Reverse engineering a coil

[ricko_uk]

Hi,
I need to reverse engineer a potted coil to replicate it but cannot remove the potting from the original as it would void the warranty.

The coil:
- is self supported i.e. an air core
- the dimensions of the potted unit are: 24.5 ID, 57.7 OD and 23.7 height/length
- the potting is just over the coil, so I can assume the actual coil is 2mm smaller in every direction (just a rough assumption)
- I can measure the resistance
- the inductance 40mH at 1KHz and 42mH at 10KHz
- DCR is 25.8R
- I could try to measure the magnetic field using the classic Trifield meter (but the sensor might be too far) or a TD8620 Field Meter (but not sure it is very reliable after running some other tests)

Is it possible to find out the wire diameter and number of turns, at least to some approximation?
If yes, how?
If no, what is the best way to get some approximation so I can replicate as close as possible?

Thank you

[Jay_Diddy_B]

Hi,

You can try a coil calculator like this one:

Link: https://www.circuits.dk/calculator_multi_layer_aircore.htm

Putting in some guesses:




This is reasonably close.

Jay_Diddy_B

[ricko_uk]

Thank you Jay,
I saw similar ones but the missing parameter I have is the wire diameter. That's why I was wondering if there are other ways/calculations to figure it out.

Thank you

[daqq]

The calculation Jay provided should be enough if you play around with the values.

As to other means: Well, if you have access to an X-ray, you could scan it? Doesn't need to be industrial, I think I read somewhere that they managed to convince a local vet/dentist to take a few shots of some electronics and it was useful enough.

[TimFox]

Thank you Jay,
I saw similar ones but the missing parameter I have is the wire diameter. That's why I was wondering if there are other ways/calculations to figure it out.

Thank you

Using the drawing he posted, and the dimensions you measured, there should be a unique solution to the wire diameter that simultaneously agrees with the DC resistance and the calculated inductance:  playing around with this calculator should get you to the inductance.

[Jay_Diddy_B]

Using the drawing he posted, and the dimensions you measured, there should be a unique solution to the wire diameter that simultaneously agrees with the DC resistance and the calculated inductance:  playing around with this calculator should get you to the inductance.

I am not sure how close the OP will get to a unique solution.

The wire gauge could be:

• awg
• swg
• metric

Then there is winding tension or the way that the wire fits on the former.

It depends on the application, but using the calculator, or a similar calculator they should be able to make a coil that works.

Jay_Diddy_B

[Conrad Hoffman]

Armchair thoughts- can you add a winding so you can measure it like a transformer and get a turns ratio and thus the turns of the coil? Once you have the turns you can get the wire gage by resistance.

[TimFox]

My point was that matching the inductance and resistance to the measured values, since the two parameters don't scale the same way with the dimensions shown in the figure, gives a "unique solution" in the mathematical sense of the phrase.  Iterating through the calculator application should result in a solution, similar to that posted, limited by the accuracy of the inductance algorithm (the resistance algorithm should be reasonably accurate).
Once you have that solution, you can translate it into practical terms, including your preferred wire gauge.

[james_s]

An xray of the coil ought to give you a good idea of the internal details. It can be hard to tell the precise details of the windings but you should be able to get a rough idea.

[Ian.M]

The dimensions seem practical to allow a core to be added (e.g. a ferrite one salvaged from an old CRT TV line output transformer) so a turns count could be determined by voltage ratio measurement using a 100 turn test secondary wound on the outside of it, and a signal generator + audio amp to drive it at a high enough frequency to avoid saturation.

With a turns count one can estimate the length of the winding wire from the dimensions of the coil, (or at least set upper and lower bounds on it) and with the resistance, set upper and lower bounds on the wire gauge.

That should give you a much smaller solution space to search through by TimFox/Jay_Diddy_B's method.

[bob91343]

I would take a hands on approach and wind a few coils with different size wire and see which comes closest.  You have the overall dimensions so you shouldn't need to wind more than two or three coils.

[geggi1]

If you manage to find a coil calculator that can calculate the length of the wire in the coil you can estimate the length of wire by measuring the resistance. A good pinpoint for the size of wire is also the current handling capacity.
If you also can find some information about maximum watts generated in the coil you will get very close.

[ChristofferB]

Knowing what its use is in the circuit would be helpful to gauge the probable inductance ranges and how critical the value is.

A choke could probably be out by an order of magnitude and still sorta do its job. An RF frontend filter probably could not

[ricko_uk]

Thank you all, much appreciated suggestions!!