Hi,

I'm trying to measure an inductor by applying a 4V p-p sinewave across an RL circuit (circuit.png). Increasing the SW frequency until the voltage across R drops to 2V p-p, noting the frequency, I can then calculate the inductance.

What I expect is that at a given frequency, the V p-p across R plus the V p-p across L = V p-p signal generator. This appears not to be the case. Approximately V p-p R + V p-p L = 5V > V p-p siggen. Should it be? Any ideas on what I'm missing here?

Thanks,
hrs

 

hi hrs,
What is the output impedance of your sig gen.?
E
***

 

Hi Eric,

My sig gen is per attachment. The NE5532 datasheet says it typically .3 ohms of 10kHz and a 600 ohm load.

 

hi hrs
Does this help, shows the -6dB and Amplitude compare plots
E

 

hi hrs,
This should give you a clue in what you are not allowing for.
It shows the Vdrop across the 1k.
Clue: the Inductor is an active device.

E

 

Thanks Eric, I'll think it over. I thought maybe I dropped a sqrt(2) somewhere, but there's the phase shift. Your amplitudes are spot on for my measurements, though R and L are swapped.

 

I see it now, thanks for the hint!

 

Hi,

There are three different formulas depending on if you know the internal resistance of the generator or not.
If you do not know or are not sure, then just measure the voltage across both R and L to determine what E is, then use the formula (w=2*pi*f):
L = (sqrt(3)*R)/w

That's once you adjust the voltage across R to be equal to E/2.
Remember E is the voltage measurement across R and L not the internal voltage of the generator.

Since you have a scope, if you want to double check you can also check the phase shift across R. It will be:
Ph = -atan((w*L)/R)

 

See this:

https://forum.allaboutcircuits.com/...ductance-of-a-coil.203763/page-3#post-1946661