12V AC lighting - How will LEDs respond to SCR dimming?

I'm setting up a 'twin cable' track light system in my living room, while attempting to buy as few parts as possible. I'd like to integrate LEDs into the picture, but I've got some concerns...

I realllly want dimming, because of the number of lights involved, and because I like to have a little mooood lighting, if you know what I mean. Most of the lights will actually be 12V MR16 Halogens, since I've got a bunch of the bulbs in a junk box, but I'd like to add some LEDs for color.

I've got an SCR dimmer pack I'd like to use for dimming, and a 12VAC transformer for the power supply. The SCR will be pre-transformer. The LEDs would probably be set up in small groups, with a bridge rectifier, a capacitor, and a resistor to create a fullwave mini-supply.

My big question is... How will these little LED/rectifier units respond to the SCR dimming? SCRs dim by delaying the rise of each half wave of the input, so, in effect, its a bit like a 120hz PWM. My thought is that the rectifier will continue to rectify, and that the cap will smooth this split sine wave into more of a DC saw wave. This should produce reasonable dimming for the LED. Perhaps a little more flicker than perfect, and a little more color distortion, but decent.

Is my vauge idea correct? Since this is a 'junk box' project, I want to purchase as little as possible, though I will need to buy some rectifiers. I'm not concerned about perfect performance, but I don't want to damage the LEDs (since, if they work, I'll get some Luxeon IIIs for it).

Thanks in advance for your suggestions!

-Sam

First, be sure your dimmer can deal with an inductive load (the transformer) Some SCR based dimmers can't. You probably don't need the cap, the transformer should smooth out the wave forms a bit. Wire the LEDs in series to get the combined Vf as close to the 12 volts as possible so less power will be lost in the resistors.

It should work.

It will flicker like hell if you run LEDs at 60Hz.

Handlobraesing said:

It will flicker like hell if you run LEDs at 60Hz.

Click to expand...

The only way to get 60Hz flicker that I can think of would be to use the LED itself as a half-wave rectifier. Each group of LEDs has a bridge rectifier (converts AC to pulsating DC, this would give 120Hz flicker) and a filter capacitor (pulsating DC to constant voltage DC, should greatly reduce flicker). When run at full power there should be no flicker. I agree though that your thought is probably right about the dimming -- it may create noticeable flicker at the dimmest setting, but it shuldn't be as bad as some of the awful products at stores that appear to use two parallel sets of LEDs as half-wave rectifiers that look like strobe lights.

One thing that I'd expect though is that if you have these in parallel with Halogen bulbs, the LEDs won't dim nearly as much by comparison -- for example, at 2/3rds the power, the Halogens will be under 50% brightness, and the LEDs will be at closer to 75% brigtness simply because incandescent efficiency goes down due to dimming, and LED efficiency actually goes up. This will be even more noticeable than the phenomenon of putting dimmable CFL and incan on the same circuit where CFL efficiency is constant when dimmed (not increasing like LED). Also, color temp will be more or less constant on the LEDs, while it will decrease on the inandescent. That could look very weird at lower intensity.

mahoney said:

First, be sure your dimmer can deal with an inductive load (the transformer) Some SCR based dimmers can't. You probably don't need the cap, the transformer should smooth out the wave forms a bit. Wire the LEDs in series to get the combined Vf as close to the 12 volts as possible so less power will be lost in the resistors.

It should work.

Click to expand...

Downside is that small resistor value makes it difficult to control. End result is probably something like that 1/10 turn of dimmer is full on/off.

I'm not convinced by that. Surely the dimming is happening by effectively decreasing the voltage on the lines? In which case you get into the non-linearity between voltage and current on an LED which means the current (and hence power) in the LED decreases a lot faster than the voltage. I suspect the LEDs may actually get dimmer faster than the halogens.

Of course all the above depends on what really happens with the combination of the SCR, transformer and smoothed full-wave rectifier - not that simple!