isolation transformer

Does your ohmeter say there is a connection across both sides? ie an autotransformer.

Most voltage-converter transformers are auto-transformers and do not provide isolation. As Mr. Cook suggests, see if you get continuity from the primary to the secondary.

This is an auto-transformer.

Peter Wieck Melrose Park, PA

Hello, and do you have a particular make and model number of device in mind? An marketed as an AC isolation transformers usually doesn't provide voltage conversion with input (primary) to output (secondary) energy transfer solely via magnetic coupling. There are also adjustable autotransformers (e.g. the venerable Variac brand) that, for example, takes 120 VAC at input and can provides 0-130 VAC output. The primary and secondaries of these devices aren't conductively isolated (as an ohmmeter test will confirm). There are also devices which combine both isolation and adjustable autotransformer function in one enclosure. Or you can simply take, say, a 120 VAC-to-120 VAC isolation transformer and follow it with an adjustable autotransformer, keeping in mind the maximum AC current/power ratings of the devices. Sincerely,

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J. B. Wood	            e-mail: arl_123234@hotmail.com

here's the model Norstar ST-750.. it's on ebay for half price used. so more or less this just a surge protector.?

That unit appears to be intended for those situations where, say a 120 VAC appliance sold in the U.S. is to be used in another country like the UK on 240 VAC (also assuming the U.S. appliance can cope with 50 Hz vs

60 Hz). Vice-versa for using a UK-marketed appliance in the U.S. Most likely it's a single winding transformer with a tap(s), which makes it an autotransformer (no input-to-output isolation). But it could also have separate primary and secondary coils. No way to tell from a picture or not having a schematic. Sincerely,

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J. B. Wood	            e-mail: arl_123234@hotmail.com

I reaad on some website where someone took two 120 to 240 volt transformers, connected both 240 windings together and ended up with 120 on the secondary, completely isolated from the AC line. This makes sense, for someone who has two such transformers on hand. But would not make sense to buy two transformers.

I've always wondered why so many foreign countries have 240volts as their standard voltage. We in the U.S. do just fine with 120v, except for heating devices and some very large motors. So why do they continue to use 240volts in these foreign countries? If for no other reason, it's considerably more dangerous..... I know all too well how getting shocked from 120v feels, and although I have never gotten shocked by 240v, I know it's a lot worse. Both voltages can kill a person, but I'd think that 240 kills faster...

Very simplistic:

240 VAC Hot/Neutral is much more efficient than 120 VAC Hot/Neutral. Euro ( residential) distribution is primarily three-wire ungrounded three-phase wi th varying phase-to-phase voltage. Grounding is done at the distribution tr ansformer. Whereas North American distribution is 4-wire three-phase with s ingle-phase 120/240 secondary distribution. Grounding in NA is done at mult iple points.

Bottom line, Euro distribution allows either much more power on the same di stance/size conductor, or the same amount of power to be transmitted twenty (20) times further for the same size conductor.

Again, a benefit of learning from others. Post War Europe - when power-to-t he-people became the norm rather than the exception was not rich in resourc es. So, a system that permitted more efficient distribution became a signif icant advantage.

Very simplistically.

Peter Wieck Melrose Park, PA

I think that if 240 volts is really that much more dangerous it wouldn't be so popular. And think of the advantages. The wires can be much smaller. And for the same wattage resistive losses are 1/4 that of 120 volts.

Working at Northrop Aircraft, I had to deal with

120, 240 and 480 volt systems.

To put it simply, 120 will get your attention and you let go of it.

480 will knock you on your ass. 240 however with grab hold of you and hang on.

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Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

it's safer and cheaper It's also barely practical to run a 3kW heating device on 110v. Our 240v 13A delivers 3.1kW per socket no problem.

it sure does. Deaths from electrical fires greatly outnumber electrocutions, and less current reduces the risk of those.

NT

If you prefer a slow death, stick with it!

Mike.

There was an old patent using two neon sign type transformers back to back. The problem with isos, you get higher voltage with no load than normal, and droop 10-20% with load..

Greg

On Sunday, December 10, 2017 at 3:48:23 AM UTC-5, GS wrote: make sense to buy two transformers.

Properly designed Isos are vastly over-sized for the rated load. Which is why a good isolation transformer is not cheap, and a cheap isolation transformer is not good. Can't have both - new, anyway.

The Iso in my Heath is rated at 4A, fused at 3A, weighs around 10 pounds and does not sag. Some of the hospital-grades units I have worked with in the past were heavy enough to be a major PITA to lift or move, and rated in the 8 - 10 A region.

Peter Wieck Melrose Park, PA

I can't imagine how that could be worth patenting. Many patents have no value.

It would take an unusually tiny transformer to be that badly regulated.

NT

"Neon sign transformer" They are DESIGNED to be baldy regulated. High voltage to strike the initial arc, then sag down to a sustaining voltage.

Same design with fluorescent ballast transformers.

Both are designed for a specific purpose, not to be used in some bastardized lash up.

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Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

On connecting transformers back to back to make an isolation transformer.

A pair of 6.3 filament transformers, back to back, was a popular kluge back in the '60s, but NOT for use as an isolation transformer for testing radios. Yes it was an isolation transformer, but it was to provide an isolated 120 VAC for a B+ supply on small projects.

That 5 amp secondary at 6.3 volts translates to only 270 mA on the primary side. Two transformers back to back is only going to be good for about 200 mA load on the "isolated" side.

AA5 "hot chassis radios from "back then" were typically 30 to 40 watts. That's 250-349 mA. This would be marginal at best.

It never fails to amaze me the number or ways people can come up with to do things the wrong way. Then the amount of time they waste trying to justify it.

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Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

which is one reason why no sensible person would use such a lash up

NT

In my experience 10% droop is pretty good. 15% more common. I'm just talking transformers in general. A Feroresonant one will do good.

Greg