Do I need an isolation transformer when using a VARIAC?

[schoolie]

I bought a sweet, used Tenma 72-100 off Ebay, and it's perfect for testing new builds.  However, people mention that one should use an isolation transformer with a VARIAC, for safety.  This VARIAC has a grounded plug and outlet, and my amp has a 3-prong plug, so do I need an isolation transformer.  Isn't the VARIAC an isolation transformer. 

Just want to make sure that I'm not being stupid...again:)

Thanks,

Rob

[kagliostro]

The Variac is an Autotransformer so isn't an isolation transformer

for safety it will be better to have an isolation transformer

for that purpose you can use also two transformers back to back ( 110v / 12v - 12v 110v as an example)

doing that you must consider the VA rating of the transformer respect to the request of current of the apparatus you have on test

some people I know didn't use an isolation transformer but is unsafe from the wall outlet till the 3 cord plug

Kagliostro

[schoolie]

Thanks, Kagliostro.  I'm still trying to understand why the utility ground wouldn't protect against shock. 

[RicharD]

http://en.wikipedia.org/wiki/Autotransformer

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where electrical connections are made. An autotransformer can be smaller, lighter and cheaper than a standard dual-winding transformer however the autotransformer does not provide electrical isolation.

A ground does not protect against shock, it protects against short circuits.  If you want protection against shocks, use a GFCI receptacle.  This still won't help you if you get tangled up in a HT or B+ circuit whereas your body resistance simply looks like additional load and not a fault.  Caution and rubber sole shoes are a good thang.  Don't put your hands where they don't belong and by all means, don't wear metallic jewelry when working on live circuits.

Since autotransformers only have 1 winding, they are more susceptible to catastrophic failure in an over current situation.  A simple lamp limiter is much better for lighting up an unknown tube circuit.  If you must use a variac, a secondary AC current meter is your best friend.  Watch that meter like a hawk when energizing the circuit.  I gave away my bench variac to a good home.  I do have a variac built into my protoboard which is very helpful when designing circuits.  Math errors can result in dangerous voltages. 

[kagliostro]

the secondary winding of an isolation transformer give you a floating voltage (no connection to earth) so if you touch one side of the secondary winding is more "safe" also if your body touch earth

when you touch the phase of the line if you are touching earth all the current of the circuit can cross your body

also

If something go wrong with an isolation transformer the max disposable current is rated by the transformer winding

If something go wrong without an isolation transformer the max disposable current is the max current of the line

Hope someone can explain you better what I'm trying to say

Kagliostro

p.s.: I was writing the same time as RicharD do

[schoolie]

 Thank you very much for the clarification, RicharD and Kagliostro!  I  use a light bulb current limiter with the VARIAC for first power and use a Kill-A-Watt meter to monitor current.  I'm not touching the amp when I bring up the voltage.  Once it checks out, I plug the amp into the Kill-A-Watt so I can continue to monitor the amperage at the socket.  I'll look into buying or making an isolation transformer for future use.  They are more expensive than the VARIAC:)

[jjasilli]

A transformer usually has 2 coils - a primary & a secondary.  There is no physical connection between the 2 coils.  The primary induces a voltage in the secondary winding, but does not physically touch it.  Thus the secondary side is isolated from the primary side.  But the isolation is imperfect due to capacitance between the coils, etc.

An true isolation transformer adds shielding between the coils for more complete isolation between the 2 sides.  The shielding has its own connection to earth ground.

A variac is an "auto" transformer -- "auto" meaning "self" or "one".  That is, it has only one coil.  Hence it is confusing for it to be called a transformer.  It is more like a variable inductor, with only one coil and a variable center tap.  Because there is no secondary winding, it provides no isolation for the DUT (Device, or amp, Under Test) from the AC wall power supply.

From what I can gather, isolation of the DUT is, or was, important for at least 2 reasons.  Old transformer-less amps might have a live chassis, which is dangerous to work on without isolation. Also, some old test equipment like VTVM's and 'scopes, if they have a 3-prong power plug, will create a short circuit when their ground test lead is connected to the chassis of the DUT.  Isolation prevents this short circuit if the ground test lead is used. Modern test equipment avoids this problem by having an internal floating ground.  Floating grounds are dangerous to the user; so the test equipment is housed in a plastic, not metal, case.  This protects the user from a ground fault in the modern test equipment. 

[sluckey]

No need for two fuses (breakers) on a simple single phase 120VAC variac. Most 'enclosed' variacs already have a fuse.

[bobmegantz]

I recently got shocked when I used a variac on a circuit where neutral and hot were switched.

[schoolie]

I Just want to thank everybody for educating me on the subject!  I understand the possible dangers now.

Best,

Rob