RE: Control LEDS with Volts

["Bricknell, Tony" <tony.bricknell@xxxxxxxxxxxxxxxxxxx>]

What you say is, essentially, correct.  However, you get much finer
control and much better LED-to-LED brightness matching if you PWM a
current through them.  It all depends what you want to achieve.

LEDs are semiconductors and react to voltage in a completely different
way to conventional incandescent lamps.
http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/
seems to
explain it quite well.

They also give you a calculator for working out what resistor you need
for a fixed voltage:
http://led.linear1.org/1led.wiz

If you simply connect a resistor in series with an LED across a 4-12V
supply, you'll find that the light output dimming range of the LED will
be bunched up at one end of the supply voltage.  Hence why people make
dedicated LED dimming modules that PWM a constant current into the LED.
(Being constant current also removes any part-to-part variability of the
LED forward voltage, which would otherwise be visible as a difference in
brightness between LEDs.)

HTH,

-----Original Message-----
From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf
Of Tracey Gardner
Sent: 19 November 2008 18:08
To: ukha_d@xxxxxxx
Subject: Re: [ukha_d] Control LEDS with Volts

Surely a voltage variation results in a current variation?
I have varied the brightness of normal LEDS purely by putting resistance
in series with them thereby reducing the voltage across them.
Your variable voltage will result in variable output from the LED.

Tracey

>I have a potential project but have hit a snag,
>
> I want to replace a 12V halogen lamp with a LED - There is already
> provision for dimming the halogen by varying the voltage, and a
> voltage slider is already in place, so is it possible control a LED or

> a LED array with an analog voltage (say 4-12V) - is there a device
> that will convert the voltage variance into current variance and allow

> me to dim the LEDS using the slider?
>
>

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