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 Post subject: Re: Isolation Transformer Needed
PostPosted: Jan Tue 19, 2021 1:17 am 
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I had one of those hospital grade isol. transformers around here for years but was not using it for anything. Started investigating an RC device on the output terminals and discovered that it had neut to gnd on the output. Quickly fixed that so that in case I ever used it for anything it would not be an issue.

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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Thu 24, 2022 6:40 pm 
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I'm hoping this thread is still in circulation after all these years. I'm convinced an isolation transformer, where the secondary has no earth ground connection, provides the safety needed to work on equipment. However, I'm only convinced because an incredible amount of people say it's so. But, I confess to struggling to understand exactly how this is safe. From a physics standpoint, the earth is an endless supplier and receiver of electrons. So, if you are touching one of the iso transformer's secondary leads, even though it's ungrounded it has potential on it, so if you are personally grounded, how does the current not flow through you? It doesn't necessarily have to flow back to the other secondary lead, does it? If someone can offer an explanation to my belief that the earth is always happy to receive all elections, I'd be appreciative. Many thanks, and Happy Thanksgiving......


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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Fri 25, 2022 4:08 am 
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frontier wrote:
From a physics standpoint, the earth is an endless supplier and receiver of electrons. So, if you are touching one of the iso transformer's secondary leads, even though it's ungrounded it has potential on it, so if you are personally grounded, how does the current not flow through you?

A potential with respect to what? That's the point. Voltage is always referenced between two measurement points. Yes, there is a low impedance AC potential between the two ends of the secondary winding of an isolation transformer, but the whole point is that there is no (or to be precise, only a high impedance potential) between either end of the secondary winding and ground.

frontier wrote:
It doesn't necessarily have to flow back to the other secondary lead, does it?

Yes, it does have to flow back to the other secondary terminal, primarily. Since there is a small capacitance between the secondary winding and the primary winding, there is also a high impedance path through that small capacitance between the hot end of the primary and neutral (which is also grounded), but that is negligible in a properly functioning isolation transformer.

frontier wrote:
If someone can offer an explanation to my belief that the earth is always happy to receive all elections, I'd be appreciative.

I've run into this misconception among the linemen in our local electric company. I suppose it's easier to just tell them that current always seeks ground (since the electric company power feed has a grounded neutral) rather than educate them about circuit theory. You should pick up a book on basic electronics and read up on circuit theory. The ARRL Handbook is one good choice.

Current only flows in a complete circuit. Any voltage source has two terminals. Current from that voltage source only flows between those two terminals, and no where else. Any current leaving one terminal must return to the other. This follows from the basic physical law of conservation of charge.

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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Fri 25, 2022 1:50 pm 
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Thanks for these replies. My takeaway from all three is that the secondary current must indeed flow within it's loop. There is essentially no potential difference between either lead on the secondary loop and earth ground, so there is no current flow. I also take away from the replies that yes, there are no absolutes. There's some capacitance between the windings, etc., but essentially, the impedances are such that there is no risk of current flow through you if you are connected between the secondary winding on an isolation transformer and earth ground, providing neither lead on the secondary winding is tied to earth ground.

I'm not yet in possession of an isolation transformer to do some experimentation, but I would appreciated commentary on the following experiment my son and I did yesterday: We plugged in an extension cord to a properly wired outlet. With one meter lead stuck in the neutral opening of the extension cord and the other meter lead connected to the positive terminal of an ordinary 12V car battery, the meter reads 12VDC. You get the same result if you stick the negative meter probe in the ground opening of the extension cord, which is no surprise, because the neutral and ground should be tied to the same earth ground. Amazingly, there is no potential difference between the negative battery terminal and earth ground.

So, back to the 12V reading between the positive battery terminal and earth ground, how can this be? It's still a head-scratcher to me.


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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Fri 25, 2022 2:54 pm 
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And what was the car battery sitting on when you did this experiment? If it was a concrete garage floor, concrete with an average amount of moisture in it is conductive and the earth is underneath it so it makes a good ground. The plastic or hard rubber cases of automobile batteries are hygroscopic, so they are slightly conductive as well. Thus you have a circuit from the positive terminal of the battery, through the meter to the neutral or ground of the electrical system, back through the earth, garage floor, and case of the battery. Even if the battery was sitting in a car it could still produce a reading, since car tires contain carbon to dissipate static (which is why they are black), and that makes them slightly conductive. But those circuits likely have so much impedance or resistance in them that it would be impossible to draw any usable amount of current through them. There's enough there to cause a sensitive meter to give a reading, but that does not mean you could draw a spark, power any kind of practical device, or get a shock that way.

Thing is, if you make meters that are sensitive enough, you can measure current flows in everything. Conductivity is only relative, there are no perfect conductors nor perfect insulators at ordinary temperatures. And when it comes to AC, or DC currents that vary over time, the effects of magnetic and electric fields have to be considered as they too can cause voltages to appear where you don't expect them. The point to isolation transformers and electrical insulation generally is not to completely block any possible currents from flowing because that is not really possible. The purpose is to make sure those currents are so weak and so limited they cannot cause any harm or impact what is being accomplished. Needless to say, this becomes a tremendous challenge as one tries to work on devices that are more and more sensitive.

Those of us who collect old test equipment have often noted that certain meters, frequency counters, signal generators, and even some oscilloscopes were made with battery power supplies either as optional features, or by design. We often grit our teeth and mutter under our breath about the inconvenience of having to replace those batteries which are often expensive and hard to find--if they are even available today--to get the instruments working again. Some people try to make AC power supplies to replace the batteries with. But when you dig down into it, you begin to realize that battery powered instruments were and still are the only way to make extremely low level measurements without interference from the AC line. No matter how well engineered, filtered, regulated, and shielded a power supply is, some unwanted voltages and currents always bleed through. About the only thing you can hope for is keeping the unwanted "noise" low enough that it can safely be ignored, but that is not always possible.

Fortunately nowhere near that level of isolation is required for fixing antique radios! As long as an isolation transformer is putting hundreds of megohms of insulation resistance, and less than 0.01-uF of capacitance between you and the AC line, it's fine for our purposes here.

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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Fri 25, 2022 3:55 pm 
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I understand and greatly appreciate your reply. With enough sensitivity, we can measure almost anything, but is it potentially harmful, or even useful? As I mentioned in my original post, I accept that iso transformers give us the insulation to a ground fault we need to be essentially safe. But since my curiosity about this, along with my decades-long understanding that the earth is an endless supplier and recipient of electrons, has made it hard for me to understand how exactly this works.

I'm starting to think that the answer lies in my potentially flawed belief of the earth's role/capabilities, electrically. In my experiment, the battery was completely isolated from the earth, way too distant for any static/capacitive readings, on a bone-dry day. But the fact that I get 12V to earth ground on the positive terminal, yet zero on the negative terminal, has me really baffled. Can the earth only send electrons, but not receive them, or vice-versa?

I was using a mid-range (nothing expensive) Fluke VOM. My next experiment will be to measure the actual current available through this "circuit." I'll also confirm the reading with my Simpson 260 and report the results.

Thank you again, and the others, for weighing in. Stay tuned......


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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Fri 25, 2022 7:39 pm 
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This little illustration someone posted a few years ago, might make sense to you.

The SECRET SAUCE, so to speak, is in the "High Resistance" material between the primary and secondary windings of the transformer.

So long as that insulating material doesn't break down, AND you only grab one or the other of the two output leads, you're safe.

Image

Of course, should you grab both output leads, YOU THEN BECOME "R" with very unpleasant results.

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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Sat 26, 2022 3:09 am 
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Location: Corinth, TX, USA
Bill Eccher Jr wrote:
Why not just buy a metered variable isolation factory made AC supply all in one neat package?
If one is not in a hurry they can be had for $150 or less and there are better ways to save money.

I did just that. I picked up a BK 1653 which combines a 2 amp variable transformer and a 2 amp isolation transformer with rudimentary metering. Physical size seems to indicate that the 2 amp rating is pretty conservative.

John


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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Sat 26, 2022 3:00 pm 
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"The SECRET SAUCE, so to speak, is in the "High Resistance" material between the primary and secondary windings of the transformer."

Some fancier isolation transformers have one, sometimes two, electrostatic shields between primary and secondary. In the one with two shields, the manufacturer shows one of them tied to the incoming hot lead.


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 Post subject: Re: Isolation Transformer Needed
PostPosted: Nov Sat 26, 2022 3:51 pm 
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jim rozen wrote:
...... Some fancier isolation transformers have one, sometimes two, electrostatic shields between primary and secondary. In the one with two shields, the manufacturer shows one of them tied to the incoming hot lead.


Manufacturer and what model?

RRM


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