I have an idea for wiring the lights in a room, which I have run past a few electrician friends, and have received contradictory and unsure answers.

There is a large room with two sets of lights - one on each side of the room. Each circuit consists of two 50W 12V halogens. There are currently two switches and two dimmers controlling the circuit.

I like being able to switch on each half of the room individually, but I have realised the dimming levels I want from each side are always the same.

My idea, when I get some other work done, is to put one dimmer upstream of the two switches, so one dimmer controls both circuits.

(i.e. active -> dimmer -> two switches in parallel -> each set of lights -> neutral.)

The sensible advice I have received:

  • Check the lights are of a type that can be dimmed. (Yep, they already are being dimmed.)
  • Check the rating of the dimmer can handle the extra load. (Yes, with a good safety margin, because they get hot in a wall cavity.)

The conflicting advice I have received:

  • If one set of lights is on, and you turn on the second set, (a) yes, the original lights will remain the same brightness, and (b) no, the variable resistor will heat up a little, affecting resistance and the brightness will change slightly.

  • More importantly: leaving a dimmer upstream of the switches, permanently connected to active will (a) be perfectly fine, (b) will be more susceptible to damage due to spikes (e.g. lightning strike) or (c) will make the dimmer burn out more quickly and should be discouraged.

Would have a dimmer attached to active but disconnected from a load cause it to fail more quickly than having it wired normally (i.e. downstream of the light switch.)?

  • \$\begingroup\$ What does the dimmer manufacturer say? It's well known that electricity tends to flow clockwise in the Antipodes, so an answer direct from the maker may be more reliable than an opinion here. \$\endgroup\$ Commented Mar 31, 2014 at 0:10
  • \$\begingroup\$ I read the spec and found nothing relevant (except perhaps that, as there will be one less dimmer in the same wall cavity, it can actually be safely loaded higher.) \$\endgroup\$ Commented Mar 31, 2014 at 0:35
  • \$\begingroup\$ I note another solution: Replace the switches with double-pole variants: active -> switches in parallel -> dimmer -> the other poles of each switch controlling separate circuits -> neutral. For a few extra bucks, the second question becomes moot. \$\endgroup\$ Commented Mar 31, 2014 at 0:38

1 Answer 1


For Q1: (a/b) are both sort of correct. A dimmer should never be a variable resistor as that would consume a large amount of power and would cause fires or fail easily. Dimmers are made from SCR's which are active devices which save power and don't heat up nearly as much. So long as the dimmer is rated to handle both lights, then it shouldn't dim when the second is turned on.

"Modern dimmers are built from silicon-controlled rectifiers (SCR) instead of variable resistors, because they have higher efficiency."

On the other hand, the rest of the circuit (wires) will have to carry more current when the second light turns on. If the wires aren't large enough they'll heat up causing a slight increase in resistance and a corresponding slightly lower amount of current to reach each device.

For Q2: (a) is correct. The dimmer will be wired in series with the lights/switches so lightning would still have to travel through the open (off) switches to damage it which is the exact same even if the dimmer is wired after the switches.

You look golden to me.

  • \$\begingroup\$ Thanks. While (modern) dimmers are not variable resistors, they contain variable resistors to control them, right? I believe that is what was being referred to. \$\endgroup\$ Commented Mar 31, 2014 at 0:22
  • \$\begingroup\$ Sure they do, but even those resistors are in series with the load as seen here: circuitstoday.com/modified-lamp-dimmer-circuit which means that they won't turn on until the load turns on (when the switch is flipped). The switch can be thought of as part of the load (or specifically in series with the load). \$\endgroup\$
    – horta
    Commented Mar 31, 2014 at 0:24
  • \$\begingroup\$ It's conceivable that some kinds of dimmers might not like being switched suddenly onto that kind of load (switching halogen supply). Perhaps not very likely, but still possible. Personally, I'd probably try it, and switch dimmer models if by chance it dies. (Assuming it meets the local electrical code to begin with, of course). \$\endgroup\$ Commented Mar 31, 2014 at 0:30
  • 1
    \$\begingroup\$ The resistors in the dimmer circuit aren't load resistors. They're their to regulate the SCR. They're making measurements off of the voltage which should stay relatively constant regardless of what you switch on. Not much current goes through them, and the current that does shouldn't warm them up at all. The current through them does NOT depend on the load current. I hope that makes sense. \$\endgroup\$
    – horta
    Commented Mar 31, 2014 at 0:35
  • 1
    \$\begingroup\$ Tried it. Brightness doesn't (perceptibly) change. Will need to wait and see for the other question. \$\endgroup\$ Commented Apr 2, 2014 at 0:06

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