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Electronics => Projects, Designs, and Technical Stuff => Topic started by: drakejest on June 15, 2021, 07:07:56 pm

Title: constant current circuit for LED with 12-24v inputs
Post by: drakejest 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 (https://pasteboard.co/K6Lif8q.png)

am i doing something wrong?

Title: Re: constant current circuit for LED with 12-24v inputs
Post by: John B 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.

Title: Re: constant current circuit for LED with 12-24v inputs
Post by: rdl on June 15, 2021, 10:05:48 pm

I use the second circuit (Restricting LED Current using Transistors) here:

https://www.homemade-circuits.com/universal-high-watt-led-current-limiter/ (https://www.homemade-circuits.com/universal-high-watt-led-current-limiter/)

Title: Re: constant current circuit for LED with 12-24v inputs
Post by: SmallCog on June 15, 2021, 10:18:47 pm

You may find this an interesting read

https://www.allaboutcircuits.com/technical-articles/need-a-current-regulator-use-a-voltage-regulator/ (https://www.allaboutcircuits.com/technical-articles/need-a-current-regulator-use-a-voltage-regulator/)

Title: Re: constant current circuit for LED with 12-24v inputs
Post by: drakejest on June 15, 2021, 10:32:38 pm

Quote from: John B on June 15, 2021, 09:45:03 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.

Title: Re: constant current circuit for LED with 12-24v inputs
Post by: rdl 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).