12VDC vs 12VAC for GU5.3 LED Spots

[Seb101]

As I explained above, LED controllers control brightness by switching the 12 Volt line on and off at high frequency. (Reducing the voltage to an LED lamp has little effect on brightness, as (unlike an incandescent bulb) the colour temperature of an LED is constant.)

The output from the controller is effectively a square wave, which creates nasty harmonics at much higher frequencies. Adding a relay coil will result in 'ringing' every time the 12 Volt output is switched off. This happens thousands of times a second, and could be seen very easily on an oscilloscope.

HF radiation from this set up will almost certainly interfere with the touch sensitive switch, and possibly with nearby radio recovers, although the switch probably wouldn't be as sensitive once installed.

Adding a reservoir capacitor across the relay coil will almost certainly resolve the problem. (Observe polarity of the capacitor). However, don't add a capacitor directly across the controller output as this will prevent the lamps form dimming, and may damage the controller.

HTH.

Great! It's made it infinitely better, there is still a slight ringing from the relay as the controller switches on and off, but the screen works fine now. There is no dimming on the white feed however it does fade them in and out as they are switched, which is what causes the ringing.

Would a different value capacitor fix this completely? I can take some measurements with a multimeter if it helps.

It's now like this - does the order of the parallel cap and diode matter?

 

[bernardgreen]

The fading of the white will be ( almost certainly ) pulse width modulation. A large capacitor load could over strain the drivers so it would be sensible to reduce that effect by ading a resistor in series with the series diode.

It might be worth using a smaller relay such as the Finder 40.525 range. The 12 volt coil has a 300 ohm resistance so will be "smoothed" by a smaller capacitor. The relay and base fit nicely into a 2 module MCB housing

 

[NuaghtyNigel]

Firstly, sorry for the delay in replying as I have been working non-stop on my bathroom project!

I would agree with Bernard about adding a resistor into the circuit as he has indicated.

However I doubt that a large value capacitor will 'overload' the circuit as such, as once charged (in a second or two) it will present a very small load. However, perhaps we can look again at capacitor values:

I initially suggested a 470 µF reservoir capacitor, which should be adequate for the job without being oversized. You could try a lower value (say 220 µF) to see if it made ay improvement, but I would doubt it.

However, 'large' electrolytic capacitors don't perform particularly well at high frequencies, so it could be worth adding a second, much smaller capacitor (say 0.22 µF or 0.47 µF) in parallel with the first to help decouple the higher frequencies.