LED Fixtures Do Not Dim on a Dimmer Channel
The single most expensive mistake in small-venue lighting is plugging an LED fixture into a dimmer pack. Here is what each box does, and what to plug into what.
~7 min read · Updated August 2026 · By the Stage Supply team
In this guide:What a dimmer does · Why LED fails · Which box do I need · Switch packs · Labelling · Picks · FAQ
Someone inherits a lighting rig with a dimmer pack in it, buys a set of LED pars, plugs them into the dimmer and turns the fader up. What happens next ranges from nothing, through flickering and buzzing, to a fixture that never works again.
It is a fair mistake to make. The plug fits, the desk is the same, and nothing on the outside of either box warns you. The difference is in what the dimmer does to the electricity.
What a dimmer pack actually does
A conventional dimmer does not reduce voltage in the way a tap reduces water. It chops the mains waveform, switching the supply off for part of every cycle and on for the rest. A tungsten lamp has a filament with thermal inertia, so it does not care about the chopping. It sees the average energy, glows less brightly, and the effect is a smooth fade.
That chopped waveform is a violent, spiky thing. It is fine for a filament and hostile to anything with a power supply in it.
Why an LED fixture fails on it
An LED fixture is not a lamp, it is a computer with lights attached. Inside is a switch-mode power supply expecting a clean sine wave, a processor, and a DMX receiver. Feed it a chopped waveform and one of several things happens: it flickers, it buzzes audibly, it resets repeatedly, it fails to respond to DMX, or the power supply is damaged.
LED fixtures dim themselves. They take full, constant mains, and the brightness is a DMX channel inside the fixture, controlled by the desk over the data cable. That is why an LED par has a DMX address and a tungsten par does not.
The rule
Anything with electronics in it takes constant power. Only a resistive load, which in practice means a tungsten lamp, goes on a dimmer channel.
Dimmer, switch pack, or neither
| Box | What it does | Use it for | Never use it for |
|---|---|---|---|
| Dimmer pack | Chops the mains to vary brightness | Tungsten parcans, floods, birdies, practicals | LED fixtures, movers, hazers, anything with a plug-top power supply |
| Switch or relay pack | Switches a socket fully on or fully off from DMX | Turning practicals, mirror balls, hazers and non-dim loads on and off from the desk | Dimming anything |
| Plain distribution | Constant power, no control | LED fixtures, moving heads, media servers | Nothing to control, so no use for tungsten dimming |
Most small rigs today need distribution and possibly a switch pack, and no dimmer at all. Keep a dimmer if you still run tungsten, which many theatres and older venues do, and if you do, keep the two worlds clearly labelled.
What a switch pack is for
A switch pack, sometimes sold as a relay pack, gives you DMX control over a socket without touching the waveform. The channel behaves as a switch: below about 50 percent it is off, above it is on. That is exactly what you want for a hazer, a practical lamp in a set, a mirror ball motor, or anything that needs to come on with a cue but has electronics inside it.
It is also how you stop a rig drawing power all night. Fixtures on a switch pack can be shut down from the desk at the end of the show.
Labelling, so it does not happen again
The reason this mistake keeps happening is that nothing on the outside of the socket tells you what it is. Fix that.
- Mark dimmer outlets clearly, at the pack and at the far end of the extension.
- Use a different colour of cable tie, tape or label for dimmed and non-dimmed circuits, and be consistent across the whole rig.
- Write it on the rig plot, so the next person to touch the rig inherits the information rather than the mistake.
- If you have retired the tungsten, retire the dimmer with it. A dimmer pack sitting in a rack with nothing to dim is a trap for whoever comes next.
For how the control side works, see DMX explained. For the power side, see event power distribution explained.
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Frequently asked questions
Can I put an LED par on a dimmer channel if I leave the dimmer at full?
It will often work, but you are still feeding it through the dimmer’s electronics, and someone will eventually move that fader. Some dimmers also do not pass a genuinely clean waveform even at 100 percent. Use a non-dim circuit or a switch pack instead.
What happens if I do it by accident?
Usually flicker, buzz or a fixture that keeps rebooting. Turn the channel off, move the fixture to constant power and it will normally be fine. Repeated or prolonged use can kill the power supply, and that damage is not covered by warranty.
Do I still need a dimmer pack at all?
Only if you run tungsten. Many venues keep a couple of dimmer channels for house practicals, worklight and the odd tungsten special, and run everything else on constant power.
What about mains-dimmable LED lamps from a hardware shop?
Domestic dimmable LED lamps are built for domestic leading-edge dimmers and are a different product from a stage fixture. They may work on a theatre dimmer and they may flicker badly. They are not a substitute for a DMX fixture on a stage.
