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From: N1BUG <paul@n1bug.com>
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Date: Fri, 9 Mar 2018 08:29:48 -0500
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Subject: Re: LF: A different inductor heating question
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Thanks Chris,

I think the wire NOT wound on the toroid is warming also, but it is 
difficult to know. The leads are very short and soldered to large 
pads on the circuit board which tend to act as a heat sink. I don't 
have a thermocouple and I can only just get the tip of a finger onto 
those short leads.

Barring someone explaining otherwise, my assumption remains that 
core heating must heat the whole core, including any portion which 
does not have windings around it. But, I've been wrong before... ;-)

Paul


On 03/08/2018 12:37 PM, Chris Wilson wrote:
> 
> 
> Hello N1BUG,
> 
> 
> Paul, I would have thought what wire was NOT wound on the toroid would
> also  get  hot  if  the  current  carrying  capacity  of  the wire was
> inadequate.  Is  that  the case? Have you got a thermocouple to put on
> the external wire?   Just my guess, I have my asbestos pants ready....
> :)
> 
> Thursday, March 8, 2018, 3:00:53 PM, you wrote:
> 
>> I have a question about iron powder toroidal inductor heating...
> 
>> I built an experimental PA for 137 kHz. Details can be seen here:
> 
>> http://blog.n1bug.com/
> 
>> For the three inductors I've used T157-2 cores with 20 AWG wire.
>> This PA works well enough. With a 34N20 FET I have been running 150W
>> output mixed WSPR-15 and WSPR-2 for more than one week already. Last
>> night I increased power to 175W and all was still OK. The output
>> looks as clean as expected on a spectrum analyzer.
> 
>> Two of the three inductors heat up (L1 and L2). I have the wire
>> wound over about 80% of the core so there is 20% core with no
>> winding. If I transmit for 2 minutes, I can feel no heat touching
>> the open 20% of the core. If I touch the wire it is quite warm, with
>> L2 being more warm than L1. My first assumption was that the wire
>> must be heating because the empty part of the core does not feel
>> warm. I do not know what the peak or average current in these
>> inductors is, but I am having some trouble believing it can be high
>> enough to cause the wire to heat to this extent.
> 
>> What do you think? Is my theory that this must be wire heating
>> valid? I do not know the thermal properties of the core material but
>> I thought if this is core heating the entire core should get warm.
>> Is this a wrong assumption?
> 
>> I do not plan to rebuild the PA as it works OK already and it is
>> temporary. But I would like to better understand this.
> 
>> 73,
>> Paul N1BUG