Variac Modification for 140 Volt Use

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Yurtle Turtle by way of Terry Fritz <twftesla-at-qwest-dot-net>" <yurtle_t-at-yahoo-dot-com>

I had three, 120 volt variacs, none of which output
more than 120 volts. Two of them had only three
terminals, while the third variac had four. This
variac had a tap halfway through the windings to allow
60 volts in or out.

Whenever I wanted a quick and dirty 140 volts, I'd
simply hook one of my three terminal variacs up
"backwards", as seen below:

line        load

N-----)-------N
      )
      )
      )
H---->)
      )
      )-------H


At 120 volts in, this setup yields a minimum of 120
volts out, up to ???. Although I had to be careful, as
turning knob the wrong direction could quickly result
in disaster, I never had any problems.

Since I ran like this from time to time with no
problems, I decided to make a more fail-safe permanent
modification to the four terminal variac. I simply
counted the number of turns, did some quick math to
determine volts per turn, located the tap point which
would give me 140 volts out, confirmed this point with
my DMM under load, then stripped the winding and
soldered a wire in place. I already had a spare
terminal, which made it easy. I simply abandoned the
60 volt tap point.

After I had counted turns and confirmed the location
with my DMM, I noticed that the turn I was going to
strip and tap, already had some sort of insulating
material around it, identical to the existing tap in
the middle. It appears that this variac was in fact a
120/140 volt variac, which was only wired for 120 volt
use. Mods can be seen at:

http://www.geocities-dot-com/yurtle_t/tesla/controller.jpg

If you can't access it, try again later. Geocities
get's uptight and shuts you down when you get too much
traffic.

The moral of the story is that if you think you only
have a 120 volt variac, you may in fact have a 140
volt one, that was simply not equipped with the proper
tap for 140 use. Also, I pushed my two 120 volt
variacs up to 140 volts with no problem. I'm sure I
could have made a similar modification to make them
140 volt out.

Adam

=====
Adam Minchey
yurtle_t-at-yahoo-dot-com
http://www.geocities-dot-com/yurtle_t/index.htm