# LEDs

Discussion in 'Electronic Basics' started by zooz, Oct 5, 2007.

1. ### zoozGuest

Hello
I need to know about controlling current through LEDs in order to
control the illumination
I've searched, but all I found was of commercial content, I need to
know the theory.
Thank you

2. ### Tom2000Guest

PWM is simplest and pretty much the standard these days.

I suppose you could get away with a pot, in series with a fixed
resistor, in series with a LED. That might work for small LEDs,
drawing 20 mA or less. If you go this route, check the power
dissipation of the pot. Otherwise you might build a variable current
source from a regulator IC or a transistor.

To figure the LED's current limiting resistor requirement, subtract
its forward voltage drop from the supply voltage. Then determine the
current you want to push through the LED. The dropping resistor value
will be (V - Vf) / I. (For small LEDs, you'd typically drive them
with 5 to 20 mA. Often less than that will serve you well.)

For manual control, a pot, feeding the ADC input of a Picaxe08M chip,
driving a small LED via its PWM output through a series resistor is
pretty simple and reliable. Just a few lines of simple code makes it
happen.

I guess it all comes down to what you want to do. Change the
brightness of a small indicator LED or control 100 3 watt RGB LEDs to
light a nightclub? Vastly differing requirements.

HTH,

Tom

3. ### John PopelishGuest

There are two common methods.

One is a DC method that uses either a resistor or a more
complicated active current regulator circuit in series with
the LED. Changing a resistor value changes the current
setpoint.

The other uses a pulse width modulation scheme that varies
the duty cycle (percent on time) of a fixed current
amplitude pulse to vary the average current. In either
case, the intensity is pretty close to the average current
through the LED.

Either scheme can be varied with a variable resistor (having
the resistor vary the duty cycle of some timer circuit for
the PWM scheme), but the PWM scheme is most often used when
a digital processor will be in control of the intensity.

4. ### Jonathan KirwanGuest

There is no "one theory" for this. Theory comes in many varieties.
(1) Any one of a number of current source/sink topologies exist --
some requiring a resistor change to adjust the current, some
controllable by setting a control voltage or current, etc. I'm no
expert on these, but I know a few ways, and they require somewhat
differing ideas to follow them. (2) There is PWM (pulse width
modulation) that can be used. Might be an electronic circuit or
controlled from a microcontroller (with software.) Different theory
here. (3) You might be interested in controlling the change in
illumination from the point of view of a human viewing it... if that's
the case, there is more theory on that subject, as well.

What do you intend? Be as detailed as you can, so that others can
make judgments about what theory you need to understand. Or else
state categorically what commercial example you find meets your needs