Simple Variac Question

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

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.

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

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

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

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.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

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

Thanks.

Matt

I would have to disagree there. A variable AC source in the form of a traditional variac, is an adjustable autotransformer, which means that one 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

ABSOLUTELY CORRECT.

It is NOT a transformer. = NO isolation.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

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

Not sure offhand what current the transformers are good for, but usually a few amps.

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

- 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.

- 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

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?

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

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.

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.

GET out your trusty ohmmeter and measure it before you make a fool of yourself.

It is a single coil of wire with a tap.

Call it what you will it is NOT two coils of wire insulated from each other == transformer

It seems youre confused as to what constitutes a transformer.

NT

Have you never heard of an auto transformer? Most all variacs are auto transformers.