I just came across this circuit meant for trailing edge dimming of LED bulbs but I was wondering what else could it be used for and what its limitations are during general use.

To my understanding by triggering the FET this circuit could either act as trailing edge dimmer / leading edge dimmer / normal switch.

This would be ideal to not only dim some LED bulbs but also to be able to control inductive loads and dim regular incandescent bulbs. Of coures a zero crossing circuit is necessary and a controller for switching the optocoupler (not included here.)

However, this circuit uses a rectified mains voltage compared to designs with two opposing FETs and there is no further protection but the built in diode of the FET.

I have a few questions regarding this circuit:

Dimmer circuit

This was originally taken and modified from here.

  1. Is my understanding correct that the circuit can be used as trailing edge dimmer / leading edge dimmer / normal switch by changing the timing of the FET switching?
  2. Do inductive loads e.g. a heater pose a threat when dimmed due to the induced reverse voltage or is this covered by the bypass diode?
  3. Are there devices known that can not cope with the rectified AC voltage or does this not matter since it reaches U=0v during each half period anyways?
  4. Are there further protections required to switch inductive / capactive loads with leading- or trailing edge dimming or switching at U=0v?

EDIT: Datasheet of used FET


1 Answer 1


You can certainly use a FET and AC bridge to switch a load, but not blindly consider leading or trailing edge dimming on any load with reactance. A zero-crossing trigger Triac is the best switch for minimum surge current and low current cut-off. Phase controlled dimming is a reactance issue , specific for each type.

But unless you have a specific load for a leading-edge zero-crossing trigger with mid-phase cut-off, this application has inductive load issues! Missing Datasheet in your question

RdsOn 0.19 Ω max @ 25'C
RdsOn 0.43 Ω typ @ 125'C ... thermal parameters require adequate sink for Rja = ~ 1 'C/W best ~
EAS 690 mJ abs. max.
EAR 1 mJ abs. max.
I pulse 62 A limited by T
I(AR) 20 A . . . "

With these parameters you can choose what is your maximum reactive energy load L or C and real energy load R for the best possible heatsink. (CPU + fan)

Consider AC motors typ. have peak surge current of 8x rated max current and light bulbs have a worst-case peak sine current on start ~ 10x rated.

Reply to previous comments:

For a std 6A Triac 100uA typ. @ 600V you can suppress the leakage below LED visible threshold with 3.3M shunt R to 0V (1/16thW) on 240Vac and perhaps tiny RF cap.But only on dimmable LED lamps.

Or you may consider to use an Alternistor (snubberless) (matched to load) .

Right , the FET diode is across the Vds switch and not across load for Drain switched loads. Like a motor (ok on some) or some transformers not rated for dI/dt switching on primary and might fail from eddy current losses and mid-current flyback damage.

  • 1
    \$\begingroup\$ Thanks for the elaborate reply. My approach would be zero crossing switching as standard and then trailing or leading edge dimming if I know what load is attached. Leading for capacitive loads and trailing for inductive loads. The idea with the shunt resistor is good but I'd like to use this circuit behind a socket so I can't know what will be plugged in. \$\endgroup\$
    – B. Ueno
    Commented Apr 3, 2021 at 8:11

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