Author Topic: constant current circuit for LED with 12-24v inputs  (Read 228 times)

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drakejest

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constant current circuit for LED with 12-24v inputs
« on: June 15, 2021, 07:07:56 pm »
So i was looking for a simple constant current circuit for my LED indicator i would like to power the LED to somewhere around  7mA with a voltage from 12-24v but when i tried to simulate the circuit i could not make the current constant across different voltage

https://pasteboard.co/K6Lif8q.png

am i doing something wrong?

John B

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Re: constant current circuit for LED with 12-24v inputs
« Reply #1 on: June 15, 2021, 09:45:03 pm »
When you bias the base of the transistor using a resistor divider, the voltage at the base will vary with the applied voltage, therefore the current that is sunk will also vary. You want to maintain a constant voltage at the base, so try replacing the lower resistor on the base with a reverse biased zener. Better still, replace the whole circuit with a current sink IC or a linear LED driver to save on parts, space and time.

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rdl

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Re: constant current circuit for LED with 12-24v inputs
« Reply #2 on: June 15, 2021, 10:05:48 pm »
I use the second circuit (Restricting LED Current using Transistors) here:

SmallCog

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Re: constant current circuit for LED with 12-24v inputs
« Reply #3 on: June 15, 2021, 10:18:47 pm »

drakejest

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Re: constant current circuit for LED with 12-24v inputs
« Reply #4 on: June 15, 2021, 10:32:38 pm »
replace the whole circuit with a current sink IC or a linear LED driver to save on parts, space and time.

"LED DRIVER" is the keyword, thank you for that very much i was able to find suitable components .

I was actually looking for pre made IC of that but i was using "constant current source" as a keyword thus i was getting expensive chips which are too pricy for me for just indicator leds.

rdl

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Re: constant current circuit for LED with 12-24v inputs
« Reply #5 on: June 16, 2021, 03:33:34 am »
You can build the circuit I referenced in a 5 by 4 section of a breadboard or perfboard. It just needs two resistors and a couple of 2N2222 transistors. It will hold the current fairly steady over a wide range of input voltages (2.2mA change over a range of 12 volts).

Smf