Hi Fred et al,

In a recent refurb for a friend, I found a Rola Choke (550ohms, 12H from memory) 'open' circuit. Extracted a second identical Choke from a scavenged chassis .... 'open' circuit also. It was time to read Fred's article in Special Projects about winding transformers ....

I am only winding a Choke, but much of the advice was 'spot-on'. I used 7.5thou wire instead of 5thou (including insulation), so the number of windings, DC resistance and inductance were expected to be lower. No problems, the thicker wire will carry a higher current. I didn't count the windings as I needed to get as many on the bobbin as possible. Unfortunately, the 'bobbin' was a rectangular shaped, cardboard tube ... no edges. So I had to add edges to support the windings.

Now my effort is not a work of art ... no Lathe, just a hand-drill held in a vice and a roll of wire on a rod ... drill speed and winding positioning done by hand ... However, the result is good ... 150ohms DC resistance, 3.0H inductance ... okay for other projects.

Now for my question ... the second Choke has a thin layer of black, brittle 'stuff' between the E's, along one edge. It broke up and spread the E's as I removed them from the bobbin. At a guess, it might be an insulating coating on the E's which has become brittle with heat. It will need to be removed to be able to get the E's back through the bobbin.

What do you suggest I do with this?? Thoroughly clean and re-coat one side of each E?? Or only remove the material which has flaked off??

Hi Ian, congratulations, you are now a fully certified coil winder!

The subject of laminations is interesting.
The laminations are usually "insulated" from each other primarily by the oxide coating on the surface from the final annealing process.
In the winding shop we would get lamination by the ton weight and use them straight from the box with no additional surface preparation.
The sheet steel going into the suppliers' punching press was not bright looking but dull grey.
The laminations produced were not cleaned/polished in any way.
One thing to note is the punched lamination invariably had a sharp edge on one face and the suppliers like Sankey usually boxed them with the edges facing one way. When you were stacking Rotor, Stator, Field Pole or Transformer assemblies for a lot of reasons you followed suit.
Most radio power trannies I pull apart do that as well.
So when you belt a transformer apart store the lams facing the way they came off.

The coating on lams is another subject.
As above most lams have just the heat treat oxide and air gap to provide the magnetic "insulation".
However, I have pulled apart trannies that used BRIGHT sheet steel stampings and either COATED one side with varnish or drenched the lot in OIL hoping that and the natural air space will do the job.
I noted the last couple of chinesium trannys I pulled apart were like that, no trouble dismantling as they were oiled!
They made humming noises and were not nice trannys.

Any way , with your old choke without actually seeing the lamination I suggest just brushing off any loose crap and restacking with the lams facing the same way as they came apart.
That is exactly what I have done in quite a few of the radio re-winds, The old bits of laquer/crap, oxide, rust and air gap will insulate the sides well enough.
NEVER EVER sand, polish, scrape or oil the lams!

Large industrial power transformers are a different game entirely and may have deliberately coated or insulated sheets of iron and are not part of this conversation.

Hope this helps.

Cheers, Fred.

Hi Fred,

Thank you ... I will follow that advice.

As to "a fully certified coil winder" ... my wife and friends agree with the "certified" part of that description ...

I think that stuff that crumbled off might be the gap spacer.

When you pass DC through a choke the core magnetises. If the current is too large the core saturates and it ceases to function as a choke.

Saturation needs to be avoided. This is usually achieved by adding an "air gap" in the magnetic path. (The "air" is usually a paper or plastic sheet.)

So, filter chokes and single-ended (not push-pull) output transformers are stacked with no interleaving, i.e. with the "I"s and "E"s stacked separately. A layer of poly insulating tape can function as the air gap and is fitted between the I and the E stack.

A thicker gap reduces the inductance but also increases the DC current that can be passed without saturation.

Hi Ian,

Timely post ... my initial re-assembly of first Choke yielded only 1.5H of inductance. Close inspection showed that the Bobbin wasn't seated correctly and there was a larger than appropriate gap between the stacked E's and separately stacked I's. After proper assembly, the Choke measured 3.0H.

Is there a reasonably simple test that a hobbyist can perform to check whether a Choke is saturating??

I had a close look at the second Choke yesterday. The crumbly stuff seems to me like something was spilled onto the 'long side' of the E's and penetrated them for about a cm or two. It does not cover the whole E or any of the I's. The air gap material between the E's and I's was intact. some of this crumbly stuff simply fell off, some rubbed off between my fingers, some came off when scrapped gently with a finger nail. Maybe the radio was stored upside-down and something spilled on it ..... ???

The crumbly stuff seems to me like something was spilled onto the 'long side' of the E's and penetrated them for about a cm or two

If it's not original lacquer from the factory, then maybe someone's DIY attempt to stop buzzing?

I'd expect it was factory applied varnish of some kind. Applied once the E laminations had been fitted to the bobbin, but before the stack of I's and any clamp was applied, with the aim of suppressing audible noise. Perhaps there was a pre-formed aluminium clamp press-fitted to the core stack.

The Rola chokes like the 14/60 have a DCR of around 540 ohm, and the one or two I've checked with open-circuits had the fault buried below the top layer or two of windings, and so were likely caused by heat dissipation from a failed filter cap or output audio stage. There is a photo of a 14/60 in the link below, along with a relatively simple way to test chokes with both AC and DC current.

https://www.dalmura.com.au/static/Choke%20measurement.pdf

Hi all,

Thanks for the comments ...

My 3 Chokes were all the 14/60 as per Trobbins Post ... one had several breaks in windings, somewhere near the centre ... the other break was towards the inner windings .... cause unknown ... third Choke was okay and is now in a working old radio ...

The article posted by Trobbins had me scurrying back to review my knowledge (that is code for "lack of") of inductors ... a good read ... thanks. I will test one of my re-wound Chokes if my variable ac supply is up to it.

Hi Trobbins et al,

Got to testing my rewound Chokes as per article in Post #7 above, posted by Trobbins.

I will admit to not understanding all the approximations included in that article. The test setup is relatively easy if one has an appropriate, variable AC supply .... which unfortunately I don't. I do have a supply which gives me 5 x AC voltages, in the range required ... just!!

Both Chokes tested about the same which wasn't unexpected given the same wire, cores, 'agricultural' winding method, and approx same number of turns. The max inductance measured was slightly higher than given by my LC Tester .... 3.5H and 3.3H, compared with 3.0H and 3.0H.

Well done Ian - rewinding is quite a chore.

The original 12/60 choke had about a 2W dissipation limit. From the spreadsheet measurements you emailed me, a similar 2W capability seems apparent, so the thicker wire and fewer turns has resulted in a choke with about 3.5H at up to about 100-110 mAdc at 10Vac. Do you think the gap thickness was forced/aligned to be about the same?

As you found, using a smallish sense resistor of 12.2 ohm means you need a good voltmeter that can accurately resolve down to at least 10mV ac and dc, and in your case that would have been the main accuracy limitation. There is no disadvantage to using a higher value sense resistor, as it doesn't affect accuracy, just the applied voltage to the test choke.

I use my Paton VCT-2 valve tester for the Vac supply, as it is so simple to use and step through a large range of heater voltages from 1.4 to 117V that have sufficient current rating. In fact that is the main use I have for that valve tester!

Ciao, Tim

Hi Tim,

... Do you think the gap thickness was forced/aligned to be about the same?

Not sure if I understand what you are getting at here ... however, the Gap (between the stack of E's and stack of I's) was controlled by a piece of paper which I was able to keep intact ... so I am fairly confident that the Gap for both Chokes is the same. It did have a large impact on the inductance.

... re Paton VCT

Yes, I have one that I fully restored ... replaced crumbling wires etc etc etc .... but it has been sitting on my shelf mocking me for about 2 years as it has a cracked wafer switch, which has filled with some cleaning junk and grey stuff ... and shorting out. I now have a box of miscellaneous wafer switches, so I should be able to build up a replacement. Another "project" ...


GTC,
How do you change your "cut and pastes" to italics?? ... I don't see any way to change the Font in VR.

Tinkera -- this site responds to "BBcode" https://en.wikipedia.org/wiki/BBCode

Thank you GTC

Oh ... it works

Now I am showing off

I do like the genius behind the wiring in mine. It has the V1 rectifier & 6V Vibrator PSU. Why it was wired with pretty much 100% purple wire defies logic. It also has a meter movement which has screws that can fall out & short onto its retaining bracket, which it did.

I did get given a Knight 600 series with paperwork & stepdown transformer. I did score an original book for the VCT, also some plastic tube for the ohm meter batteries.

There are variants in these & I did take photos of another that came in for repair. It used half of a 6X5 in place of V1 albeit the characteristics of a single element in the two are similar.