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Yakk · 06-14-2004, 12:06 PM

http://www.cpemma.co.uk/dimleds.html

Ayep, looks like you are right TIO.

Quote:

Dimming LEDs

Adding some indicator lights to the Diobus design has been covered, but you can add illumination to any fan controller in various ways. One wish might be to vary a led brightness as the fan voltage is turned up or down.

Dimming leds isn't as straightforward as with filament bulbs, which glow because of their high temperature, and rapidly fade as this goes down. The eye has a non-linear response to light, and can see a wide range of dim objects but a much narrower range of bright ones, so subtlety is not on.

Say a 2v led was running at 12v with a series resistor of 1k, giving a 10mA forward current. (If = (Vs - Vf)/R)

Reducing the voltage to 6v would reduce the forward current to 4mA. The change in brightness would not be major.

A trick to getting a bright led at 12v and a dim one at 6v is to raise the forward voltage by adding ordinary silicon diodes (eg 1N4001, 1N4148, etc.) in series with the led. Each led adds about 0.6v at these current levels.

So with a 2v led, adding 6 diodes makes the Vf about 5.6v. A 620R series resistor will give 10.3mA at 12v, but only 0.65mA at 6v. Major difference!

Try various numbers of diodes to get the effect you want. In practice you can usually add an extra diode or two on top of the calculated number, as the 0.6v diode Vf goes considerably lower than this at very low currents, as does the Vf of the led.

And don't start with highish currents at the 12v setting or you'll have to turn down a fair way before you start to see any apparent change in brightness. 5-10mA is ample.

The finished string is connected across the controller output in parallel with the fan power leads.