# Can I measure inductor like this?

#### bug13

Joined Feb 13, 2012
2,002
Hi guys

I will need to measure some inductors I got from some old electronic.

So can I hook it up to a AC say 9VAC from a transformer, and measure the AC current, then calculate the L with XL=2*PI*f*L?

the value of the inductors I need to measure is in mH range (guessing), so will this give me a good indication of the actually value of those inductors?

if not, what else can I do? I only have scope and DDM.

#### GopherT

Joined Nov 23, 2012
8,009
You will also need to know the DC resistance of the coil because X(sub L) is the total impedance. Also, frequency in your application should be roughly in the same range as your 60Hz transformer. That is, it may not be valid for use in a 100kHz oscillator since eddy current and other losses will start to get very big - it will depend on the quality of your core materials. You should also have a resistor in series with your coil so you can measure a voltage drop (otherwise you will only measure the source voltage which should not change).

EDIT: I guess you said you would measure current. I recommend measuring the voltage drop and calculating based on voltage drop instead of current. There are internal resistances in current meters that would have to be corrected for and all can be avoided by measuring voltage drop.

#### Markd77

Joined Sep 7, 2009
2,806
Probably the best way is to make an oscillator with them. SgtWookie gave me a circuit using an inverter. I'll have a look tomorrow if you like.

#### bountyhunter

Joined Sep 7, 2009
2,512
we used a square wave switcher and switch a known voltage across it then look at the rate the current through the inductor rises.

V = L di/dt

#### bug13

Joined Feb 13, 2012
2,002
Probably the best way is to make an oscillator with them. SgtWookie gave me a circuit using an inverter. I'll have a look tomorrow if you like.
Hi Markd77, that will be great if you can have a look tomorrow, thanks a lot

#### bug13

Joined Feb 13, 2012
2,002
we used a square wave switcher and switch a known voltage across it then look at the rate the current through the inductor rises.

V = L di/dt
How exactly do you look at the rate the current through the inductor rises? can you give a bit more details?

#### bountyhunter

Joined Sep 7, 2009
2,512
How exactly do you look at the rate the current through the inductor rises? can you give a bit more details?
You use a current loop. You could probably also use a low value resistance.

#### bug13

Joined Feb 13, 2012
2,002
OK, my original plan doesn't work for my inductors, my main is 50Hz and XL is too small, I can't get a useful reading with my \$10 DMM

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#### Mustafatarhan49

Joined Aug 16, 2012
56
You are talking about AC. What if the input is DC step?
do you think this is the right formula to find out the RL value for DC? iL(t) = (Vmax/L)*(1-e^-(t*RL/L))

#### bug13

Joined Feb 13, 2012
2,002
Here's the oscillator. The formula did turn out to be the right one. Worth a try if you can measure frequency and have either high precision capacitors or a capacitance meter.
If you only have +/- 20% capacitors and no capacitance meter there isn't much point.