About the switch

In the UK dimmer switches with modules that look like this are quite popular: https://ssli.ebayimg.com/images/g/7f0AAOSwMvtZOrsC/s-l640.jpg It's a mains light dimmer, usually rated 50W-250W. Having searched the web a bit and disassembled a few, they appear to be triac dimmers.


I use these dimmers in rooms with three to five lightbulbs. I have recently replaced the lightbulbs with dimmable LEDs, leaving one halogen in, so load is still 50W+. Without that they flicker when dimmed (my understanding is that triacs have a minimum current requirement). All fine so far.


A dimmer switch broke, so I got a replacement.

It worked fine for a few hours, then lights started flickering (my guess is 10-20Hz) and were a bit dimmer. Sometimes flickering stops for a bit, lights get brighter, then starts again etc. IOW, behaviour changes over time, but is never fine.

Switching off/on doesn't help. Dimmer was dialed to full power when this happened. This has happened in multiple rooms (I've moved dimmer switches around to eliminate other factors), and with three replacement dimmer switches. With one of them it took a few days for this fault to develop. After fault develops, that dimmer switch does not work correctly anymore.

Now, the fix seems obvious: source dimmers elsewhere. But I feel uncomfortable not knowing the root cause, and I think this could be an learning experience.

What is happening?

It appears I'm using these dimmers in some incorrect way, although that's not obvious to me. I guess circuit in LEDs could draw too much current through triac in dimmer, but that theory seems weird.

  • \$\begingroup\$ can you add a schematic? \$\endgroup\$
    – CNA
    Commented Mar 17, 2018 at 11:32
  • \$\begingroup\$ Maybe I misunderstand your question. It's a dimmer switch from a shop. Mains wiring is straightforward, switch is on LIVE wire. \$\endgroup\$
    – domen
    Commented Mar 17, 2018 at 11:34
  • \$\begingroup\$ Sorry, I couldn't understand what you said in a comment! \$\endgroup\$
    – CNA
    Commented Mar 17, 2018 at 11:39
  • \$\begingroup\$ Schematic of what? There is none, but I'll try my best to explain the situation if you have some specific questions. \$\endgroup\$
    – domen
    Commented Mar 17, 2018 at 11:41
  • \$\begingroup\$ Where did you buy them from? \$\endgroup\$
    – Andy aka
    Commented Mar 17, 2018 at 11:48

1 Answer 1


You probably are using leading edge dimmers. When you instead need to use trailing edge dimmers. Both kinds work just fine on purely resistive loads. Use leading edge with inductive loads (halogen lamps using magnetic step down transformers as an example). And trailing edge dimmers with electronic transformers or drivers (which LEDs have)

The difference- imagine your light dimmer is set to 50% brightness. Leading edge blocks the first part of the AC waveform (going from 0V to 380V -peak of 250V RMS). While a trailing edge dimmer let's through the first half and blocks the second half.

When you connect a leading edge dimmer to an electronic power supply, (imagine the dimmer is still set to 50%). The voltage being fed to the LEDs will instantly change from 0V to 380V as the control element in the dimmer switches on. Now imagine that the storage capicitors in the LED lights are completely discharged when the dimmer suddenly puts 380V across them. Really high surge current, goodbye dimmer. Inductive loads are OK as the inductance limits the current when the dimmer switches on.

Connect a trailing edge dimmer to an electronic power supply. Same 50% setting. The dimmer starts feeding power to the storage capicitors in the LED lights, as soon as the mains voltage starts rising above the zero crossing point. The capicitors are charged gently as the voltage rises. The dimmer then switches off the power. The capicitors discharge into the LEDs, and the cycle repeats next time the mains voltage crosses past 0V. Connect an inductive load, and the back EMF "kick" from the current being switched off mid cycle then destroys the dimmer.

Also is your 1 halogen lamp a direct connected to mains type, or is it a 12V one with a step down transformer? As you can often get away with a much smaller Min load. Even a filament lamp or 20W resistor would most likely work. Reason is that during the off part of the waveform controlled by the dimmer, a very small current still flows through the load. Only around 1mA or so. Which mainly comes from the timing circuit in the dimmer. The 20W resistive load allows that current to flow without the voltage on the dimmers output terminal rising much. Without that load resistor, the timing circuit can't work properly.

The best dimmers are ones that have a neutral connection for the timing circuit. But they are very rare. And can be difficult to install. As in some countries, there is no requirement to make a neutral connection available at a light switch. Meaning a dimmer that requires a neutral can't be used. Note that there are also dimmers out there that have a neutral connection, but that neutral is only used as part of an inbuilt interference suppression circuit.

  • \$\begingroup\$ Thanks. I'll look into this (I also use leading edge dimming in unrelated DIY project - it works fine for LEDs, but maybe only by luck). \$\endgroup\$
    – domen
    Commented Mar 17, 2018 at 14:33
  • \$\begingroup\$ I think your 380V figure is a bit large (230*sqrt(2))? \$\endgroup\$
    – domen
    Commented Mar 17, 2018 at 14:34
  • 1
    \$\begingroup\$ Perhaps a more complete description of LE vs TE dimming: ledjournal.com/main/blogs/leading-edge-vs-trailing-edge-dimmers \$\endgroup\$ Commented Mar 17, 2018 at 16:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.