LEDs

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

Reply to
zooz
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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

Reply to
Tom2000

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.

Reply to
John Popelish

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 and ask about that. Or?

Jon

Reply to
Jonathan Kirwan

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