Re: 577D1 itching issue - Haven't touched since, currently in storage


Chuck, Bruce, and Arden,
All my life I assumed there was little more to learn from how a Variac works. They appear to be pretty simple and a very clever. This has accounted for their incredibly useful life spanning more than 85 years.

The practical side of me says it is a very bad idea to short out a turn on a transformer with 120 (more or less) turns and 120V across it. So tomorrow I will test this by intentionally hooking a short piece of wire from one turn to its neighbor while I am holding the wire in my hand. If you are right the wire won't get hot and I will learn something new. If I am right I will get burned and never trust the three of you again. Either way you have nothing to lose. :)

I will report the results either way.

Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of KB6NAX
Sent: Wednesday, January 01, 2020 9:35 PM
Subject: Re: [TekScopes] 577D1 itching issue - Haven't touched since, currently in storage

Hi Dennis,

I made my donation so you have to read this (just kidding :-).

This raises another observation about the brush. It must be narrow so
it doesn't span two turns at a time else it will be shorting out one
turn to the next turn and that will short them together. They would
heat up as would the brush which is causing the short. In my example
this would cause a short across two turns that differ by exactly 1VAC.
But each winding is capable of providing from 1 to 10 Amps under
normal load situations for the size Variacs we are likely to
encounter. With a brush causing a shorted winding it isn't hard to see
that more than10 Amps could flow. To minimize the likelihood of a
short the brush comes to a chisel point. The point is slightly
narrower than one turn of wire and it is as wide as the removed enamel
area of each turn. <
I studied this on variacs and came to a different conclusion. The maximum power transfer theorem says that the source and load impedances have to be equal for maximum power to be transferred. A shorted turn is far from a matching impedance for the "primary" side, the rest of the variac winding.
In order for a large amount of power to be coupled into a shorted turn the resistance of the wire would have to be much lower than what copper provides in order for the reflected impedance to be equal to the source impedance.
Think Weller soldering gun. In addition, the brush, overlapping the pair of contact points is a resistor. Intentionally, so it throws the impedance mismatch further off. The result is a very lossy mismatched transformer action, little power is transferred. Also, the brush is large enough to dissipate its heat, most of which is caused by the output load current. The losses are so small you don't realize they are there.


Dennis Tillman W7PF
TekScopes Moderator

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