[Question] Trailing edge smart dimmer

[RichyRicho]

As the title says, I am looking for a trailing edge smart dimmer. I have gone through 3 smart dimmers already, only to read here tonight that all the dimmers I have had are leading edge dimmers. I have used a no name brand, the Ewelink smart dimmer and the Sonoff D1(which keeps turning itself on&#129315
This is to be used with GU10 Osram bulbs.
I have changed the bulbs from cheap Chinese ones, to more well known ones, and now to the Osram ones, only to have the same issue each time, flickering when dimmed.
I have googled trying to find trailing edge dimmers, but I can't seem to find any.
Many thanks in advance.

[ians]

These guys have an interesting series of info about dimming.

https://www.youtube.com/watch?v=zuaQ8WIMUbQ&ab_channel=efixx

Contact your local wholesaler they can source any type of dimmer (leading or lagging)

[Justloadit]

Any dimmer which uses a TRIAC is a trailing edge dimmer. The majority of cheap dimmers in the market.
What the control circuit does is that it 'times' the rise of the sine wave, and then switches the TRIAC on for the duration of the sine wave time until it passes through zero and automatically switches off, waiting for the next cycle.
This works great for incandescent lamps.

Now the LED lamps poses a problem due to the power supply design.
The cheapie non dimmable will not work with a dimmer, as they require a minimum AC voltage of the sine wave, usually 180V before the LEDs are lit. So effectively the LED cheapies have a poor power factor because they only work in the area of the sine wave above 180V. But because the amount of energy consumed is so low, that the power factor is not an issue as far as ESKOM is concerned.

The more expensive dimmable LEDs, have a switch mode power supply, now depending on the design, they may have a capacitor to store energy during the off times to power the electronics. They then measure the amount of voltage from the dimmer, and then translate this voltage into what is called PWM, to cause the LED to look dim. Effectively PWM is a fast switching in which the off time and on time are varied. Due to the human eye, we see the average light from the On and off time, and see this as if the light is dimmed.

The cheaper dimmable LEDs prefer what is called leading edge dimming, which simply means that at the start of the rise of the sine wave a certain amount of time is passed before the sine wave is abruptly switched off and waits again for the next cycle. So there is power going through the on time, the dimming circuit then switches the LED on, then when abruptly switched off, the LED is off. To do this type of switching, a FET has to be used rather than the TRIAC, due to the characteristics of the semiconductor.

Using a FET in your dimmer, makes it more expensive because of the typical control that is required to manage the FET.

Another problem with dimmers, is that they require a minimum amount of current to make the electronics work, which is not an issue with incandescent lamps, but is a problem with LEDs, because of the low power required to make them work, and this is what causes the flickering. There is insufficient current to charge the capacitor quickly to power the electronics.

There are 3 solutions here.
1. Place a 20W incandescent lamp in the circuit that you are dimming - can be an issue
2. Place a 0.47uF 275V X2 Capacitor in parallel to each LED lamp in the circuit you are dimming. If this does not help, then check the next point.
3. Place a 2.2 kOhm 50 watt resistor on a piece of 3mm aluminium 150x180mm as a heatsink in parallel to the circuit you are dimming. Leave this in the ceiling. There is no problem with this as the heatsink is more than ample to insure the temperature of the resistor never exceeds some 50°C as it draws 24W at 230V.

If you want to learn more about leading/trailing edge dimmers, follow the embedded link.

[adrianh]

Isn't the bottom line that if one is to dim leds that you have to purchase leds that are designed for this purpose and specify that they are dimmable on the packaging.

This page is quite good

https://www.simplelighting.co.uk/led-downlights/dimmable-led-downlights/

[Justloadit]

Isn't the bottom line that if one is to dim leds that you have to purchase leds that are designed for this purpose and specify that they are dimmable on the packaging.

Correct.
They usually say 'Non Dimable' or 'Dimable'