[AMPS] switches

Tom Rauch W8JI@contesting.com
Sat, 10 Feb 2001 08:39:38 -0500


> Anyway, as I understand things, it's best to short the un-used turns
> out to prevent a high EMF on un-terminated taps and thus causing a
> flash-over or arc in the bandswitch.  At the same time, I figured out
> that by doing so, shorting the turns, this created a "shorted turn"
> situation and some lost energy and heating would result.  I guess it
> makes a difference which end is shorted.  Short from the output to the
> input side of the coil or from the plate end toward the output.

The side you short on makes no difference at all. Current in the coil 
is essentially equal all through the coil, as long as the coil is not 
so large it acts like a transmission line or antenna or there aren't 
"shunting paths" that have low impedances compared to the coil.

All inductors behave that way. Even loading coils.

Two things save us from "great power loss" in the shorted turn area.

First, the resistance in that shorted turn area is low compared to 
reactance.

Second, the mutual coupling from the shorted turn area to the 
active area is not perfect.

It always increases heat when we short sections of a tank coil (or 
loading coil), but the loss in power is usually insignificant. The real 
worry is heat, when a small amount of power loss is concentrated 
in a small area.

If you eliminate the impedance of the leads to the switch with a coil 
that is marginal in size for the application, you will notice the 
increased heating even when you barely see a change in output 
power because all the heat is concentrated in a small area. Take a 
lug and wedge it between the turns of a miniductor coil that is a 
form having high mutual coupling between turns. The shorted turn 
will have low impedance, and resistance will be a significant part of 
current limiting.

Mutual coupling effects make it a bad idea, in some cases, to 
progressively short a toroid. But most air-wound coils have so 
much flux leakage (and leads are so long) that shorting does not 
noticeably affect system loss. 

No matter what end you short, the effect is the same so far as 
circulating currents.



  
73, Tom W8JI
w8ji@contesting.com

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