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

george edmonds

Hi all

I have been following this topic for some time, why not do the simple thing and measure with a DMM the voltage between cold end of the Variac and the slider when fully counter clockwise, or am I missing a point. My UK made Variac has a zero volt pad for the brush and termination of the winding.

73 George G6HIG On Thursday, 2 January 2020, 06:14:15 GMT, Chuck Harris <cfharris@...> wrote:

On the contrary, the brush is intended to connect to multiple
windings at the same time.  To prevent the problem of shorted
turns, the brush is highly resistive

You can think of the brush as a network of multiple resistors
each connecting to a different turn, and combined to form the

Because each adjacent turn is less than a volt, or so, different
than the next, no great amount of heating occurs in the brush.

Yet another reason why a variac may not reach exactly zero

-Chuck Harris

Dennis Tillman W7PF wrote:

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.

Dennis Tillman W7PF

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