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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 11th Aug 2017, 6:40 pm   #1
McMurdo
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Default Toroidal choke failure

I've just spent a couple of days fault-finding on a class-D audio amplifier and found the fault to be the output choke gone short. The choke, wound on a small (2 coin size) ferrite toroid has rather thick ECW (18swg) and 40 turns. It's very tightly wound to get it on there, and adjacent turns have bitten through the enamel and reduced the inductance to nothing.

My question, for you coil winder experts (!) is, when I come to hand wind my replacement, is there any way of avoiding a similar failure? It's rather stiff wire going around some tight corners and over itself...this is where its shorted. It hasn't failed in any way due to heat or corrosion.

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Old 12th Aug 2017, 5:01 am   #2
Diabolical Artificer
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Default Re: Toroidal choke failure

Can you not wind some Kapton tape over the bottom layer of turns or failing that put a coat of varnish or
laquer over them. Kapton tape is quite thin and when pulled tight conforms around the windings. I got mine here - http://www.ebay.co.uk/itm/High-Tempe...A/111726072244 it is the proper stuff.

Andy.
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Old 12th Aug 2017, 6:49 am   #3
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Default Re: Toroidal choke failure

Another trick is to wind it with multiple strands of thinner wire making up the same cross sectional area of copper. This will go round tighter corners before the varnish cracks.

Is the core free from sharp bits? wrapping it in insulating material or epoxy coating can help if the core is part of the problem.

David
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Old 12th Aug 2017, 8:34 am   #4
G6Tanuki
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Default Re: Toroidal choke failure

Can you use stranded wire to rewind it (while maintaining the same CSA of the conductor)??

High-temperature PTFE-insulated panel-wire is more flexible than solid-core stuff and a lot easier to wind (the PTFE jacket is nice and slippery so you can get the coil turns tight without worrying about damaging the insulation).
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Old 12th Aug 2017, 8:52 am   #5
kalee20
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Default Re: Toroidal choke failure

Photo of the original would help!

It's unlikely to be a ferrite toroid, unless it is a common-mode choke. Iron dust or moly-permalloy powder, yes.

In the absence of a photo, is it a single-layer winding? Are start and finish turns pressed closely together? If so, maybe drop a wire gauge. It'll run slightly hotter but this may not be an issue.
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Old 12th Aug 2017, 8:20 pm   #6
Ed_Dinning
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Default Re: Toroidal choke failure

Hi Gents, I would echo Kalee. Also remember that bifilar wires and Litz take up greater space for the same copper area as the solid wire.
OP said it was a tight wind, so dropping a gauge size is probably the best option.

Ed
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Old 12th Aug 2017, 11:07 pm   #7
joebog1
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Default Re: Toroidal choke failure

Another trick is to "break the grain" in the wire!!.
To do this you need some soft material like flannelette over an edge of your bench, several thicknesses deep.
Pull the wire tightly between your hands and rub it over the edge of the bench with a steep angle.
Turn the wire over and repeat this several times. The wire will soften and make it much easier to get through the core. It still requires some tension when winding though, especially the first few turns that you should try and get as flat as possible on the toroid.
Its fiddly but arranging the wire side by side on the first layer, and then laying the second layer in the troughs of the first layer will also help.
Joe
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Old 13th Aug 2017, 3:34 am   #8
Terry_VK5TM
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Default Re: Toroidal choke failure

I would suggest caution in rewinding or making a replacement unless you have the specs of the original.

Especially using a lighter gauge wire, there can be some very high peak currents in a class D amps output stage.

Class D amps can be very fussy as to that choke.
You need to know inductance, resistance and type of core.

If it's wrong, the output transistor/s (or FET, MOSFET etc) could be toast.

Terry
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Old 13th Aug 2017, 7:51 am   #9
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Default Re: Toroidal choke failure

To the list of important parameters, I'd add self-resonances. Having one hit the switching frequency can be dramatic.

Dropping a gauge and going to 'Heavy Formvar' enamel may be the best move. It's possible that the original was made with light solder-through vanishing varnish coated wire.

David
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Old 13th Aug 2017, 11:16 am   #10
MrBungle
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Default Re: Toroidal choke failure

My father used to put the toroids in a sandwich bag and fill it with salt and shake it around for half an hour first. Takes the sharp edges off. Not sure id recommend it though!
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Old 13th Aug 2017, 1:23 pm   #11
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Default Re: Toroidal choke failure

A lot of ferrite cores are 'tumbled' like in a gemstone polisher to blunt edges.

These are likely metal powder ones, though.

David
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