Dimming LED lights

[steve66]

I've got some LED lights that run off 12 Volts AC, and some that run off 12 V DC.

Does anyone have any idea what to use to dim these? I can't seem to find anything that dims a 12 V supply. I'm hoping for an "off the shelf" solution as opposed to having to design a custom power supply.

Would a line voltage dimmer on the line side of the transformer or power supply work? Or would that violate the listing of the transformer or Power supply by not supplying it with the proper voltage?

Steve

 

[JDBrown]

Does anyone have any idea what to use to dim these?

I would start by contacting the manufacturer. They will often have a dimmer that they recommend you use with their products.

Would a line voltage dimmer on the line side of the transformer or power supply work? Or would that violate the listing of the transformer or Power supply by not supplying it with the proper voltage?

I would avoid a line voltage dimmer. It might work for the 12VAC power supply, assuming the dimmer is the type works by truncating the sine wave, rather than by using a variable resistor to reduce the voltage, but I wouldn't mess with it. It won't work at all with a DC power supply.

LEDs are dimmed by pulse-width modulation -- ideally, they would be square wave pulses, but they don't have to be. Basically, the LED is fed a series of pulses rather than a continuous DC voltage, causing it to turn on and off very quickly. The pulses occur at a high enough frequency that your eyes can't see what's happening, and you see it as the LED getting dimmer.

I'll try to illustrate using ASCII art. In the example below, the LED would be on half the time and off half the time.

[FONT=courier new]12V ----      ----
        |    |    |
 0V      ----      ----[/FONT]

In the next example, the LED would be brighter, because it's on more of the time.

[FONT=courier new]12V ------    ------
          |  |      |
 0V        --        --[/FONT]

That's the basic idea of how LED dimmers work. They have to use PWM because LEDs have a threshold voltage, rather than a filament like an incandescent lamp. With an incandescent, if you reduce the voltage, you also reduce the amount of current flowing through the resistive filament, so it doesn't get as hot or glow as brightly.

With an LED, the brightness is pretty much constant for its entire operating voltage range. If the voltage drops below the threshold, the LED turns off. If the voltage increases to above the threshold, the LED turns on. That's the way it works in theory, anyway. In real life, you can get a little bit of dimming by reducing the voltage to right around the threshold, but it's inconsistent (and darn near impossible to control just how much dimming you get).

So, there's my long-winded reply. But the short answer is still, "Ask the Manufacturer what they recommend."