# Different LED forward voltages depending on the order of the limiting resistor on Arduino

I'm testing the Vf of a red LED with these two simple circuits:

Circuit (A)

5V----|R|---->LED----GND

Circuit (B)

5V---->LED----|R|----GND

R=150 Ohm.

I use analog input 1 as a probe between the LED and the resistor to measure the voltage drop.

Assuming the red LED has 2V of Vf, the 150 Ohm resistor should limit the current at 20mA.

According to this, analog input should read 3V in circuit (A) and 2V in circuit (B), but that's not the case. I get 3V in circuit (A), but 4.5V in circuit (B), indicating a Vf of only 0.5V for the LED (?!).

What am I missing? Is it normal. I'ts the way I'm measuring the Vf (using analog input) what it's wrong?

Thanks!!

• Let me guess. The led is not burning. Reverse the led to fix your problem. – martijndb Jan 14 '17 at 21:09
• But, if the multimeter measures are as supposed to be, then it's the Arduino's input which I'm using wrong? – Angel Jan 14 '17 at 21:21
• Do you have a multimeter available? If so, can you post its measurements across the LED and the resistor for both (a) and (b). Also, how are you converting the ADC result to a voltage reading, is it using some third-party programme or one you wrote? – TonyM Jan 14 '17 at 22:23
• Your assumptions are wrong. Circuit (A) should read 2V between the resistor and LED (relative to GND) and circuit (B) should read 3V. That's all assuming the LED actually has a forward voltage of 2V. – Dan Laks Jan 14 '17 at 22:45
• If the LED were reversed, he should read ~0 V, not ~5 V. – The Photon Jan 15 '17 at 0:56