I have been battling LED dimming in my house for a few weeks now and I have come across some inconsistent information.

According to my research trailing-edge dimmer switches are the correct technology for dimming LED bulbs - in that case, why are there so many supposedly LED compatible switches with leading edge instead? (https://blog.lightbulbs-direct.com/led-dimmer-switch-compatibility/)

Example https://www.screwfix.com/p/british-general-nexus-metal-1-gang-2-way-led-dimmer-switch-brushed-steel/53506


Similarly on this switch it clearly states it's LED compatible however it says it only supports inductive and resistive load - https://download.schneider-electric.com/files?p_enDocType=Product+Data+Sheet&p_File_Name=GGBL6012LSBS_DATASHEET_WW_en-GB.pdf&p_Doc_Ref=GGBL6012LSBS_DATASHEET - neither of which seems to be used for LED (https://www.lyco.co.uk/advice/dimmers-trailing-vs-leading-edge/)

  • Leading edge dimmer switches were in use for dimming before led lights came on the market - that’s why so many switches have it. – Solar Mike Jul 18 '20 at 5:48
  • Plain and simple, leading edge is simpler, cheaper to implement, and provides acceptable performance for most people (see: annoying buzz with many LED bulbs). A trailing edge dimmer would be at least twice as expensive based on existing technologies and the average Joe Schmoe often goes for what’s cheapest. – statueuphemism Jul 18 '20 at 6:19

leading edge dimmer will cause repeated current surges into input capacitors on electronic ballasts. this will damage the ballast or the switch.

trailing edge dimmers will experience voltage spikes when they interrupt the circuit to inductive loads like iron ballasts and transformers possibly causing malfunction or damage.

What style of dimmer is right for your lamps depends on the lamps.


A dimmer that reduces voltage properly as you'd expect, is called a variac. It is about an 8" cube (0.2 m) and weighs 50 pounds (23,000 g), and probably costs $300.

The second most obvious option, a resistive dimmer, is called a rheostat. It is cheap enough, but it makes an insane amount of heat, so the provisions for heat removal will be quite expensive.

As it happens, the semiconductor revolution has given us a rather cheap device called a thyristor or triac. It insulates (blocks current) - until you send a "gate" signal to turn it on. Then it conducts, forever. This device is pretty useless, since it cannot be turned off. Ever. However, if current stops flowing, it turns off by itself, and then awaits another gate signal. The triac has very close to zero voltage drop across it, which means it runs cool, which means it can switch a LOT of current at a cheap price.

Now, electric power is AC. (see where this is going? I bet you do.) AC reverses itself 100-120 times a second - voltage crosses zero, and with a resistive load (incandescent light), current goes to zero at the same time. This "dimmer" practically builds itself, AC's zero crossing turns it off, you just need to know when to turn on, and that's a simple timer circuit.

That is the leading edge dimmer you are complaining about, and it's cheapie cheapie cheep cheep.

OK, you want a different kind of dimmer? Now you need power electronics able to forcibly interrupt AC power under load, when that load could be enormous, and potentially inductive. Which is really hard to interrupt. And it needs to fail non-deadly or UL won't approve it. We're no longer in the realm of the 3-cent triac. And the triac has set the customer expectation for cost and heat dissipation.

  • 1
    Yeah, a TE dimmer requires a HV power FET or IGBT and a rectifier bridge in place of the triac, as well as a neutral connection to provide an accurate zero-crossing reference – ThreePhaseEel Jul 18 '20 at 16:21

Your question asks way two many questions to be answered with one answer! Electronic switches need power to cycle the output. No matter leading or trailing edge look to se if a neutral is required!!! If a neutral is required it makes no difference electrical wise. Old school dimmers used SCR’s silicon controlled rectifiers, back with old school incandescent lamps no neutrals were needed because you cannot see the tiny current needed to make them work with out a neutral. Now with LED’s a tiny Very tiny amount of currant called gate current lights up LED’s . So leading edge and trailing edge are gimmicks in my opinion if no neutral is required.

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