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"amdx" wrote in message
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"amdx" wrote in message
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"Jeff Johnson" wrote in message
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"Jeff Johnson" wrote in message
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"amdx" wrote in message
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Several years ago while making inductors for tuning a class E
amplifier we
end up with a 4-1/2 turn inductor. The inductor used a 3F3 potcore,
gapped I think,
but it has been awhile. The inductor was driven hard but below
saturation.
The problem; the 1/2 turn got HOT. Four turns or five turns were
ok.
My physicist friend had an EE verify the problem 2000 miles away.

So, can anyone tell me why 1/2 turn would make such a difference in
the heating?

Thanks, MikeK



Resonance? (I assume by 1/2 you mean the 4.5 inductor?)
Yes, a 4 and 1/2 turn inductor had the 1/2 turn overheat.


huh?

The half turn and the other turns were ok? This is impossible!?!?!
One has the same current through the whole coil and if the wire was
uniform then it should heat heally well. Not only that copper is a
good heat conductor so if the 1/2 turn was heating up then he heat
should spread pretty quickly.

This assumes everything else is uniform along the coil. Something has
to be going on that your not telling us? Ideally the heat should be
uniformly distributed along the coil.

By "HOT" I assume you mean much much hotter than the other coils?

Heat is generated by the current, is it not? and the current should be
uniform throughout the wire? The resistance of the wire itself should
also be uniform. This suggests that the heat dissipated per unit
length is independent of position.

Were both ends hot? If not then something else is going on. Because
you are saying the .5 end of a 4.5 coil got HOT. Yet which end? the
4.5 coil has two .5 ends and should in theory be symmetric and hence
both get equally HOT. If they wern't then something is aloof.

I got some confirmation from my collaborator, The 1/2 turn as were
describing it
was the LAST 1/2 turn wound on the bobbin.
MikeK

What you describing is,

L1 V L2
V1--/\/\--+--/--0

on the same core?

L2, the 1/2 turn part of the coil is behaving wildly different than
expect?

After all, ideally L2 has resistance in direct proportion to the total
winding which in this case 0.5*/4.5 ~ 11% of the total resistance of the
coil.

In a single coil the power dissipation of the 1/2 turn would also be ~
11% and this is quite easy to see. You are saying that it is much more
than this as if the ratio's were turned up side down. Something like the
1/2 part dissipating 90% and the 4 turns part dissipating 10%?

if you agree with the layout of

L1 V L2
V1--/\/\--+--/--0

Then it is easy to see that in ideal circumstances the current through
L1 should equal that of L2 and the I^2R heating would produce the
results I described first.

Instead of assuming the two inductors are on the same core we can assume
they are on different cores. This will help us understand if differences
in flux could cause such problems.

You mentioned in another post that you were using AC to drive the coil?
Did the effect happen with DC?

Never tried it with dc.

Did you measure the voltage on the 1/2 turn?

No.

Was the core itself generating the heat or was it due to the 1/2 turn?

It's been ten years, all I recall is the insulation on the last 1/2 turn
of the coil
got charred.

What was the total current through the coil?


I think about a litle over 2 amps at 600khz.

Was it possible the coil could have been shorting out on another turn or
the core?

No

Was it really 1/2 a turn getting hot or "just the end of it"?

It was the 1/2 turn.

Was the coil tested by itself outside the circuit? If so did it exhibit
the same phenomena?

Not by us, but someone else confirmed our observation.

What was the end of the coil that generated the heat connected to? Was
the connection itself possible cause for the heat?

Naw.

Was the coil reversed in the circuit? If so did it exhibit the same
problem with the ends switched?

No

You mentioned that you tried 4 turns and 5 turns and the problem went
away. Did it go away completely or partially? Was those two coils made
exactly the same way and connected the same or were they in any way
different besides just the turn difference?

We shipped the product, as far as I know the didn't it back for
replacement smoke.

Was the coil one continuous piece? the 1/2 turn was not spliced on?

One continuous piece.

Did you try a 5.5 and/or 6 turn coil?
No


Did the 1/2 turn end get hot very quick or did it take a while?
Basically seconds or minutes?

Minutes.

Did you try to change the direction of the core relative to the coil? If
so did it make any difference?

No

Is there any possible way the core itself could have been generating the
heat at the 1/2 turn and the effect you experienced was just the core
heating up that 1/2 first?

I don't think so.

Did you allow the inductor to run a long time? If so, What was the
effect still? Was the whole coil hot or still just the 1/2 turn?

The whole think would have got hot, we were pushing limits.

Hopefully you can answer some of these questions. It sounds to me like
you didn't do much troubleshooting so I expect most of them can't be
answered so the true reason probably will not be known.

Probably, won't know the answer, once we learned the 1/2 turn overheated
we didn't do it again.
But it stuck as a curiosity. I had one of the overheated 4-1/2 turn
bobbins hanging on a cord over my
bench for years. It's been ten years but my old bench is still the same,
next time I stop in I'll see if it
the bobbins is still hanging.



I don't know what to say. Just not enough information. If your looking
from some exotic possibility I don't think you'll find one.

The reason is that the current through the wire will almost be surely the
same through out all points along the wire. That precisely means that the
heat generated per unit length will be the same. Even if you have some
crazy resonance or core flux/transformer effect going on, that current in
the wire has to be the same through the whole wire(assuming no shorts).

You can't have electrons bunch up on part of the wire and not on any other
part. Even if you could, that bunch would have to flow through the wire
from one part to another and on average it would heat it up uniformly.

The reason is simple, We all know that for a power supply when one
electron is supplied on the wire one electron leaves it so there really is
no way to "bunch" it up.

Now, if the wire is not uniform in size or resistance then the heating
effect is not uniform. As smith mentioned about the joint connecting the
coil. In this case it is possible that the joint could be heating up and
it was being passed along to the 1/2 turn and it just happens that you
didn't let it run long enough to heat up any more turns.

The 1/2 turn was really a point at the joint and the heat just spread
about about 1/2 turn.

To me this is the most plausable case. I'm not sure if it's physically
possible any other way that involves the coil itself generating generating
the heat. (I gave a good reason about about electron flow being uniform in
the wire)

It may be possible using some type of standing wave or transformer like
effect but even here I can't see it possible if the wire's restance per
unit length is constant.

I'm sure it was not a bad connection.
I know all about heat caused by I^2 x R at connections.
In fact just yesterday I was checking for bad connections on my electric
gokart and I burned my finger! I found a loose connection between a
5/16" post and a ring terminal connected to a 6 gauge wire.
It was some oddity about the 1/2 turn in a potcore. I suspect the low
turns count is also important to the phenomena.
I thought I'd ask and see if anybody else ever had the problem.
Closest I got was when John Larkin said,
"I've seen it do strange stuff, too."
John, would you care to elaborate?
MikeK