# Autotransformer and Back-EMF

Assuming that potential is due to a static electric charge then it'll be the same as touching a doorknob on a dry winter's day.

You mean touching a doorknob on a dry winter's day can produce 1 million volts potential? any reference so I can just read about this huge value. Thanks.

gneill
Mentor
A million volts was your suggestion. I thought you were exaggerating for effect. No, typical static electric buildups are automatically limited to be less than about 35 kV since after that coronal discharge will dissipate the charge into the air. You wouldn't get anywhere near that high in a typical household environment where the air has some humidity. More typically you can expect less than 12 kV.

Wikipedia has an entry on static electricity which covers the matter.

https://en.wikipedia.org/wiki/Static_electricity

Merlin3189
Homework Helper
Gold Member
So there is no way to measure electric potential by itself.
The point about static charges (in everyday life) is that although the potential can be thousands of volts wrt earth (how else would you get a spark when you touch it ?) the capacitance of objects is very small and the total charge stored on them is very small.
That makes it difficult to measure, because all measurements draw some charge from the object and therefore change its potential. The usual rough and ready way to measure static potential is with an electroscope (gold leaf), but even this accepts some of the charge and reduces the potential a bit. Valve and insulated gate fet voltmeters can have ultra high resistance inputs with very small capacitance. These sorts of things could give you a ball park measurement (or maybe better.)

In practice I think it unlikely that you would ever build up very big static potentials in the sort of situation you're thinking about. We have to try very hard with our insulation in static electric experiments. In your transformer, wire and TV the insulation has only to prevent significant current flowing with 250 V emf. I'm not sure what the test criterion is (I'd guess in domestic low voltage work, you can have μA currents), but to preserve a static charge you need much better insulation than that. A leakage current of 1 μA would discharge a 1 nF capacitance charged to 1000 V in one second.

To get charged in the first place it would need something to be charging it faster than it was losing charge. When you walk across the carpet and touch the TV maybe you'd charge it up instantly. Then you sit and watch for a few hundred seconds while that charge leaks away, before touching it again to recharge it. The charge is never likely to build up.

While you're questions have raised some interesting comments, I can't help feeling that your fundamental worry is misplaced. You have bought an isolating transformer, whose main purpose is to prevent your getting electric shocks and your equipment getting damaged by line faults perhaps. So then you ask how that transformer can fail in its primary objective. It is very unlikely that it will fail. It is the solution of choice for this application, so much so that it is a legal requirement in many workplace situations. That wouldn't be the case if this were just some unreliable gimmick. So you're now asking us to come up with some outlandish extreme circumstance which might circumvent or prevent this transformer from doing its job. What is the point?

The point about static charges (in everyday life) is that although the potential can be thousands of volts wrt earth (how else would you get a spark when you touch it ?) the capacitance of objects is very small and the total charge stored on them is very small.
That makes it difficult to measure, because all measurements draw some charge from the object and therefore change its potential. The usual rough and ready way to measure static potential is with an electroscope (gold leaf), but even this accepts some of the charge and reduces the potential a bit. Valve and insulated gate fet voltmeters can have ultra high resistance inputs with very small capacitance. These sorts of things could give you a ball park measurement (or maybe better.)

In practice I think it unlikely that you would ever build up very big static potentials in the sort of situation you're thinking about. We have to try very hard with our insulation in static electric experiments. In your transformer, wire and TV the insulation has only to prevent significant current flowing with 250 V emf. I'm not sure what the test criterion is (I'd guess in domestic low voltage work, you can have μA currents), but to preserve a static charge you need much better insulation than that. A leakage current of 1 μA would discharge a 1 nF capacitance charged to 1000 V in one second.

To get charged in the first place it would need something to be charging it faster than it was losing charge. When you walk across the carpet and touch the TV maybe you'd charge it up instantly. Then you sit and watch for a few hundred seconds while that charge leaks away, before touching it again to recharge it. The charge is never likely to build up.

While you're questions have raised some interesting comments, I can't help feeling that your fundamental worry is misplaced. You have bought an isolating transformer, whose main purpose is to prevent your getting electric shocks and your equipment getting damaged by line faults perhaps. So then you ask how that transformer can fail in its primary objective. It is very unlikely that it will fail. It is the solution of choice for this application, so much so that it is a legal requirement in many workplace situations. That wouldn't be the case if this were just some unreliable gimmick. So you're now asking us to come up with some outlandish extreme circumstance which might circumvent or prevent this transformer from doing its job. What is the point?

Last part of questions. You discussed a lot about static charge.. but what has it got to do with the transformer secondary floating to any potential. I mean how does this static charge related to the isolation transformer secondary floating and getting any potential value? When you touch the door knob. The transformer is elsewhere, unless you mean the transformer secondary can take up static charge and become millions of volts in potential? Does the transformer absorb static charge via it's two leads or the windings? How does it able to absorb it by itself since it is not moving or can touch any knobs.

And you missed this question. For a similar rated 500VA 240v-120v step down isolation transformer vs 500VA 240v-120v step down autotransformer. Are their primary windings same sizes? Or could either one be larger and why?

Thanks a lot!

gneill
Mentor
A grounded device won't develop a static charge with respect to ground. The ground connection provides a path for that charge to escape. An isolated circuit has no path to ground, so static charge has a possibility to build up (but that requires that some effort like rubbing the device with silk or wool, or other ways and means of depositing a charge on it). The total amount of charge will be very small despite the potential, due to the tiny capacitance of isolated bodies. Also, millions of volts is out of the question. Maybe 12 kV or so in a typical household environment with relatively dry air.

Step-down autotransformers typically use less copper in their construction, so they may be less expensive for a given application, but they do not provide isolation (from the mains); There's a physical connection between the mains and the load.

You will need to tell us about your local electrical codes with regard to ground and neutral in order for us to give you any ideas about doing something to mitigate static charge buildup on isolated devices.

**Note that we are not licensed to give electrical wiring advice for any jurisdiction!**, so we cannot give you (code certified) instruction on what to do for your particular case. We can only point you to information that you may find helpful in your own researches. Do be sure to read the Wikipedia article on static electricity and in particular the section titled "Removal and prevention".

A grounded device won't develop a static charge with respect to ground. The ground connection provides a path for that charge to escape. An isolated circuit has no path to ground, so static charge has a possibility to build up (but that requires that some effort like rubbing the device with silk or wool, or other ways and means of depositing a charge on it). The total amount of charge will be very small despite the potential, due to the tiny capacitance of isolated bodies. Also, millions of volts is out of the question. Maybe 12 kV or so in a typical household environment with relatively dry air.

Step-down autotransformers typically use less copper in their construction, so they may be less expensive for a given application, but they do not provide isolation (from the mains); There's a physical connection between the mains and the load.

You will need to tell us about your local electrical codes with regard to ground and neutral in order for us to give you any ideas about doing something to mitigate static charge buildup on isolated devices.

**Note that we are not licensed to give electrical wiring advice for any jurisdiction!**, so we cannot give you (code certified) instruction on what to do for your particular case. We can only point you to information that you may find helpful in your own researches. Do be sure to read the Wikipedia article on static electricity and in particular the section titled "Removal and prevention".

My electricity is identical to USA and I'm very familiar about the ground wire needing to be bonded to neutral only once at service entrance. I can easily connect the ground to the secondary output of the isolation transformer, but the isolation would be lost. Hence concerned how the potential could rise by its own. I was concerned the potential could even rise to the vev (vacuum expectation value) of the electroweak plasma potential and I don't want this happening near my room, but you commented it won't happen and the potential rise could occur by rubbing the transformer in the door knob and the amount of charge will be small.

But then, if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v?

gneill
Mentor
My electricity is identical to USA and I'm very familiar about the ground wire needing to be bonded to neutral only once at service entrance. I can easily connect the ground to the secondary output of the isolation transformer, but the isolation would be lost. Hence concerned how the potential could rise by its own. I was concerned the potential could even rise to the vev (vacuum expectation value) of the electroweak plasma potential and I don't want this happening near my room, but you commented it won't happen and the potential rise could occur by rubbing the transformer in the door knob and the amount of charge will be small.
No, an isolated body can gain or lose charge by rubbing with various materials (silk, wool, nylon, cat fur... the list is extensive) or by encountering another already charged body (yourself shuffling across a carpet). Nothing to do with the electroweak plasma potential is relevant here. At all.
But then, if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v?
Why would this be a concern if the transformer and its mounting is properly constructed?

I cannot provide any advice beyond what what I would surmise might happen in a given situation. I am not licensed to dispense instruction to guarantee conformance with your local electrical codes, and will not accept the potential ramifications of doing so. If this is a Do-It-Yourself (DIY) project where you are building your own step-down / isolation interface to the mains, I'd suggest consulting a local licensed electrician to approve your build.

No, an isolated body can gain or lose charge by rubbing with various materials (silk, wool, nylon, cat fur... the list is extensive) or by encountering another already charged body (yourself shuffling across a carpet). Nothing to do with the electroweak plasma potential is relevant here. At all.

Why would this be a concern if the transformer and its mounting is properly constructed?

I cannot provide any advice beyond what what I would surmise might happen in a given situation. I am not licensed to dispense instruction to guarantee conformance with your local electrical codes, and will not accept the potential ramifications of doing so. If this is a Do-It-Yourself (DIY) project where you are building your own step-down / isolation interface to the mains, I'd suggest consulting a local licensed electrician to approve your build.

Actually. I buy isolation transformer just to understand the concept of magnetic flux. In my country. We don't have any isolation transformer. We only have autotransformer. But as I learnt in this thread, autotransformer also has magnetic flux and same principle.

At least please share the hypothetical situation that if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v? The logic is that if you don't ground the secondary, and the neutral is bitten by rats and it touches a 240v source, the neutral can take that value... and secondary voltage may change, true? so if the secondary is 120v.. and one lead touches a 240v source, would the total become 360v or 240v? Of course I won't try it.

gneill
Mentor
At least please share the hypothetical situation that if one of the 120v leads of the secondary output of isolation transformer touches another 240v live, then the output would become 360v or 240v?
The potential difference variation between the transformer leads would remain 120 V. Your load device would remain happy. The potential of the "shorted" lead and the other lead with respect to your mains wiring neutral or ground would vary according to mains voltage (240 V). This would not affect the connected load, since it only "sees" the transformer leads. You would definitely have a problem with exposed wiring/chassis on your load device, since it'll share the potential of a "iive" wire. Not a happy circumstance.

The logic is that if you don't ground the secondary, and the neutral is bitten by rats and it touches a 240v source, the neutral can take that value... and secondary voltage may change, true?
No. The secondary is isolated. The neutral is on the mains side of the circuit. The rat will die, the GFCI will note the ground current and trip the breaker. The neutral is bonded to ground as you've already stated in a previous post, so it won't change potential w.r.t. ground (ignoring any small resistance in the bonding wiring). Your load is safe in this situation (in my unlicensed opinion). The rat is not safe of course .

The potential difference variation between the transformer leads would remain 120 V. Your load device would remain happy. The potential of the "shorted" lead and the other lead with respect to your mains wiring neutral or ground would vary according to mains voltage (240 V). This would not affect the connected load, since it only "sees" the transformer leads. You would definitely have a problem with exposed wiring/chassis on your load device, since it'll share the potential of a "iive" wire. Not a happy circumstance.

No. The secondary is isolated. The neutral is on the mains side of the circuit. The rat will die, the GFCI will note the ground current and trip the breaker. The neutral is bonded to ground as you've already stated in a previous post, so it won't change potential w.r.t. ground (ignoring any small resistance in the bonding wiring). Your load is safe in this situation (in my unlicensed opinion). The rat is not safe of course .

Oh I read that if the USA utility pole isolated transformer is not grounded. Then the neutral can take positive values as it touches live wire.. meaning it can reverse.. so the house neutral can become hot.. this is why they ground it at the pole and house service entrance. Similarly. If you have 3 leads in small 500Va isolated transformer and assume the middle is neutral but didn't ground it.. then the neutral can take hot value. So I guess this is importance of grounding the secondary of isolation transformer if the load would become complicated.. and also to suppress common mode surges and capacitive coupling.

I want to collect rare transformers just for collections. Most in the market now are the shell type. Know any commercially available small 500VA transformer that still uses the core type?

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gneill
Mentor
Oh I read that if the USA utility pole isolated transformer is not grounded. Then the neutral can take positive values as it touches live wire.. meaning it can reverse.. so the house neutral can become hot.. this is why they ground it at the pole and house service entrance. Similarly. If you have 3 leads in small 500Va isolated transformer and assume the middle is neutral but didn't ground it.. then the neutral can take hot value. So I guess this is importance of grounding the secondary of isolation transformer if the load would become complicated.. and also to suppress common mode surges and capacitive coupling.
If you're worried about what can happen outside of your service entrance, then there's nothing to give advice about other than to say that your wiring should meet local code specifications. This would involve engaging a local electrician/inspector. If secondaries remain isolated, nothing that happens to the primary side (other than voltage surges or dropouts) can affect it.
I want to collect rare transformers just for collections. Most in the market now are the shell type. Know any commercially available small 500VA transformer that still uses the core type?
Nope, not me. You might want to visit the Electrical Engineering forum pinned thread,

Beauty of old electrical and measuring things, etc.

and engage some of the members who hang out there.

If you're worried about what can happen outside of your service entrance, then there's nothing to give advice about other than to say that your wiring should meet local code specifications. This would involve engaging a local electrician/inspector. If secondaries remain isolated, nothing that happens to the primary side (other than voltage surges or dropouts) can affect it.

I'm not worried.. Just pointing out the analogies. In power utility poles, they also use isolation transformers.. and they ground the neutral. One very important reason is to get a grounded reference.. so equipment metal chasses with shorted live can trip the breakers. But i'm more interested in the other reasons.. why or how the neutral able to take on non-zero voltages. Here exactly the same analogy with small home 500va step down transformers (with difference the power lines can take lightning strike while the small transformer beside your tv may not). This is the reason I asked what potentials the leads can take. Because I read that in power lines, the neutral can get non-zero values. So next time in other forums. I'll simply ask about the reasons for grounding utility neutrals and use the same reasoning for small isolated transformer besides tv.

Nope, not me. You might want to visit the Electrical Engineering forum pinned thread,

Beauty of old electrical and measuring things, etc.

and engage some of the members who hang out there.

Thanks for all your help. 5 stars to you :)

I'm not worried.. Just pointing out the analogies. In power utility poles, they also use isolation transformers.. and they ground the neutral. One very important reason is to get a grounded reference.. so equipment metal chasses with shorted live can trip the breakers. But i'm more interested in the other reasons.. why or how the neutral able to take on non-zero voltages. Here exactly the same analogy with small home 500va step down transformers (with difference the power lines can take lightning strike while the small transformer beside your tv may not). This is the reason I asked what potentials the leads can take. Because I read that in power lines, the neutral can get non-zero values. So next time in other forums. I'll simply ask about the reasons for grounding utility neutrals and use the same reasoning for small isolated transformer besides tv.

What I was saying above was that if the neutral centertap in the secondary output of isolation transformation is not grounded. The Neutral can shift to Line 1 or Line 2, right. So without grounding.. say a resident user put the Line 1 to the ground, then it can become the neutral. And neutral would become hot. The scenario I mentioned above may not work, that is, neutral connecting to other live and it becomes hot. This is because if the neutral connects to hot, it is short circuit and trip the breaker or fry the primaries.

Before I was not so clear about floating secondaries potential. So far this thread makes it clear that

1. Sources of secondary potential floating is from static charges which is little and not permanent.. I thought it was like HUP that the potential can take on any value even the Electroweak potential. And since you can't measure the potential. I wondered yesterday why if the potential take on the electroweak potential, we don't feel any plasma in the room.

2. Even if static charges cause potential changes, the output leads would still be 120v. I though the output lead voltage would vary.

3. If one of the leads touches other hot wires from other transformer, the two leads would still be 120v and it is the potential of the other lead with respect to ground that can have potential differences with respect to the Primary neutral (not the secondary which doesn't have it assuming it's not grounded).

4. And above... only if one of the leads are referenced to ground that it becomes the neutral. So without grounding the centertap, the neutral can shift to the side (L1 and L2 above). Please comment if the above scenario is correct.

At least I don't have to worry about my flat screen TV exploding from potential increase to millions of volts or even the GeV. And don't worry about me trying to change the utility grounding or terminals (of course I won't do it.. i'm afraid of getting shock). I just want theoretical understanding of it. Thanks a lot for those who helped.

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What I was saying above was that if the neutral centertap in the secondary output of isolation transformation is not grounded. The Neutral can shift to Line 1 or Line 2, right. So without grounding.. say a resident user put the Line 1 to the ground, then it can become the neutral. And neutral would become hot. The scenario I mentioned above may not work, that is, neutral connecting to other live and it becomes hot. This is because if the neutral connects to hot, it is short circuit and trip the breaker or fry the primaries.

Before I was not so clear about floating secondaries potential. So far this thread makes it clear that

1. Sources of secondary potential floating is from static charges which is little and not permanent.. I thought it was like HUP that the potential can take on any value even the Electroweak potential. And since you can't measure the potential. I wondered yesterday why if the potential take on the electroweak potential, we don't feel any plasma in the room.

2. Even if static charges cause potential changes, the output leads would still be 120v. I though the output lead voltage would vary.

3. If one of the leads touches other hot wires from other transformer, the two leads would still be 120v and it is the potential of the other lead with respect to ground that can have potential differences with respect to the Primary neutral (not the secondary which doesn't have it assuming it's not grounded).

4. And above... only if one of the leads are referenced to ground that it becomes the neutral. So without grounding the centertap, the neutral can shift to the side (L1 and L2 above). Please comment if the above scenario is correct.

At least I don't have to worry about my flat screen TV exploding from potential increase to millions of volts or even the GeV. And don't worry about me trying to change the utility grounding or terminals (of course I won't do it.. i'm afraid of getting shock). I just want theoretical understanding of it. Thanks a lot for those who helped.

Let me verify if I understand the concepts with regards to floating potential very well.

When you first turn on the transformer and its building the magnetic field.. and you plug a load to the secondary side after a minute. And there is no stray static field. The potential would start from zero, right? Or is it undefined (related to quantum field potential) and it can start at any value? (I mean is it based on classical ignorance or inherently unknown similar to quantum randomness? don't blame me from asking this because the electric field is also quantum in essence)? If you still insist it can start at any random value. Then it's not far from HUP and can still reach random potential even millions of volts? Or is it undefined because you can't measure it but assuming the potential is zero. So imagine the graph starts from zero potential and fluctuating up and down perhaps due to the capacitive coupling of the human body, etc. passing by it or inherent quantum nature of fields (like electric field)?

anorlunda
Staff Emeritus
When we get to HUP in a discussion about basic electricity, the thread has gone on too long.

dlgoff