# Calculating Inductance using an O-Scope...

#### gman19

Joined Apr 26, 2007
2
What is the proper formula(e) for calculating inductance using an O-Scope.

I have a GM A/C Compressor clutch coil that when energized produces a maximum current of 4.3A. Rise time to 63.2% of max current (2.717A) is
90ms. My working voltage is 13.5VDC. Using different methods to calculate, we cannot get consistant values. Any help will be appreciated.

#### Papabravo

Joined Feb 24, 2006
13,512
Coils don't "produce" current, they are passive devices. In a series R-L circuit, a voltage step caused by connecting the 13.5V source to the "load" consisting of the coil with inductance "L" and DC resistance "r", and any external resistance "R", will cause the voltage across the inductor to be:
Rich (BB code):
V_sub_L = L*(di/dt)
With small currents and copper wire you can usually ignore the resistance of the wire in the coil. Large currents make this approximation increasingly problematical. I'm confused by the multiple methods of calculating the inductance. There should only be one way.
Rich (BB code):
Starting with the solution to the ODE, and assuming r << R
i = (V/R)*(1 - e^(-(R/L)t))
as t -> infinity we have
4.3A = 13.5 / R which implies R = 3.14 Ohms
then we have L/R = one time constant = 90 msec
0.090 seconds = L / 3.14 which implies L = 282 mH
What other way is there to do the calculation?
Unless r, the resistance of the coil is < 300 milliohms, the above analysis is inaccurate.

#### gman19

Joined Apr 26, 2007
2
Thanks Papabravo,
The 282mH is what I did come up with on one calculation...I guess I need to review the calculations in the program on a collegue's machine. I wrote the formula in my spreadsheet as you stated above, but we were getting 319mH on another system.

And yes, larger currents do prove more difficult. Comparatively, this is a small load current to other circuits I have had to build.

Simulating trailer brake magnets was my last bane...20A loads at 14VDC, inductance not specified PWM @ 250Hz 50% duty cycle. I got handed 8 magnets and an oscilloscope and was told "Plot it and simulate it"...

Not real easy, and the coils did prove a problem...