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Re: Low-voltage (12V) LED lights

To: "Randall" <tr3driver@ca.rr.com>, <shop-talk@autox.team.net>
Subject: Re: Low-voltage (12V) LED lights
From: "Arvid Jedlicka" <arvidj@visi.com>
Date: Tue, 16 Jan 2007 07:06:15 -0600
> > You are correct, a regular wall mounted dimmer will have no reliable
> > effect on the LED strips.
>
> I guess I'm missing something here ... why is everyone convinced this won't 
>work
> ?

My thoughts were related to how well it would work - hence the term reliable - 
not that it would not light, or possibly even dim
the LED strip at all.. But you are correct. Build it and see what happens.

As you mentioned, flicker might be a problem. When pulse width modulation is 
used for LED dimming, the frequency is usually a
minimum of several kHz. 120Hz is pretty slow.

Another issue might be the effective range of the dimmer. Lets look at our 
power supply - the existing AC transformer and a bridge
rectifier - mostly because we have no clue as to what the DC power supply might 
do. We agree that our dimmer is controlling where
in the AC cycle the power is actually turned on - anywhere between 0 and 120 
volts. Our step-down transformer converts that so we
have 0 to 12 volts. Our bridge rectifier has two diode drops in it so from 0 to 
1.2 volts of transformer secondary, nothing
happens - the bridge doesn't conduct - so 10% of our dimmer controls range has 
no effect, and out output voltage now is in the
range of 0 to 10.8 volts - i.e. you turn the knob for the first 10% and nothing 
happens and the final 90% of the knob actually
dims the LED. You could probably live with 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%. But what if they are white LED's, then we need 3.5 
volts minimum - the effective range gets even
smaller. And if they wired the LED's in series to get more light and to use one 
or two less resistors, the threshold voltage goes
to 7 or 10.5 volts. In the end, the area of dimmer knob rotation that actually 
has any effect could be pretty small.

But so much for the theory. Build it, see what happens [note that we are not 
responsible if 'happens' involves the word smoke],
let us know.

Arvid




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