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?
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.
Half a circle, as used at say 450 MHz and up (the one end is soldered to one point on a circuit board, goes straight up say 1/4", bends over 180 degrees and the other end soldered to another pad on the PC board...just like this "U" but flipped over (upsidedown U)...
This kind of odd-ball result can be caused by "parasitics" - in other words, a high overtone excited by just the right length of inductor paired with just the right stray capacitance, and energized by something that can hit the high notes....
Maybe, but we had it confirmed by someone else and I doubt very much they developed the circuit we were using to do the test. We used it to cancel out the capacitance of a piezo transducer. Driving about 250 watts into a
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?
Did you measure the voltage on the 1/2 turn?
Was the core itself generating the heat or was it due to the 1/2 turn?
What was the total current through the coil?
Was it possible the coil could have been shorting out on another turn or the core?
Was it really 1/2 a turn getting hot or "just the end of it"?
Was the coil tested by itself outside the circuit? If so did it exhibit the same phenomena?
What was the end of the coil that generated the heat connected to? Was the connection itself possible cause for the heat?
Was the coil reversed in the circuit? If so did it exhibit the same problem with the ends switched?
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?
Was the coil one continuous piece? the 1/2 turn was not spliced on?
Did you try a 5.5 and/or 6 turn coil?
Did the 1/2 turn end get hot very quick or did it take a while? Basically seconds or minutes?
Did you try to change the direction of the core relative to the coil? If so did it make any difference?
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?
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?
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.
It's been ten years, all I recall is the insulation on the last 1/2 turn of the coil got charred.
I think about a litle over 2 amps at 600khz.
No
It was the 1/2 turn.
Not by us, but someone else confirmed our observation.
Naw.
No
We shipped the product, as far as I know the didn't it back for replacement smoke.
One continuous piece.
No
Minutes.
No
I don't think so.
The whole think would have got hot, we were pushing limits.
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.
There isn't a cold solder joint at the heating end of the coil? This would result in localized heating to one end of the coil ... strange is all I can say.
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