Choke test - are these reasonable values?

[SurreyNick]

I have picked up this Igranic C40 Choke but without the benefit of any literature I don't know whether the test values seem reasonable. What do you think?

The Henries reading fluctuates between -3.65 and -4.01H. I didn't expect a minus figure. The reading won't hold steady either.

The resistance reading is 15.22M ohms. I was shocked at how high the resistance reads.

I know very little about chokes and would appreciate the benefit of your knowledge.

Thanks

Nick

[GMB]

Would that be what your meter does if connected to nothing, perhaps with something touching a lead to add a little mains hum?

Either your meter is broken or your choke is broken, or your are not connecting to it correctly.

I would just add that negative inductance is capacitance.
If your meter is looking at an open-circuit choke then maybe that is the kind of reading it would show.

But check you have the clips attached to the right thing!

[SurreyNick]

The meter seems fine I have tested it against a known item and it was true. With no continuity, whether something is touching one probe or not, the meter returns to 1.

My meter only reads to a maximum of 20H, but when I test it on a 100H choke it just reads 1, so I don't think it can be caused by that either.

Addition
Yes. I have just checked the meter on a capacitor on the Henries scale and it gave a minus figure. So the choke is almost certainly open circuit too.

[Station X]

If the 15.22 Mohm reading is correct then the choke is open circuit.

I'd be inclined to remove the crock clips and probe the inductor terminals directly just in case there's a dodgy connection to the clips.

[SurreyNick]

Thanks Graham. I did what you said and the 15.22 Mohm reading is correct. It seems the choke is duff.
Nick

[Station X]

Are you testing the right points? I guessing the two brass lined holes are for mounting screws, but what about the two studs? I'm assuming that only one stud is visible in the picture.

[SurreyNick]

Yeah I checked those studs Graham. Nothing on them. I came to the conclusion that Igranic used the same casing for a transformer and those would have been used for the additional two terminals. The two brass lined holes are indeed mounting points.
Nick

[Station X]

Does it look like it'll come apart easily? If so rewinding looks like a simple job.

[SurreyNick]

Just about to have a look. I'll let you know. I may be lucky and find the break near the end of the winding.
N.

[SurreyNick]

It was so clean inside I'm surprised it's open circuit. I'd swear it was brand new and unused. Just a light layer of rust on the outermost stamping and a slight discolouration to the top layers of cotton thread covering the wire. The cotton was still pure white underneath the outer layers. Sadly I can't find a break near the end of the winding so I suspect it's a rewind job or a new life as a paperweight!
N.

[Station X]

That looks like reasonably thick wire, so rewinding by hand is a possibility. You've nothing to lose.

[SurreyNick]

A task for a winter's evening in front of the telly I think
N.

[Skywave]

Originally Posted by Station X

That looks like reasonably thick wire . . .

Indeed: which makes me a trifle suspicions of the diagnoses so far.

Nick - this remark may be too late, but better late than never.
In my experience, trying to read the resistance of iron-cored inductors of a substantial inductance can often produce a high-resistance indication. Try measuring the resistance again - with a moving coil ohmmeter this time - such as AVO. If that produces a reading in the megohms range, then yes, that choke is definitely O/C.

Al.

And try measuring it now the coil is out of the iron.

[SurreyNick]

Thanks guys. Did a quick check of the coil with my DMM free of the iron and still o/c. In light of this I'm guessing it isn't worth digging my AVO out of the garage?
Nick

[Skywave]

In that case, Nick, I'd be inclined to state that the coil is O/C.
Of course, there is always the Ultimate Test: connect it to a low-voltage PSU - say 12v. d.c. (you might need a current-limiter in series) - and measure how much current flows in the coil. Zero current obviously proves an O/C: no ifs; no buts; no maybes.

Al.

[julie_m]

Or, to throw a curve ball, test the coil on a capacitance range .....

A short-circuit is equivalent to an infinite capacitance (since the voltage across it will never increase). If there is a break, the two continuous bits of winding either side of it will act as the plates of a capacitor, with the cotton or enamel insulation acting as the dielectric. So if it shows a sensible capacitance, you will know that there is definitely a break.

[SurreyNick]

I'm pretty sure it's O/C, but I will still try the last two suggestions in the morning and report back. No stone left un-turned.... Thanks.
Nick

[PJL]

If you have a piezo electric lighter, try connecting it up to the winding and seeing if you can see a flash in the dark. It's only ever worked for me once but worth a try.