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

hi all,

i'm testing vf of red led these 2 simple circuits:

circuit (a)

5v----|r|---->led----gnd


circuit (b)

5v---->led----|r|----gnd


r=150 ohm.


i use analog input 1 probe between led , resistor measure voltage drop.

assuming red led has 2v of vf, 150 ohm resistor should limit current @ 20ma.

according this, analog input should read 3v in circuit (a) , 2v in circuit (b), that's not case. 3v in circuit (a), 4.5v in circuit (b), indicating vf of 0.5v led (?!).

¿what missing? normal. i'ts way i'm measuring vf (using analog input) it's wrong?

thanks!!

i don't know you're doing wrong because your "theory" correct.    kirchhoff's laws the voltages have sum-up, , in case voltages must sum 5v  (and ohm's law, it's law of nature).

quote

i 3v in circuit (a), 4.5v in circuit (b), indicating vf of 0.5v led.

but, led on , same brightness, know there's still 2v across it, right?

quote

¿what missing? normal. i'ts way i'm measuring vf, using analog input it's wrong?

i dunno, thing that's tricky have measure relative arduino's ground, whereas multimeter, black lead doesn't have grounded.   so, meter can measure voltage across resistor , voltage across led, no matter way connected.

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