To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
From: N1BUG <paul@n1bug.com>
Message-ID: <e581319f-a807-0507-427d-6d4f8f595f66@n1bug.com>
Date: Thu, 8 Mar 2018 10:00:53 -0500
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Subject: LF: A different inductor heating question
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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