Inductor meter

Resonate with known value capacitor. f=1/(2*pi*sqrt(LC))

Yes, at least for the ideal model of an inductor.

Yes, although as you've surmised to "put a sine wave in" you need to be aware of the voltage source's resistance.

What are you trying to do?

If you just have a few inductors you'd like to measure, the simplest options are:

There are plenty of fancier methods for doing all this, but there's little point unless you need that additional accuracy. (And with inductors, there are so enough non-idealities that the "effective" inductance is often noticeably different from the "actual" inductance anyway.) The other trick to all this is that you have to have some vague idea as to what the inductor's value is in the first place to size those resistors and capacitors in the first place, or at least plan on iteration a bit. As you'd expect, particularly small and large values of inductor tend to be prone to plenty of measurement errors: It's pretty much impossible to measure small (100mH) inductors you often have so much wire resistance that needs to be subtracted out first.

---Joel

As BobG points out, this should be f=1/(2*pi*sqrt(L*C))

you also have the resistance and capacitance of the unductor itself.

Bob

If significant, you could get 'off' results from the interwinding reactance. I say sine tests are ok if you stay far below the inductor resonant frequency.

I think pulse techniques that measure inductor di/dt are better at getting the inductive portion.

D from BC British Columbia Canada

Here you can also yield inductance as a function of frequency. Just change the generator frequency ;-)

Of course that doesn't work for really lossy inductors. Also, mind saturation since some of those 0805 cores can't take much.

Now, now. Thanks to Heathkit I had a dip meter when I was 15.

That's where the dip meter comes in. For really small inductance you'd have to build one because the usual versions won't go past 250MHz. But I have seen self-built ones that went to almost 1GHz.

On Mon, 31 Mar 2008 18:44:47 -0700, Joerg wrote: ...

Do you know of any Internet location that has a description for such a version?

Joop

In USAF tech school, we learned this the hard way - we had benches with caps, resistors, and at least one inductor; we did a calculation for an RLC circuit, figured out what all of the voltages should be, and went to the bench and they weren't even close.

Turns out we had neglected to account for about 2K of plain ol' wire resistance. :-)

Cheers! Rich

Here's a few:

Have Fun! Rich

What I'd like to see is a well-written guide to *using* a grid dipper. I know, I know, Google has that too... but I guess I'm looking for something from the likes of Joerg that gives hints and tricks beyond the "obvious" uses. :-)

Yeah, right. I tried that already. Like Joerg said most do not go beyond 250MHz or so.

This site however has a nice article:

Joop

Yes, here:

About half way down there is one that goes to 1400MHz. It's sad that Jan-Martin had to die so young but he left us a nice collection of circuits. Remember that most of these ideas are from decades ago. Nowadays you can buy RF transistors with 65GHz ft for under a Dollar. Such as the BFP620.

I think I've got another one in a book called "UHF Unterlage" but it probably won't do you any good since that's in German.

Hmm, maybe I should write one and place it on the web. When there is time which there never seems to be. Just got a call that the CEO of one of my clients passed away suddenly. Very sad. Probably means more work soon but I am more concerned about his family.

WRT dip meters there isn't all that much to say about them. They are basically a simple oscillator where the feedback is adjustable so you can place it "just on the edge". The supply current to that oscillator or in the old days the grid current now moves quite significantly when you approach anything that conducts or is in any way RF-lossy. In the case of resonant circuits that lossiness is frequency dependent so it'll "dip" where the resonance is. That can be used for all kinds of jobs, as long as you can make a resonant circuit with the component you want to test.

For an RF guy a dip meter is what a tractor is to a farmer. You do all sorts of jobs with it without thinking much about the instrument. IOW it doesn't have any glitz to it, it's just one of the tools.

Here is a brief tutorial:

My dip meter is transistorized but I've also got that vintage "Megacycle Meter" in figure 5. More as a souvenir, just like the old 50's pickup truck that everybody in the west seems to keep.

Thanks for the links, Joerg... great post!

I guess our posts just crossed. The same link popped up in my search. German is no problem. Whenever you find the time (seen you other post..) I would be happy with that section of your book. A detailed photo would be fine if that makes things easier for you.

Cheers,

Joop

I've got one of those RF Measurements "Megacycle Meter" dippers too, as well as a Heathkit transistorized dipper, and the dip attachments for my MFJ 259/269.

The Measurements dipper is the most sensitive one I've ever had the pleasure of using. I made up a test circuit using a piece of Airdux coil wired in parallel with an air-variable capacitor. The Measurements dipper, properly adjusted, gets a good sharp dip even when its probe tip is 4" or more away from the end of the coil.

The Heathkit is less sensitive - I had to get the probe to within about an inch of the coil to get a good dip. This close proximity would probably lead to additional loading of the circuit by the dipper, I think.

The MFJ with its dip-sense attachment was essentially useless... the probe had to be right up against the test coil to get any sort of readable needle deflection.

I've seen an interesting circuit for a dipper based on a "lambda diode"... two JFETs back-to-back, in a circuit which exhibits negative resistance akin to that of a tunnel diode. Maybe one of these days I'll buy a junked transistorized dipper at a swapmeet, rip out the circuitry, and replace it with a lambda-diode oscillator... just for the halibut.

Looking for the UHF Unterlage I found an offer for a second hand 1980 issue named "Gesamtausgabe". Also I found this list: # 1978 - UHF Unterlage Teil I # 1980 - UHF Unterlage Teil I+II # 1982 - UHF Unterlage Teil III # 1984 - UHF Unterlage Teil IV # 1987/88 - UHF Unterlage Teil I-V Leinen # 1987/89 - UHF Unterlage Teil V

Are they all interesting reading material? Which one contains the article of the GDO?

Joop

Just checked my UHF Unterlage, that one only goes from 270MHz to 500MHz so I guess the one in Jan-Martin's link is better. If you want it anyway let me know. Then I'll try to scan it in and email. It's a heavy and thick book, might not scan well and my camera is only 2MPixel which won't work well either.

The concept of the dipper is the same, a low frequency box and then plug-ins which contain the LC circuit and the RF transistor.

This is the one I have.

Could be V but hard to tell because to be honest the big book which has them all isn't very well organized IMHO. More like a collection. But for someone wanting to learn about real high frequency stuff this is good reading. Goes well into the GHz range. However, keep in mind that the devices they had available back then are a far cry from what we can buy now. So I always that the good old days are right now.

If you want to buy one get the newest. Shipping will be expensive, it is very heavy.