Electronic – Using diodes to limit current to LEDs


I'm setting up an attiny2313 to drive an array of LEDs and blink them in different patterns. Rather than putting a series resistor in line with each LED (they'll be controlled separately, so I can't do tricks like putting them in series with each other), I was wondering about just using a couple diodes to get the voltage down to the right range.

The system is running at 3.3V (with a regulator to keep it pretty constant), and two 1N4001 diodes will drop that down to 1.9V – right in line with the lower end of the 1.8 – 2.2V these LEDs are specced for. Since the diodes will always want to maintain the same voltage drop across themselves, I don't have to worry about that voltage changing as I turn the LEDs on or off. Overall I save 6 components.

Has anyone else tried something like this? Any gotchas I've overlooked? I realize the diodes will have to dissipate the full current from all the LEDs, but for this small amount of current that doesn't seem like it will be an issue.

(edit: I'm putting these diodes between the common cathode of the LEDs and ground)

Best Answer

I must confess I never tried this. But the series resistor has an important role: it is there to limit the current through the LED. If there's no resistor, the current may be limited in the end to a value which is too high for the LED or for the driver transistor. Theoretically you should graphically add the U-I characteristics of the diodes and the LED, and see on the the resulting characteristic what's the current for your Vcc value. But the main problem is that this current cannot be reliable predicted, since the U-I catalog characteristics of the diodes and LED give you an typical curve, and this curve will also shift with the temperature.

So while it may work, I wouldn't count on it working in every instance. But you may have some help from an unexpected place: the IC driving your LEDs. Sometimes the digital outputs have internal resistors or other ways to limit the output current, in order to avoid overloading them. So check the spec sheet for your attiny2313.