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 Post subject: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 7:43 pm 
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Joined: Jan Fri 29, 2021 10:08 pm
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I read in an ARRL handbook that an isolation transformer should be used even with a variac, and that some variacs have built in isolation.
Since I always ground all equipment that I connect to my variac, would I still need (or benefit from) an isolation transformer?

Larry


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 7:54 pm 
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isolation transformer is handy if you're powering AC/DC radios or equipment and want to poke around with a piece of grounded test equipment. or ground the DUT and lift the ground on your test equipment as you see fit. Handy to have one or three of everything, IMHO.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 8:23 pm 
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If your "variac": is not made by General Radio Company or ISE, Inc., then it is not a Variac, which is a registered trademark for a brand of variable autotransformer.

It is possible to buy a variable autotransformer packaged with an isolation transformer. Some, including myself, have built their own including metering and fusing.

Image

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 8:57 pm 
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Here is a schematic for a deluxe isolation and variable transformer power center with metering.

This builder chose the meter the incoming AC voltage as well as the variable voltage and current.

You can see that a variable transformer has continuity from the incoming AC line to the output.

Some builders like to use a Kill-a-watt for the metering.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 9:24 pm 
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The two types of transformers serve different purposes. They are often used together but don't have to be.

An isolation transformer is a two-winding device used to give a source of AC power that is not referenced to ground. In most normal residential and commercial wiring, one side of the line, the neutral, is at or near ground potential. Thus it becomes possible to complete a circuit from the hot side of the line to anything that is grounded. This has several consequences in electronics. First, it makes it easier to get a shock if you happen to be in contact with some kind of ground and you touch a chassis or circuitry connected to the hot side of the line. Second, it could lead to a situation where the neutral side of the line is inadvertently shorted to the safety ground through a piece of grounded test equipment. Since neutral is a current-carrying conductor but safety ground is not except under fault conditions, they are not necessarily at the same potential and a lot of current can flow unexpectedly through your test equipment if you connect its ground lead to neutral. This is especially true if there is a fault in the building wiring or if you are at the end of a long branch circuit. Finally, even if the neutral to ground currents are just a few milliamps, they can still cause hum which will upset whatever you are trying to work on.

A Variac or variable autotransformer is essentially a variable impedance that you use to raise or lower the power input to a load on AC power. In radio repair it is used for a few things such as setting the line voltage to a specific number voltage readings match what is on the schematic, operating things above and below the nominal line voltage at your bench to show up intermittent faults and ensure the gear will work under abnormal line conditions, and "smoke testing" equipment by bringing up the voltage slowly to see if any smoke appears, and if so, shutting it down before any serious damage occurs.

Thing is, the vast majority of Variacs or variable transformers are single-winding devices with a movable arm that contacts taps in the winding in a commutator-like structure. Being just one winding there is no electrical isolation from input to output so they cannot do what isolation transformers do. There are a few Staco variable transformers which have two windings and do provide isolation but they are very expensive and seldom seen. Unless you happen to have one of those rare birds, if you need an isolation transformer to do the job correctly, then you still need an isolation transformer with a Variac.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 10:46 pm 
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xmo wrote:
Here is a schematic for a deluxe isolation and variable transformer power center with metering.

This builder chose the meter the incoming AC voltage as well as the variable voltage and current.

You can see that a variable transformer has continuity from the incoming AC line to the output.

Some builders like to use a Kill-a-watt for the metering.

It seems to me that the AC outlets on the output side of the isolation transformer should NOT be connected to earth ground the way they are shown. Doesn't that just defeat the purpose of having an isolation transformer in the first place? One side of the AC line is grounded back at the breaker box in the house or at a common ground point .. so by grounding the DUT, you've just reconnected it to the common electrical ground of the house, no ?

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 11:21 pm 
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There should be no physical connection between the input of an ISO and the output, ground or otherwise.

A simple effective ISO can be made by connecting two power transformers back to back (secondary's to secondary's). One primary plugs into the mains, the other is the output to the equipment under test.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 11:22 pm 
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The grounded outlets are correct per the National Electric Code which requires a grounded outlet to have its ground connected so if anybody plugs something in expecting the ground to be there they won’t be disappointed. I looked to see if I could squeeze the word ‘ground’ into that sentence one more time but didn’t see a way. :wink: My opinion is nothing with a hot chassis should ever have a three wire grounding cord and plug installed so if you only plug in two-prong plugs the third is not going to be connected so it’s not bothering you anyway.

Yes it is possible that with modern switching supplies and similar things that ground connection could increase the shock hazards a bit but your test equipment is going to re-introduce ground into the picture anyway so it’s something you already had to live with.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Thu 27, 2021 11:30 pm 
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They are absolutely shown correctly.

By code and common sense - the ground prong of any three prong outlet must be grounded..

By definition - any hot chassis appliance is not going to have a three prong cord so you plug it into the two wire prongs and it is completely isolated.

An isolation transformer doesn't isolate 'ground' - it isolates the current carrying conductors from ground.

Look at the schematic again and think about it. A connected 'hot chassis' unit under test can have its chassis connected to ground through your test equipment or any other way without the possibility of a current path from the AC circuit hot conductor.

Now, look at this diagram of electrical service. You can see that the neutral conductor - the wide prong of the outlets - and one side of every two wire connected appliance - has a path to 'earth' ground because the neutral and the protective ground are bonded together at the service panel.

Because of that, there is always the possibility of a path from the 'hot' conductor on the narrow prong to create a circuit through 'something' (maybe you) that has a path to earth ground.

The test lite shows that path. You can connect anything you want - a test lite or a radio - to the isolated outlets and there is no path to earth ground wheteher it's the concrete floor under your feet - or that same outlet's ground prong.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 12:34 am 
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Joined: Jan Fri 29, 2021 10:08 pm
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Thanks for all the responses. I think I got it now.

Larry


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 12:45 am 
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It follows that test equipment with a three prong plug should be grounded.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 1:22 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
someone pointed out a while back that a hot chassis two wire radio might not behave the same through an isolation transformer as it would connected directly to the wall outlet, so there may be a time when one would want to have the test equipment isolated and the ground on it isolated, rather than the radio. I haven't run into this, but I can see how this would function in theory if the RF ground for the radio was also lifted in addition to the ground for the power coming in.

I highly recommend this link here that gives the best descriptions/explanations of what "ground" is, and the many forms it takes:

https://www.angelfire.com/electronic/fu ... ounds.html

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 1:25 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
for me, the smart money is on isolating the power line coming in with the isolation transformer, and grounding the chassis of everything you'll be working on in. This has been the "smart thing" to do on just about every piece of equipment I've ever worked on. I try to think through what should be happening, and what could be happening in circuit under test, so that I can make good choices about how to connect things to it. Make sure the procedure you are using makes sense, and isn't going to make things more dangerous.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 3:03 am 
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For powering a Variac, or anything else with AC, I have only one isolated source on the bench. All test equipment is powered from the wall and properly grounded.

Image

The isolated power is available at the output receptacle of an Un-Grounded Kill-A-Watt multi-meter. Even though it is a 3 prong receptacle, the ground pin is not connected to anything.

Image

Attaching the ground from any piece of test equipment establishes the ground on the device being tested.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 4:18 am 
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Mike, why not have the ground pin on the killawatt grounded too, to the same ground as your test equipment, so that the device under test is always properly grounded if it has a three wire cord?

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 4:54 am 
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Joined: Jan Sat 16, 2016 4:15 pm
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As Mike said, all the test equipment should be properly grounded and the unit under test floated.

Then, when you connect a scope probe ground lead to the chassis of the unit being tested, there won't be a dangerous 'boom'

After you do that, the equipment chassis is at ground potential through the test equipment's ground but the AC supply to that unit has no path back to ground.

If you use only a variable transformer without isolation - well - this could happen to you:


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 5:39 am 
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It usually follows that a hot chassis and newer stuff which is "double insulated" will either have a two pin plug top, or one where the grounding pin is not connected.

I will use one of my isolation transformers on transformer sets as it is set up with a kill "switch": Earth /ground is connected.

As points to be noted: Where there is a GFCI /RCD aka Earth leakage trips. There is no protection on the secondary of a transformer: It will only see the primary and only act if there is a fault on the primary side. Fuses and circuit breakers will also be ineffective unless there is an overload.

In the case of an auto transformer, be it Variac (tm) / Dimmerstat (tm) aka (Slide transformer) when the output voltage falls below a certain point it may fail to trip due to lack of current. I did do this deliberately using a tester, by holding down the test button. @240VAC line voltage, it did not trip at the required 30mA until the Slide Regulator output got to 130V.

Marc


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 12:05 pm 
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Anchorman asked;
Quote:
...Mike, why not have the ground pin on the killawatt grounded too, to the same ground as your test equipment, so that the device under test is always properly grounded if it has a three wire cord?...
Not being an alarmist, or screaming "the sky is falling"....But. Do you know FOR A FACT, that anything you drag home from who knows where, having a 3 wire cord, actually has the ground connected to ground? Will you, for the next 20 or 30+ years, check each and every 3 wire corded piece of whatever, to verify that? I think not, and, depending on circumstances of course, it'll only take once, for the big "LIGHTS OUT".

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Fri 28, 2021 1:37 pm 
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Mikeinkcmo wrote:
Anchorman asked;
Quote:
...Mike, why not have the ground pin on the killawatt grounded too, to the same ground as your test equipment, so that the device under test is always properly grounded if it has a three wire cord?...
Not being an alarmist, or screaming "the sky is falling"....But. Do you know FOR A FACT, that anything you drag home from who knows where, having a 3 wire cord, actually has the ground connected to ground? Will you, for the next 20 or 30+ years, check each and every 3 wire corded piece of whatever, to verify that? I think not, and, depending on circumstances of course, it'll only take once, for the big "LIGHTS OUT".


Having worked in OH&S and not leaving it at the doors of the factories. The above raises questions and comes back to the understanding of what you are doing. In around five decades I have seen massive damage done by exuberance.

As I have written many times. Be it run by mains, or battery. If you know nothing of its history then applying power is not ever: Step one. An isolation transformer is not going to save an apparatus which is "unfit to power" from serious damage to itself, or even the transformers. My unit has circuit breakers both sides of the isolation transformer windings to help protect it.... But!

It is therefore, wise counsel to inspect the "unknown's" chassis for obvious damage, tampering, and the condition of the wiring: I call this "assessment". Many a radio here has been repaired, after being assessed as unsafe to power.

Marc


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 2:20 am 
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I always keep a handful around


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