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Re: [Amps] questions on my tranformer test

Subject: Re: [Amps] questions on my tranformer test
From: "Will Matney" <>
Date: Tue, 14 Jun 2005 02:30:15 -0500
List-post: <>

Yes there is a long drawn out formula to figure temperature rise and the 
appromimate temperature it will run at. The problem is it isn't real exact as 
the construction has a lot to do with it, especially if it's potted in a hard 
block of expoxy, the tightness of the windings, etc.  What it takes into 
account is the heat rise in the wire by it's size, number of turns per layer, 
number of layers, the mean diameter of the winding in question, and the 
insulation thickness of the paper between the layers, and the way it's made. 
The formula works around an open type transformer where the heat can readily 
escape the coil by natural convection, etc., with at least one open side of the 
coil facing upwards (horizontal mounting). Without a computer program, it can 
take a few hours to figure out, or at least it does for me, heh.

The coil gets its hottest at its center or at its mean radius for each winding. 
The outside coil runs the coolest. The deeper you go in, the more layers they 
are with insulation, the hottoer it will get in the center of the inner 
winding. There's ways around this buildup of heat by using a 2, or 3 part 
bobbin and winding one coil on the top half and one on the bottom half. 
However, this raises the losses a little due to magnetic coupling where one is 
not on top of the other like standard layer winding. Each coil will still get 
the hottest in its center though or at its mean radius. Another trick is to use 
the thinnest bobbin you can use between the inner coil and the core. This 
allows more heat to be transformed into the iron and act as a heatsink. Heat is 
the major killer of any indictor whether an inductor or transformer. The hotter 
one runs, the shorter lifespan it will have. That's why I like running EI cores 
over the cut cores. I hope to put this information up on the we
 bsite I started and also in the e-book I started.

The formatting from this e-mail at I have no control of. My main 
e-mail still isn't right and using the one on the main server has not been 
sending some off even though it shows it did. I'm trying to find an alternative 
provider now.



<blockquote style="border-left:solid #1010ff 2px; margin-left:5px; 
padding-left:5px;">----- Original Message -----
From: "David Kirkby" <>
Subject: Re: [Amps] questions on my tranformer test
Date: Tue, 14 Jun 2005 07:53:17 +0100

> Will Matney wrote:
>   > The only way you can tell for sure is find the wire size used
> > in the coil. If you knew what the wire insulation was,
> > so to know its temperature rating, you could snake a  thermostat 
> > wire down in the center of the winding
>   > and run it under load to
> > see what the coils innards are heating to. Or do
>   > the hope-so test and see how hot the core and
>   > coil gets by touch which isn't accurate. If its right
>   > for the load, the core and coil should just be warm to the
> > touch, and not close to hot enough as you couldn't keep your hand 
> > on it. 180 degrees is way too hot for any to run. Something in 
> > the order of 100 degrees to maybe 110 is about maximum for one in 
> > open air.
> >
> > Best,
> >
> > Will
> Hi,
> Can you not infer the average temperature rise by measures the DC
> resistance cold and then after some time the DC resistance when hot? The
> temperature coefficient of copper will give you the mean rise. If there
> are multiple taps, you should be able to find the mean rise on different
> bits of the transformer.
> If the secondary is wound over the secondary, you could be pretty damn
> sure the primary would get hottest.
> I guess you would need a pretty decent DVM (preferably with 4-wire
> resistance measurements) to do this on, but it might be practical.
> Although I have not sat down and worked out what the change in R will
> be, and so if its easily measurable.
> PS,
> your email client is sending one long line of text, with no line breaks.
> It's common to set a line break of about 72 characters. Some mail
> readers will have problems with a very long line. Hence I reformatted my
> reply.
> --
> David Kirkby,
> Please check out
> of if you live in Essex
> _______________________________________________
> Amps mailing list

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