Simple LED driver/constant-current source 20 mA


Another Instructable by Jan Henrik,  a constant-current source which gives 20mA at the output.  He writes:

It is a circuit, which limits the current to 20mA, what allows us to drive any normal led, without calculating a resistor, you can just plug the led in and see it emitting light…
Also this allows us to test LED´s, to test the polarity(if there are no marks on the LED) and also it allows us to figure out the perfect forward voltage of the LED.
We can measure it, when we plug the LED in and measure the Voltage drop at the LED. Then you will have the perfect forward voltage…

This is a Great project for beginners, it´s easy to solder, it doesn’t require expensive parts, no ic´s…
Or you can use it for circuits with a variable input Voltage, because the input voltage does not change the output voltage, until it reaches the limits of the circuit

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  1. Interestingly, I have found myself rarely needing to try out LEDs at 20mA. When I buy white LEDs for lighting projects, I already know their ratings, and most of the time I use a switching circuit that can be easily dimmed, thus allowing various current values to be set. As for indicator LEDs, I would never drive them at 20mA anyway. Too bright. 1mA is often enough for modern diffuse reds. Then for the 7-segments, I like the quite popular habit of dropping all the resistors and directly connecting them to pin outputs. So I have a multi-resistor board to try LEDs on rather than a constant-current circuit.

  2. While LCD modules are available that run off 3.3V, the backlight is typically multiple LEDs – in series.
    To provide 20 mA, a 3.3V source needs to be boosted to over 21V…
    I was hoping to see such a circuit. No such luck.

    I suspect there’s a voltage independent version on the Adafruit RA8875 LCD Driver board, but I can’t find a schematic to know for sure.

    1. Most TFT LCD boards from China that has MCU 8/16-bit I/O usually have the backlight LEDs in parallel, so no boost is needed. If you stick to those (3.5″ or under) then it’ll save you a lot of trouble. Bigger panels often need to be driven using RGB signals, that need a biggish SMD MCU and many signal lines, or you’d need a driver PCB. Those might have white LED strings for backlighting.

      A few years ago I’d be enthusiastically building an MCU-based boost converter for white LED lighting. Now, boost ICs specifically for white LED strings may be better — very cheap, very high efficiency, and relatively simple and small circuitry. For 21V output, it’s about 7X of your 3.3V Vin, or 140mA. That will be ugly for 2X alkaline batteries, and even for power supplies, you’ll have to pay attention to the current capability. So making your own boost circuit will be more difficult than using boost ICs for white LED lighting.

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