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isolating transformer and variac - what order?

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I have an isolating transformer and a variac.

Would it be best to have the isolating transformer before or after the variac?

My practice has been to put the isolating transformer before the variac. I can't see any real reason to favour one way over the other.

On the other hand, my isolating transformer has earth wired straight through, which is clearly a problem if neutral or live get shorted to ground on the DUT.
 

Tha fios agaibh

Aug 11, 2014
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Doesn't having earth (neutral) wired straight through negatate the whole (safety) reason of an isolation transformer?

I agree with the practice of putting the Isolation xformer first.
 

Martaine2005

May 12, 2015
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upload_2020-1-3_12-36-33.jpeg
Mine has the transformer first. It is earthed but through the front large switch I can float.
I use the back output for my series bulb current limiter. Works like a charm.


Martin
 

Martaine2005

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If the variac was connecred first, wouldn’t that enable higher current at lower voltages?.

Martin
 

(*steve*)

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Doesn't having earth (neutral) wired straight through negatate the whole (safety) reason of an isolation transformer?

Not a straight through neutral, a straight through earth.

However, as I suggested, a short from either side of the isolated mains to the chassis will render the isolation moot. Actually it's worse than that, because now you're not expecting to receive a shock through ground.

This is the reason why many isolation transformers deliberately break the ground connection.

With a broken ground connection, it probably pays to have the isolation transformer last so that the maximum about of equipment is earthed.

If the variac was connecred first, wouldn’t that enable higher current at lower voltages?

It may, but the current rating of an autotransformer is determined to a large extent by the moveable tap. This the current limit should apply to the output.

Interesting issues can arise if you place a dim bulb tester before an autotransformer. You can get surprisingly high fault currents if you start from a very low voltage.
 

Martaine2005

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I have never actually reduced the voltage while using the series bulb. I just set 240v, plug in the DUT and turn the series bulb unit on.
I’ll try to add a picture of the unit I made later. It has two bulbs and switches in parallel. I can obviously change the bulbs for different wattages but normally just keep 2x100W in it.
My isolation transformer has no earth to the transformer itself. It only has earth from the mains input cable through a switch and direct to the output sockets. The metal chassis is earthed but the transformer and variac are isolated from the chassis. It’s certainly not a portable unit. It weighs a tonne.

Martin
 

DJL33B

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Grounding issues aside, the isolation transformer first is the better way. If the variac is first, you will starve the magnetic field of the isolation transformer, causing poor voltage control, especially at lower voltages.
 

hevans1944

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Youse guys livin' in England, Europe, Asia, Australia... anywhere except here in the United States of America... have the ability to buy something that is now forbidden to us: 100 watt incandescent light bulbs. Even smaller wattage incandescent lamps are slowly being pushed off our shelves in big-box home-improvement stores by LED lamps! Curly bulbs (compact fluorescent lamps) also seem to be yielding to LEDs... good riddance to curly bulbs. Almost every lamp I could find in our "new" home in Venice, FL had either an incandescent or a curly bulb in it when we moved in. I have replaced all except for two (out of six) bathroom vanity lamps with LED lamps. The two hold-outs get replaced Real Soon Now. However, none of my LED lamps will work with a Variac, and an isolation transformer (even in a bathroom) seems a bit of overkill for shock protection. ...Just sayin' my two centavos for this thread.

For what it's worth, I've always wired the Variac to the secondary of the isolation transformer. Been doing it that way since the 1960s.
 

Bluejets

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Youse guys livin' in England, Europe, Asia, Australia... anywhere except here in the United States of America... have the ability to buy something that is now forbidden to us: 100 watt incandescent light bulbs. Even smaller wattage incandescent lamps are slowly being pushed off our shelves in big-box home-improvement stores by LED lamps! Curly bulbs (compact fluorescent lamps) also seem to be yielding to LEDs... good riddance to curly bulbs. Almost every lamp I could find in our "new" home in Venice, FL had either an incandescent or a curly bulb in it when we moved in. I have replaced all except for two (out of six) bathroom vanity lamps with LED lamps. The two hold-outs get replaced Real Soon Now. However, none of my LED lamps will work with a Variac, and an isolation transformer (even in a bathroom) seems a bit of overkill for shock protection. ...Just sayin' my two centavos for this thread.

For what it's worth, I've always wired the Variac to the secondary of the isolation transformer. Been doing it that way since the 1960s.

Nope...been gone from here at least 10 years now in Aus.
Bit of a joke really when it was introduced.
Contractor mate of mine attended some convention where a bulb maker from Hungary was in attendance.
My mate mentioned the "then" upcoming ban on incandescent bulbs and the Hungarian replied " Our factory operates 24/7 and we make 1 year worth of bulbs for Australia in 1 day. The rest of the year for the rest of the world".

Curly bulbs gone 99.9% the way of the dodo also, good riddance.
Now we have the LED with claims of 20,000 hours or more. Problem is the driver kacks itself so the whole shebang gets chucked in the bin after 12 months anyhow.

Don't know why an earth is brought into play in the above, as said, defeats the purpose.
If fault current is a factor in any of the above, then fit an appropriate breaker calculated to retain the fault current below a predetermined level.
Operating theatres use isolation transformers with leakage detection but cost would be prohibitive in a home workshop I'd imagine.
 

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(*steve*)

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Youse guys livin' in England, Europe, Asia, Australia... anywhere except here in the United States of America... have the ability to buy something that is now forbidden to us: 100 watt incandescent light bulbs.

Believe me, they're really hard to get here. I have resorted to buying some from China. I'm generally stuck with weird bases like E27...

Many of the ones available here are the halogen type that are considered "efficient", and which (apparently) don't like operating at reduced voltage.

Some types of incandescent bulbs are still explicitly permitted, such as bulbs for use in ovens, where the environment is very hot.

Even smaller wattage incandescent lamps are slowly being pushed off our shelves in big-box home-improvement stores by LED lamps! Curly bulbs (compact fluorescent lamps) also seem to be yielding to LEDs... good riddance to curly bulbs. Almost every lamp I could find in our "new" home in Venice, FL had either an incandescent or a curly bulb in it when we moved in. I have replaced all except for two (out of six) bathroom vanity lamps with LED lamps. The two hold-outs get replaced Real Soon Now.

I'm trying to think of a light that's not LED at home, and apart from my dim bulb tester, a flourescent light in my shed (that's supplimented by led lighting), and maybe some small bulbs in very old equipment and wien bridge oscillators I've made...

In a hot climate, the reduction in heat from incandescent bulbs, while not huge, is a positive.
 

Bluejets

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In a hot climate, the reduction in heat from incandescent bulbs, while not huge, is a positive.

One canefire, bushfire or simply a small motor vehicle would make that pretty insignificant.
Other considerations are the dust to dust factor and hidden costs.

While an LED light may use less energy over it's lifespan, I have yet to see an LED last anywhere near that of an everyday fluro tube or it's associated components. My bedroom bed lights, while used perhaps 1 hour a day in total, have been there since 1986.

Then, if one decides to go LED, in comes a contractor to fit the "now" 413 sockets to the existing wiring at maybe $50-$60 an hour, probably a lot more in the big smoke.

A 60 cent bulb replacement has now become a minimum $10-$15 fitting replacement and most consumers have no idea it plugs in or inclination to change the fitting themselves. So back comes the contractor.
 

WHONOES

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Doesn't having earth (neutral) wired straight through negatate the whole (safety) reason of an isolation transformer?

I agree with the practice of putting the Isolation xformer first.

I agree with your reasoning.
 

Martaine2005

May 12, 2015
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Ebay. I bought a box of 17 bulbs for £12. Yes, pricey, but still available.
 

hevans1944

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I have never considered the "cradle to grave" cost of lamps whether LED, curly-bulb, incandescent, halogen incandescent, neon, or whatever. Years ago electroluminescent strips were popular "night lights" that apparently lasted forever and consumed very little energy. You can still buy them, but the reason they never took over the lighting market was simple: they were just not bright enough and seemed to come in only one color, a sort of green or turquoise. These handy night lights didn't require any electronics and were inexpensive to manufacture. Electrically they behaved like capacitors and ran fine from household line voltage. Later versions provided a range of colors, but brightness was still lacking.

Electrically powered lighting has never been energy efficient, whether it be arc lamps or LEDs, because the total cost to manufacture and distribute lamps, as well as the infrastructure required to power them and eventually dispose of them, is never considered as a contributing factor to the cost of ownership. How much would it really cost to recover the minute quantity of mercury present in every discarded curly-bulb? What if, instead of mercury, the curly-bulb contained a highly radioactive or extremely toxic substance that absolutely HAD to be removed from the environment? And what about the true cost of all those "green" solar-powered garden-path lamps? Lessee... whup up a mess of silicon, dope it properly, and make some photovoltaic cells and light-emitting diodes. Then figure out the chemistry to make a rechargeable battery to store the electrical energy absorbed during the day and release that energy to a white-light LED during the evening. If you can do all that without cost, congratulate yourself on being "green" and go hug a tree in celebration.

So, unless I learn the fine art of blowing glass bulbs, winding very small diameter tungsten wire into coils and winding those coils into slightly larger coils, then attaching the tungsten ends to wires passing through a glass stem... yada, yada, yada. Even "simple" incandescent light bulbs require more technology to manufacture than is available in my back yard. Goodbye yellow brick road and tungsten filament incandescent lamps. We loved your warm yellow color for awhile and the soft heat you provided. I guess I will now go looking at Wally World for an LED covered heating pad. I wonder if those will be available in a soft yellow or violet color?
 

Tha fios agaibh

Aug 11, 2014
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You can still get whatever bulb you want here in USA provided your willing to pay the price.
I do agree with the dept of energys ruling on the law stopping production of the common incandescent bulb. The benefits are huge. Regardless of law. Supply and demand will always rule.

Btw, we still make incandescents in 3 way wattage, heavy usage, and other lamp shapes such as candle, globe, tubular, par.. etc.

To me, Halogen bulbs are really just a glorified incandescent. Aside from the fact they run hotter, they are roughly 30% more efficient and will last 3 times as long.

So no worries, you will always find a replacement when your inline current limiter bulb burns out.
 
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