[duncanlowe]

I wonder if some kind of inductive loop held near the lamp, connected to your 'scope' might pick up enough to see the waveform? I guess you could always look at the PCB of the controller, too.

I think part of the reason for the switch, is that newer controllers aren't simply controlling the power (leading edge dimmers essentially wait for the half cycle to get to a certain voltage before switching on using a diac and triac) where newer stuff often has some kind of microcontroller, probably with some kind of RF link (either wifi or just a receiver) that means they actually need power. The thing is with that, that you can't simply use any kind of normal power supply. That's because in the vast majority of UK installations, the Neutral isn't available in the light switch. There's only phase, switched phase, and earth. So the only way for the electronics to get power is by 'stealing' it from the trailing edge switch off.

I'm not sure why this method is used trailing edge, and can't be done leading edge, but it does seem to be why the change has happened. It does mean, though, you can never get 100% because there has to be an off period, however small, for the electronics to steal the power.

LED compatibility with dimmers to be honest is a nightmare. I have a few replacement lightswitches so they can be remote controlled. They are trailing edge to steal power as described. Because of the issues the manufacturer of the dimmers / switches published a compatibility list. The wall lights use a total of 6 lamps, the ceiling light 3 lamps. Apart from the supplier sending some wrong versions in a couple of orders, and some not working straight out of the box, the wall lights (6 lamps) work fine and the fade up and down as they come on and off is no problem (though I strongly suspect they would be significantly brighter on a straight switched supply, given how bright they are at the first flicker of power) but the 3 lamps in the ceiling sometimes have a bit of a fit until they get switched off and on again.