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RE: Low-voltage (12V) LED lights

To: <>
Subject: RE: Low-voltage (12V) LED lights
From: "Randall" <>
Date: Tue, 16 Jan 2007 11:02:59 -0800
> We agree that our dimmer is controlling where
> in the AC cycle the power is actually turned on - anywhere 
> between 0 and 120 volts.

Well, except that this is a half-sine wave, with a peak value closer to 170
volts.  And, as I mentioned before, the dimmers I have don't have enough
phase shift in the trigger circuit to trigger the triac all the way out to
180 degrees ... in effect, they can only dim the lights so far and then the
voltage suddenly drops to 0.  I've not tried to measure it, but judging by
how bright my incandescent lights are at that point, I'd guess it's
something like 20-40 volts rms, meaning the peak is much higher than that.

> But we now have to consider the circuit in the LED strip. If 
> they are wired as a single red LED and a current limiting resistor,
> our 0 to 10.8 volt output voltage must go over 1.5 volts 
> before the LED will conduct and give light. So our dimmers effective
> range just lost another 15%.

The other thing you are disregarding is that modern transformers typically
aren't very efficient ... their turns ratio provides for a rather high
output voltage to make up for the resistance of the windings.  Most "12vac"
transformers actually put out more like 15-18 volts unloaded.

> But what if they are white 
> LED's, then we need 3.5 volts minimum

I believe if you'll check, you'll find that even white LEDs start to conduct
at around 0.7v, like standard diodes.  They just have relatively high
resistance, so it takes more voltage to force full current through them.

> Build it, see what happens

Yup.  And while the results may not be what you desire, if it's built
properly this has no chance of smoking your LEDs.


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