# Simple Variac Question

Discussion in 'Electronic Repair' started by Matthew Rossiter, Mar 2, 2007.

1. ### Matthew RossiterGuest

I built a test fixture for doing all kinds of different things. One of the
ideas I had in mind was using a Variac to adjust AC voltage for certain
types of monitors, one which requires two 45VAC inputs.

Would I be able to use a VARIAC to set the voltage to 90 VAC and have both
terminals deliver equal 45VAC in reference to ground?

Thanks

Matt

2. ### Arfa DailyGuest

No, because there is no centre tap on a variac. Electrically, it is just a
variable auto transformer. One output leg is common with one of the line in
legs.

Arfa

3. ### Matthew RossiterGuest

What if you used a secondary transformer with a Center Tap and used the
VARIAC to control the AC input? I guess that's a pretty simple question,
but just want to make sure I'm not missing something.

4. ### Arfa DailyGuest

Yes, that would work. You would need to find a transformer with a 50v - 0 -
50v secondary ( or two 0 - 50v windings that you could hook in series ) that
could supply the current demand of the monitors that you want to power.
Then, as you say, you would connect the variac to the primary. This scheme
will have the added advantage that it will provide safety isolation that the
variac on its own, won't.

If you couldn't find such a transformer, you could use two single-secondary
transformers, with the secondaries hooked up in series, and the primaries in
parallel, to arrive at the same end result.

Arfa

5. ### Meat PlowGuest

The output of a variable AC supply is almost always isolated from any
reference to any kind of "ground" you might be thinking of.

6. ### Meat PlowGuest

If you used a 1:1 transformer rated at 110 volts or better with a center
tapped secondary you should be able to obtain the desired result.

7. ### Matthew RossiterGuest

I might as well ask. Can you recommend a good place to find a 1:1 Center
Tapped "High Current" Transformer?

Thanks.

Matt

8. ### Arfa DailyGuest

I would have to disagree there. A variable AC source in the form of a
of the line legs - lets call it "neutral" or "cold" - is common to both the
input and output, which means that the variable output is referenced to the
incoming supply, which is itself tied to ground at the local substation in
most countries, as far as I know. A variac does not provide any form of
electrical isolation, so the output from it *must not* be treated as
'bench-safe', unless its input is plugged into a proper isolation
transformer.

Arfa

9. ### Guest

ABSOLUTELY CORRECT.

It is NOT a transformer. = NO isolation.

10. ### Meat PlowGuest

Was thinking of my Sencore PR57 which is a VARIAC as in (variable AC
supply) and it does have an isolated output, no big fucking deal.

12. ### James SweetGuest

Well it is a transformer, it's just one that has one side of the
secondary tied directly to one side of the primary, hence no isolation.

You can get cheap 1:1 isolation transformers, they're sold for use with
older video arcade monitors. Any place that sells amusement supplies
should have them. I get most of my parts here
http://homearcade.org/BBBB/ Not sure offhand what current the
transformers are good for, but usually a few amps.

13. ### Guest

Yours then is exceptional. To assume a variac has an iso output is
asking for a nasty accident, as most have outputs connected direct to
line.

NT

14. ### Arfa DailyGuest

Sorry old boy - wasn't meaning to dis you ... I just wanted to make sure
that no one had a safety issue by not understanding the difference between a
double wound and an auto tranny. Thanks for the support in that other thread
further up though. That Aussie twat pops up on other groups as well. I think
it must be every time he runs out of money to buy his meds ...

Arfa

15. ### Matthew RossiterGuest

I'm having a little hard time finding a 1:1 Center tapped transformer with
some good power.

Hammond makes an 80CT transformer @ 4 amps
http://www.hammondmfg.com/165.htm - 165N80
but that would only give me 40VAC each tap, but pleny of current. Maybe
that's all I'd need, I don't know.

I think their 300 series would be a little under-powered.
http://www.hammondmfg.com/300series.htm - or am I reading the specs wrong?

I've been doing searches on google (and on ebay) but it seems finding a 1:1
center tapped transformer with plenty of power is hard. unless maybe I'm
not looking in the right place.

Just looking for a little help.

Thanks!

Matt

16. ### James SweetGuest

You can use a pair of 1:1 transformers in series to achieve that.
Primaries in series and then secondaries in series. That will give you
center tapped 1:1.

What sort of motor is this that requires two transformer windings?

17. ### Matthew RossiterGuest

Doesn't the Electrohome G08 monitor require two 45Volt AC inputs?

18. ### Guest

Actually, it is not a transformer.

It is ONE coil of wire connected across the AC supply with a variable
tap. Output is tied between tap and (Hopefully) neutral of the AC
input.

19. ### James SweetGuest

I think you should read up on autotransformers. Variac is a trade name
for a variable autotransformer, and they are in fact transformers. The
output of most of them can be adjusted *above* the incoming line
voltage, standard is about 10%. The one I have sitting here accepts 120V
input and outputs 0-140V so it is clearly a transformer.

For a bit more detailed info on the subject here's an article.

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

20. ### Meat PlowGuest

Understood. I've got to remember to always consider the rookies
who read SER. As far as Phil goes, he's got issues with those whose
knowledge far surpasses his own.