[sdiy] isolating variac unit

[Roy J. Tellason]

On Friday 02 November 2007 20:02, anthony wrote:
> Hello, All
>
> I am updating a handy Variac box I made, putting a Powerstat in a pretty
> good sized enclosure, with fuse and back and three-prong grounded jack and
> toggle switch on front.
>
> The first update is to add an isoltation transormer. I did not know at the
> time that Variacs do not isolate the circuit they are powering from the
> powerline. There's no room inside for a 1:1, so I'm using 2 transformers in
> a step-down-step-up configuration. My question is, should I just use non
> center-tapped transformers, or would it be better to use center-tapped
> transformers and ground the tap to the chassis along with the grounds from
> the plug and outlet.

I don't see where center taps get you anything...

FWIW,  my setup has the isolation transformer and the variac box as two 
separate items.  They can,  of course,  both be used when warranted,  but 
that's not too often the case.

> Second update is an AC voltage meter. Does it really matter for Variac-type
> uses to have a true RMS meter or could I just rectify and filter it and
> show that voltage.

I don't think metering matters much here.  Mine has a dial below the knob that 
goes zero to 100%,   and that's usually good enough for what I want to do 
with it.

> Thirdly, I want a neon bulb for an "ON" indicator, but those always end up
> flickering or not working. Why does this happen and how can I prevent it?

They do seem to do that,  sometimes.   Mine aren't flickering too badly at 
all,  mostly, but that also depends on ambient light levels.

I have a few more goodies stuffed in the box:

1.  A current-limiting light bulb in series with the load.  I actually used 
one of those DPDT-Center-Off toggle switches for this,  and have a pair of 
binding posts on the panel that also connect to the bulb points.  In one 
position the switch shorts out the bulb,  labeled "BYPASS",  in the other it 
has the bulb in circuit,  labeled "LIMIT",  and in the center-off position 
only the binding posts decide what's connected,  labeled "METER" -- I can run 
a dual-banana jumper cable from the unit to a meter and have a continuous 
reading of what current the device under test is drawing.  A neon indicator 
here tells me I blew the fuse in the meter.  :-)

2.  Another toggle switch that selects the "normal" output or the "boosted" 
output of the variac -- this gets me 0-120V or 0-135V,  for stress testing 
stuff.  :-)

3.  A third toggle switch that performs the on-off function.  This is a 
double-pole switch,  so that when it's off _both_ sides of the power line are 
disconnected from the DUT,  for safety reasons.  And it saves wear and tear 
on the outlet from repeatedly having to unplug something.  A neon indicator 
here tells me power is on.

It's not a very large variac,  only 2.5 or 3A output,  so I managed to stuff 
all of this into a 5x7x3 metal chassis box.

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin