# Measuring Change in Inductance

Discussion in 'General Electronics Chat' started by joepublic, Aug 24, 2010.

1. ### joepublic Thread Starter New Member

Aug 24, 2010
2
0
Hey everyone, I've looked around the net as well as here and cant seem to find an answer to fit my needs.

I have to measure the change of inductance over time as external exposures effect the DUT however I can't use an LCR (or other standard means) due to the wide range of operating frequencies and cost of accurate high frequency capable meters.

What I'm hoping to do is have a function generator as the input (set to the intended operating frequency of the DUT), a circuit to which the DUT can be attached to, and measure an output.

What the output is(voltage/frequency), doesn't matter as long as the Δoutput = Δinductance.
I don't need to know what the actual inductance is, just the change.

The real question is, what is a simple circuit that can be used for this?

Thanks in advance for any help!

2. ### tom66 Senior Member

May 9, 2009
2,613
214
How about an LC oscillator? You could base the oscillator around your DUT and a very stable, low temperature coefficient capacitor, and measure the change in frequency.

3. ### timrobbins Active Member

Aug 29, 2009
318
16
Joe - it doesn't quite make sense when you say you can't use a commercial meter and the frequency range is tooo wide! What meters have you tried and what wasn't working?

Is the inductor just a pure inductance? Is it a linear component (ie. will its inductance change with frequency according to Z=wL)? Is it electrically connected to anything? How would you propose to calibrate the 'new' measurement method - do you know what the measured value will be for a given set of conditions?

Ciao, Tim

4. ### joepublic Thread Starter New Member

Aug 24, 2010
2
0
Thanks for the replies so far!

Tom-
An LC oscillator is one of the methods I'm looking into, one of the challenges of this method is to make sure that one test setup can be used for the entire range of inductors I'll need to test. What I'm hoping to do here, is use the function generator to sweep around the expected Q point of the circuit and see where it actually is while the inductor is being tested.

Tim-
Lets take a few meters i have in house as an example. Agilent 4263, upper limit is 100k. 4192, upper limit is 13mhz. 4287 upper limit is 3ghz, but it's lower limit is 1mhz. 4287's sound great, but they are \$\$. I'll need to run 6 tests concurrently, and that would add up. Now the operating freqs of the DUTs range from 10k to 2ghz so having one setup that can accommodate any inductor without switching meters or test circuits becomes difficult.

5. ### timrobbins Active Member

Aug 29, 2009
318
16
I don't get the rationale - you appear to have a variety of DUT types to put through the hoops. But I think you are saying that each DUT would only be tested in a limited frequency range, or just at one centre frequency, but you don't indicate the span of frequencies that might have to be tested for a DUT?

Is the requirement to use only one test jig/system just a commercial one, or is it based on a practicality/difficulty perception, or ?

I think you will end up using more than one meter/system due to the 5 decade span - so compromise may be in order such as using the same jig head, etc.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
Since you only want change in inductance how about including the DUT in the feedback loop of an (op) amp?

Another method is 'current sensing' method to add a low values resistor in series (preferably on the earthy side of the DUT) with the DUT and measure the voltage across the low resistor. This gives the current through it and therefore the current through the DUT. Again a change will show up. This method can be pretty good across a wide frequency range.