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Strange inductance readings

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LAN_403

Xray

Sat Mar 03 2012, 07:15PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Hey guys -- I pulled a ferrite transformer out of dead ultrasonic knife power supply, which I plan to re-use in a different circuit design. I used my digital LCR meter (Tonghui model TH2821B) to measure the windings, and I got some confusing inductance values. Refer to my picture of the transformer below (I added pin numbers for reference). There are two rows of pins. In one row, there is only one winding that goes to pin #5 and #6 in the picture. In the other row, there is a multi-tapped winding that goes to pins #1 through #4. I do not know which row is considered the primary, and which is considered the secondary, but it doesn't matter for the purpose of this post.

All inductance readings taken with meter set to 1KHz:
Between pins #1 and #2 I measure 15mH.
Between pins #2 and #3 I meause 195 uH
Between pins #3 and #4 I measure 195 uH

So far, so good. But here's the strange part. When I meause the inductances across multiple taps, the inductance values do not make sense. I know that if you double the number of turns in an inductor, its inductance will increase by four times (am I right?). But in this case, the inductance values are MUCH higher than what seems reasonable. Can someone please explain why I'm getting these strange readings?

Between pins #1 and #2 I measure 15 mH (same as previous reading shown above).
Between pins #1 and #3 I measure 20 mH (this doesn't make sense!)
Between pins #1 and #4 I measure 24 mH (ditto!)

On the other row (pins #5 and #6) I measure 20 mH. (there is no connection to any other pins)

Thanks!

Ash Small

Sat Mar 03 2012, 07:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The only thing I can think of is that the series windings may be wound in opposing directions, which would lead to some unexpected readings, but this is only a wild guess.

Xray

Sat Mar 03 2012, 09:20PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Ash Small wrote ...

The only thing I can think of is that the series windings may be wound in opposing directions, which would lead to some unexpected readings, but this is only a wild guess.

Ash -- I suppose that's possible, and I could confirm that by applying a signal across two of the pins, and looking at the phase relationship between the various windings. If the signal is 180 degrees out of phase between any two windings, then that would confirm your suspicion. I'll try that later tonight if/when I have time. Thanks for your reply!

Experimentonomen

Sat Mar 03 2012, 10:49PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Could be a centertappd winding + a feedback winding

Reverse engineer the rest of the circuit around the transformer.

Xray

Sat Mar 03 2012, 10:59PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Experimentonomen wrote ...

Could be a centertappd winding + a feedback winding

Reverse engineer the rest of the circuit around the transformer.

I already know that it's a multi-tapped winding. I began to reverse-engineer the circuit, but it was taking way to much time so I gave up on it. I can tell, in general, that the ultrasonic knife circuit is basically a "Royer Oscillator" that uses a pair of NPN transistors (not mosfets) to drive the hand-held knife. It operates at approximately 40KHz and puts out 30 Watts of ultrasonic power. Rather than try to repair it, because there are too many burnt components, I'm going to use a basic ZVS circuit using a pair of high power mosfets.

Proud Mary

Sun Mar 04 2012, 12:50AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Inductance meter readings tend to be more dependent on the test frequency than we might hope for - and if only one test frequency is used, the readings can sometimes be very wrong indeed.

Your transformer is not pure L after all, but will also be home to a good deal of parasitic capacitance between layers and windings and so forth. The windings also have resistance, so there you have the famous triad of LCR whose impedance will vary and might even resonate according to the test frequency applied.

The permeability of the core will also vary according to the drive level and the applied frequency.

And don't forget that a high turns ratio will multiply error effects - including in an auto transformer where the effective primary and secondary are in series.

These are some explanations for anomalous inductance meter readings. In industry, inductance is often given with reference to a standard measuring device, such as the HP4342A Q meter for example, to create at least one fixed point in this ocean of variables that refuse to be pinned down.

Xray

Sun Mar 04 2012, 01:19AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Proud Mary wrote ...

Inductance meter readings tend to be more dependent on the test frequency than we might hope for - and if only one test frequency is used, the readings can sometimes be very wrong indeed.

Your transformer is not pure L after all, but will also be home to a good deal of parasitic capacitance between layers and windings and so forth. The windings also have resistance, so there you have the famous triad of LCR whose impedance will vary and might even resonate according to the test frequency applied.

The permeability of the core will also vary according to the drive level and the applied frequency.

And don't forget that a high turns ratio will multiply error effects - including in an auto transformer where the effective primary and secondary are in series.

This are some explanations for anomalous inductance meter readings. In industry, inductance is often given with reference to a standard measuring device, such as the HP4342A Q meter, to create a fixed point in this ocean of variables that refuse to be pinned down.

Some good points, Proud Mary! Besides the fairly new digital LCR meter, I also own an old GenRad analog LCR bridge that I'll run the same tests with. It requires a number of size D batteries to power it, which I'll have to purchase, and then I'll see if the analog bridge shows me different results from the digital meter. I'll let you guys know how that turns out.

Thanks.

Xray

Sun Mar 04 2012, 02:18AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

UPDATE: I ran the same inductance checks with my old GenRad "Impedance Bridge" and I got nearly identical results as I did with my digital LCR meter! There was only a slight difference of maybe 1 percent, which is probably due to the GenRad not being calibrated for many years. So, I confirmed that the strange readings were not due to some funky artifact of the meter.

I guess at this point I'm not going to worry about it because I may not use this transformer anyways. It was just the strange readings that I wanted to get resolved. I'm now starting to think that the basic problem is due to all the variables that Proud Mary had mentioned, and I may just be chasing my tail if I continue to pursue a simple, concrete answer when there is none!

Thanks to all who contributed!

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