variac

[SPIDER89119]

I collect old radios and I am in the process of researching about how to restore/refurbish their inner workings.  I am going to start by working on a couple of AM 5 tube AC DC sets that wouldn't be much of a loss if I messed up, then I want to gradually work my way up to restoring an old boatanchor.  I would like to purchase a variac, but the prices vary a lot, and I am not sure what I need.  (How many amps should it be rated at, etc.)  Any suggestions?

[W5RKL]

A 10 amp variac would meet most, if not all, of your
restoration needs. A 10 amp variac will also provide a
margin of safety as well. Make sure it has a fuse,
3 wire grounded power cord, and an ON/OFF switch.
Meters are nice but many are not vary accurate. Mine
has a meter but I use end up using a DVM to check
the voltage. I use a non-surge suppressor power strip
plugged into the variac's single 3 prong outlet.



73
Mike
W5RKL

[SPIDER89119]

Thanks, Mike.  Now I know what to look for.

-Will

[AJ4CU]

HI,

I agree with the other poster on this, meter not very important but fused and three wire, yes!
 I have two and both are like that.
My addition to the thread is be sure to get yourself a good isolation transformer, remember the "5 tube all american" has a hot chassis and you want all the isolation you can get.

73    DE    AJ4CU

Hari

[SPIDER89119]

I thought a variac was a type of isolation transformer.  Am I wrong?

[SPIDER89119]

I just did some research online and discovered that I was wrong.  I thought the variac would vary the voltage AND act as an isolation transformer.  but apparently I have to purchase both separately and plug the isolation transformer into the variac.  Thanks for the helpful comment, Hari  

[K3HVG]

Looks like you've got the issues doped out OK..  I'll add that for 90% of the receivers I repair, a 5A Variac does the job with ease. The only thing I'd also add is that you might consider installing a fuse commensurate with the current requirement of each radio you test. I'd venture that 90% of the smaller radios will do OK with a 1A fuse, though.  Left to its own devices, a 5A or 10A Variac will provide a great deal more current than any radio will ever require.  If there's a problem with the radio, it'll smoke long before the Variac drops out.

[W5RKL]

A variac is just what the name implies, a device
that, when plugged into a wall outlet, "varies" the
amount of "AC" line voltage available at the
"output" of the variac, hence the name VARIAC
(VARIable AC).

A simple variac, the kind used in most ham shacks, is
nothing more than a large wire wound variable resistor
much like a rheostat.

A variac does not "produce" current. A variac's
current rating is the absolute "maximum" current that
can flow "through" the variac before the variac's fuse
will blow. The wire size and hardware used to
construct the variac determines the variac's fuse
rating, hence the variac's current rating.

A simple variac is not an isolation transformer.

A simple variac is not a transformer in the true
electronic sense of the word "transformer".

A simple variac does not "increase" the line
voltage the variac is plugged into. There are
some more sophisticated variacs that do but the
most common variac used in ham shacks don't.

When selecting a variac, the model chosen must have
a current rating that is higher than the highest
current rating of any equipment plugged into it.

Simple variacs provide no surge protection. Any surge
induced at the input to the variac will be seen at
the variac's output.

The ARRL handbook discusses how variac work. The
link below also provides similar information on
variacs.

http://www.nmr.mgh.harvard.edu/~reese/VariacPage/

73's
Mike
W5RKL

[K3AN]

I have never seen a large adjustable resistor called a Variac. A true Variac is an autotransformer with an continuously adjustable "tap." Except for the lack of isolation, it works like any transformer. If you set the variac slider at the halfway point to get 60 Volts AC output and your load draws 2 Amps at that setting, then the transformer will draw just 1 Amp from the 120 Volt input. Actually it will draw a bit more than 1 Amp, to cover core and resistive losses.

[K3AN]

To continue, here's a quote from the Wikipedia article on autotransformers. The article has some diagrams and photos as well.

"From 1934 to 2002, Variac was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for the Variac trademark for the same type of product. The term has acquired the status of a genericized trademark and is often used to describe similar variable autotransformers made by other makers."

That's autotransformer, not rheostat or potentiometer.