Unknown Inductor Testing Box

There are times that I needed to know the value of an inductor out of my junk box or build an inductor of my own. I do have an Almost All Digital Electronic L/C Meter IIB. This meter is a joy to use and has serve me well over the pass ten years. If you are looking at this article and want to built a nice piece of gear, instead of paying a lot for one of the commercial pieces of please read on.

I have always enjoyed relearning things that I learned years ago!!!! I think all of us have those moments. I am not sure why I feel that I need to review the basics but that is OK. The January QST 2011 had an article from Bob Rogers, WA1PIO building a test fixture to determine the value of an inductor. You can measure the value of an inductor by paralleling it with a known capacitance and measure the resulting resonate frequency.

This kind of goes back to the old days when we used grid dip meters to determine the resonance of circuit. As Bob indicated, it is getting harder to find grid dip meters and to really know how to use one. If you get the coupling of the grid dip meter with the tune circuit wrong you could get a false reading. This setup that Bob did is really a lot more reliable.

The box that I built from Bobs article provides a substitute for the dip meter method. A separate signal generator is used to provide the needed signal source, and switch internal capacitors are used to resonate the inductors. Bob used an internal meter so that you could indicate resonance and also using a BNC connector output you can connect it to an oscilloscope. Here is a picture of the schematic:

The circuit that I built, I also added a meter. I started off using multiple signal generators that I had in the shop. My best signal Generator is a HP-8640B. When I tried to test my first unknown inductor I realized that my generator did not go below .455 meg hertz, and I needed a generator that would go down to 1 kHz. I still tried to use the HP and also realized that the internal meter would not read. I thought I had some something wrong with the circuit or the signal diode that I used. The 1N34 diode is a germanium diode that will rectify a voltage down to .3 volts. So I turned the output of the HP generator to +20 dB and the meter did work some. Through my first tests I used the oscilloscope to find resonance. It really worked well to some extent.

After rereading Bobs article I realized that he was using a B&K 4040 that has a higher output of 10 volts. So he was using a generator that you would normally not have in an individual shop. I looked at the B&K website and that generator is $650. That would really be outside of my budget. I have a couple of old RF generators from the 50's through the 70's. I knew that these generators would not have a high enough output to work with the internal meter. It should work as a RF source and I would see the same results using an oscilloscope or a RF Meter (the RF Power meter that I built using Wes Haywards design).

The RF generator that I picked is an old Heathkit Model IG-102. I tried the knobs and the band switch was locked. That's how long ago I used it. I got the band switch working and realized that I did not have any output. It always seems like I have to fix some of my equipment before I can make any progress. I had broken a wire going to the main tuning capacitor from flexing the band switch.

The inductor tester has a switch that provides us with three settings. Placing the switch in the down position selects the 0.01 microF capacitor supporting measurements of inductors in the microH range using a signal generator with a 159 Khz to 5 mH range. Having the switch in the up position selects .22 MicroF capacitor that is used to measure larger MicroH inductors, using an signal generator frequency in the 1 to 34 kHz range. With the switch in the middle position you can substitute your own capacitor and inductor in parallel for testing.

Testing really turned out great I made a new cable for the Heathkit putting a BNC connector to match the connectors that I added for the test fixture. I had a BNC tee connector that I used on the input of the test set that connected the RF Generator and a Frequency Counter. The output connector is connected directly to the oscilloscope. Here is a picture of the setup:

The first unknown inductor came out of the junk box. I set down to hand calculate the inductance. I used a formula that all of us should know :-). It is how to calculate Resonate frequency in hertz:

F = 1 / 2pi * Square root of LC

Since we are trying to solve for L: I had to convert the formula too:

L = 1 / 39.478 * F to the 2nd power * C

I measured the frequency at resonate and it turned out to be 2.2 meg hertz. The switch on the test set was in the .01 mf setting. So starting the calculations

L = 1 / 39.478 * 2,200,000 to the 2nd power * .00000001

L = 1/ 39.478 * 4.84 to the 12th power * .00000001

L = 1 / 1910735.2

L = .0000005233587554 to the -7 power

L = .52 mH

The other thing that you can do is build a Excel spread sheet or Bob has one on the ARRL website.

Hope you enjoy this project as much as I have?