Today I learned about Vf for leds, after founding out that 3 leds in series driven by 5 V did not make them light up while 2 did.

However, I also read that it is quite bad for a power source to use more volts (like 3 times 2V) this way than the power source (5V for the Uno).

Can the Arduino be hurt this way?

  • The Arduino can only supply a few tens of ma for each output pin, so as a practical matter you present it with an open circuit when you string along too many. Essentially the same as having nothing connected to the pin at all. That won't hurt the Arduino, no.
    – SDsolar
    Mar 2, 2017 at 19:23
  • @Downvoter, can you instead of downvoting all my posts, explain WHY you downvoted, so I may improve the post. Nov 18, 2020 at 11:50

2 Answers 2


The forward voltage of an LED or diode is the applied voltage of the "knee" in the exponential characteristic at which it starts to substantially conduct, and in the case of an LED, produce light.

It is not, under ordinary circumstances of indicator-type LEDs, a voltage that would be produced internally to the LEDs, at least not with any substantial current capability.

As long as you are supplying the LEDs from a voltage within the Arduino's I/O range, it should not be hurt by a series strings of too many LEDs. However, in theory (if rarely in practice) a series string of too few LEDs that did not include a current limiting resistor would stress the Arduino beyond its ratings - and possibly (and more commonly) also the LED(s) beyond their current limits.

  • Thank you very much ... about your last paragraph ... I already found that out ... luckily by only destroying one LED and not my Arduino. Mar 2, 2017 at 2:14

Placing 3 "typical" LEDs in series and then driving them with the Arduino will not produce any light because the Arduino can only output 5V on any pin. It would require approx 6 volts to drive current through the LEDs when connected in series. This is for typical green, yellow, red type LEDs that have a 2 volt or so forward voltage drop. Note: some other colors require more forward voltage.

When you place two of these LEDs in series the forward voltage drop required to light them is only about 4 volts, that's why they light up when 3 in series didn't. However, since the Arduino outputs 5 volts on its pin and the LEDs only require 4 volts the extra 1 volt can cause more current to flow which, if excessive, can damage the output pin. Thats why when driving LEDs in this way its usually a good practice to place a current limiting resistor in series with the LED(s). This resistor limits the current induced by any driving voltage which is over the voltage needed to drive the LEDs to conduction.

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