Re: 577 AC collector supply not reading zero on display

Chuck Harris

The first turn on a variac, and the last turn on a variac are both
very vulnerable to damage... physical and electrical.

When properly adjusted, the brush should fully cover (and connect to)
the first through about the 5th turn, and about the 5th to last through
the last when at full scale.

The brush is highly resistive to avoid shorting out the transformer
when it straddles turns. If you were to sharpen the brush to a point,
so that it covered only a single turn, it would burn right through
that turn when operated anywhere near its full current rating.

A pointed carbon brush, pulling very little current will heat to
incandescence long before the skinny little copper wires used in this
variac get to their rated current.

[I used to engrave tools using a pointy carbon rod and a 2V transformer.]

The variac's design requires that the carbon brush be covering 5 or 6
turns, and resistively combining each turn's contribution, to achieve
its full current rating.

This is also required to give the variac seamlessly smooth changes in
output voltage as the voltage control knob is adjusted.

Historically, the variac was invented as a replacement for huge highly
lossy carbon pile rheostats that were used in stage lighting. It was
a real boon, as a variac could be very small, and not get hot... unlike
the rheostats that were being used.

It allowed the light's brightness to be changed smoothly, without jumps.

The variac wasn't meant to go all the way down to zero, only all the
way down to where the flood light is dark.. a couple of volts here or
there didn't matter.

The Tek curve tracer takes this imperfect lamp dimmer device, and
multiplies its output voltages by the step up/down transformer to create
a collector supply that ranges from 0-6.5V all the way up to 0-1500V.

Tektronix never expected that you would use the 50mv per division scale
on the 1500V range. They expected that if you wanted to measure such
a small range of voltages, you would use the 6.5V range.

The only reason you can even see this flaw in your variac is because
it was too expensive to build interlocks to prevent every possible
undesirable setting. They instead concentrated on some of the unsafe

Are you sure that what you are asking your 577 to do is reasonable?

I would hate to see you do any more damage to your curve tracer.

-Chuck Harris

DW wrote:

I should note the feedback here has been noted and considered so far here.

I have avoided shaping the brush to a sharp point as I was observing how the other variac had a point but it was blunt on the end so I worked to mimic that, somewhat by slightly giving a flat roll off on on just the two ends.

Vertical lines represent contact area to windings.
---||||----------- <Winding
| |
|_ _ | < Brush

| |
| | <Top view of contact area of brush
| | winding aligns perpendicular to this

These adjustments I am making are only brief and temporary, I am just experimenting to see how I can get the collector supply to 0 volts.

Obviously as noted I want to make absolutely sure the carbon brush is making a wide enough contact patch to prolong the life of the variac for the long term.

Also the brush was running off the winding on to a metalic platform. I observed the control slightly sticks as the brush has to overcome the lip of the first winding, as Chuck noted and as I obviously noticed, I don't want this to occur to avoid premature failure. I will readjust approximately.

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