Apartment I moved into has a light with 5 halogen bulbs - rated 20W, 12V each.

I'd like to replace the current on/off switch for all of them, with a dimmer, but concerned there might be issues to be aware of.

Would something like this be safe?


  • 240V 10 Amp Single 1 Gang Light Dimmer Switch
  • Push On/Off with Rotary Dimming 400W/400VA
  • Max. Load: 400W. Min. Load: 60W. Leading edge type. Derating required with mains halogen lamps.
  • 2 Way Switching - can be used as 1 Way
  • Suitable for mains/low voltage lighting in 1 or 2 way circuits

If it has a min load of 60W, what would happen if 3 of the bulbs blew - drawing just 40W. Is this unsafe?

Many thanks.

  • 2
    If the lamps are 12v you need to do a little more investigation. What’s actually being dimmed is the transformer. Some transformers won’t dim reliably at all, some are fine with a standard dimmer, and yet others need a dimmer designed for dimming low voltage lighting loads with a transformer between the dimmer and the bulb.
    – Tyson
    Commented May 1, 2018 at 11:49
  • 2
    Don't forget that running halogen bulbs at lower than rated voltage dramatically shortens their life.
    – Chenmunka
    Commented May 1, 2018 at 12:19
  • 1
    @Chenmunka can you point us to some data on that? In practice I find reduced voltage even by a small percentage extends the life of any incandescent or halogen light bulb due to reduced heat/operating temperature. Prior to LED mass availability our company quite often installed pre-set dimmers for customers to extend bulb life especially for hard to change bulbs. Running incandescent/halogen at 88-92% doesn’t significantly decrease lumen output, it does significantly extend bulb life.
    – Tyson
    Commented May 1, 2018 at 14:06
  • @Tyson: My comment is based largely on my own experience of using halogen bulbs in or out of a dimmer. But there is an article describing it at topbulb.com/blog/dimming-alters-halogen-cycle
    – Chenmunka
    Commented May 2, 2018 at 7:28
  • Up vote for shorter life on halogen lamps with reduced voltage. I have read this in the past when trying to figure out why 200 hour lamps were lasting well less than 100 hours.
    – Ed Beal
    Commented Jan 10, 2019 at 15:14

1 Answer 1


Is a min-60W dimmer safe with a 40W load

It will be safe - it won't catch fire. But it probably won't dim reliably - it won't predictably control the brightness.

More importantly, it probably won't work well when you inevitably replace the halogen bulbs with LED bulbs.

  • Many thanks. So am I looking for a dimmer with a min load as low as possible, to make the dimming work?
    – Nik
    Commented May 1, 2018 at 10:28
  • The bulbs in question are 12v, meaning there’s a transformer in the mix. The wattage rating on the dimmer is at 240v.
    – Tyson
    Commented May 1, 2018 at 11:53
  • Thanks! I cannot find this transformer anywhere - so it must be hidden. Would it make sense to just swap out these bulbs for LEDs? Would that allow it to be easier to dim?
    – Nik
    Commented May 5, 2018 at 11:59

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