home made isolation transformer-weird scope reading

[hitachi8]

hello,
i made an isolation transformer yesterday by connecting to identical transformer wall~120-12#12-120~out.
while trying to see the final waveform on my scope i saw something very weird that i dont understand.

here's some picture:
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Neutral side of the Iso Transformer

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Hot side of the Iso Transformer

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Hot side of the wall-same volt/div setting on the scope

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Hot side of the wall- volt-div adjusted

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can someone explain?

[tekfan]

On the Isolated side of the transformer you need to put the ground clip onto either hot or neutral. It doesn't matter since it is fully isolated from the mains via the transformer. Put the probe then on the other wire of the iso transformer. If you don't put the scope ground anywhere the scope won't know what the 0V reference should be.

Warning: Don't try this on the non isolated wall side. If you mistakenly put the ground clip on the live wire you will get nasty sparks, destroy your probe and probably trip the circuit breaker in your house.

[hitachi8]

the ground probe clip was connected on the ground (wall side).
the same ground that the scope and the iso transformer masses are connected to.

[IanB]

the ground probe clip was connected on the ground (wall side).
the same ground that the scope and the iso transformer masses are connected to.

That's fine on the primary side of the transformer, but it won't work on the secondary (isolated) side because it is isolated from ground. To have a voltage difference to measure you need to have a complete circuit. If you don't have a complete circuit there is no voltage. The entire purpose of an isolation transformer is to break the circuit and safely isolate the secondary side from the primary side. So attaching one probe to mains earth and one probe to the secondary of the isolation transformer should produce (ideally) zero volts. In practice there is a bit of residual voltage but it should hardly register.

[IanB]

On the Isolated side of the transformer you need to put the ground clip onto either hot or neutral.

Reading this prompts me to mention that on the isolated side of a transformer there is no way to label either wire as hot or neutral. Each wire is equally as dangerous as the other.

[hitachi8]

thank you for your help! i now understand what i needed.

[mkissin]

Reading this prompts me to mention that on the isolated side of a transformer there is no way to label either wire as hot or neutral. Each wire is equally as dangerous as the other.

While this is very true, I would add that it is always best to also treat your house neutral wire as being live. There are numerous things that can put dangerous voltages onto it, not the least of which is the electrician having wired it incorrectly.

[Zero999]

Yes the neutral should never be presumed to be totally safe as it could break causing it to float at the mains potential but that's not the point. With an isolation transformer, both conductors have the same level of danger. With the mains, the neutral is slightly safer because it's normally near 0V which is why switches and fuses are always placed in series with the phase conductor.

I suppose one could cause the phase conductor from the transformer the one which is in phase with the mains.

[Computeruser]

>>> While this is very true, I would add that it is always best to also treat your house neutral wire as being live.

Where would this be?

While I truly understand the need from complete safety and practice such all the time, my house neutral has a number 8 copper wire from the incoming fuse box clamped securely on the water pipe through the basement floor into the earth.

From there, I ensure all outlets in the house are 3 prong and that ground and neutral are both properly grounded.

Just for clarity, there are 3 wires coming into the house: neutral and 2 live (120V each side). It is the neutral that is clamped to the water pipe.

So at the end of a run, there may be a slight (<< 1 V) on the neutral, it can always be clamped to ground.

Again, I practice complete safety. ... C

[IanB]

Where would this be?

While I truly understand the need from complete safety and practice such all the time, my house neutral has a number 8 copper wire from the incoming fuse box clamped securely on the water pipe through the basement floor into the earth.

The particular wiring arrangements in your house may not be the same for every person in every house. In some places the earthing arrangement you describe might even be against code. Since this forum has an international readership, it would be best to assume nothing about the potential on the neutral wire or any mains conductor (if there is even neutral wire at all--sometimes there might only be two live wires, like the 240 V conductors in your house).

[Computeruser]

>>> The particular wiring arrangements in your house may not be the same for every person in every house.

This is an interesting discussion. Thank you. I made certain every outlet in the house is properly grounded. I have very good tester to prove to myself that this is the case for all persons and every outlet.

The eathing arrangements are Code in Ontario, Canada and everywhere nearby where I have lived (New York and Connecticut).

Howver, I also understand your point about this being an international forum.

Thank you, .... C

[ejeffrey]

In the netherlands for instance plugs are not polarized and live and neutral are wired randomly.  The wire you think is neutral may in fact be live. The whole wiring system here is absurd, but my point is you are not just talking about backwater villages. 


However, even so, you have to treat the neutral mostly the same as live (not exactly the same: in the US you normally fuse only the live wire and are not required to switch the neutral wire).   It can and will develop some voltage relative to ground since it carries the load current.  This should rarely be more than a few volts in the US, but I understand that in some European countries the neutral-to-ground connection is shared by several houses on the same distribution circuit, rather than at each individual house.  Even though the voltage is rarely at a dangerous level, you are *not* allowed to connect the neutral to ground anywhere except at the designated point.  That means that (for instance) connecting your oscilloscope ground clip to neutral is a big no-no.  It is also likely to damage your scope.  The voltage present is small, but if you give it a low impedance path back to ground, a significant fraction of the circuit current will take that path.

[alm]

Another issue with treating neutral as less dangerous is that a wiring fault reversing live and neutral may not be noticed for a long time. Any other single fault will result in the equipment not working or the GFCI tripping. A broken ground wire will also remain unnoticed, but at least a second fault is necessary for it to shock someone. It is one reason why double isolated is safer than grounded equipment for consumer applications, though.

[mkissin]

At the end of the day, it's your personal safety at risk and you should take all reasonable precautions around mains wiring.

[vk6zgo]

On the Isolated side of the transformer you need to put the ground clip onto either hot or neutral.

Reading this prompts me to mention that on the isolated side of a transformer there is no way to label either wire as hot or neutral. Each wire is equally as dangerous as the other.

Exactly!,but of course,the only danger involved on the secondary side is becoming connected across both sides of the transformer at once,which is less likely than becoming connected between the primary active & any earthed object,OR the neutral.

The OP is doing the right thing by measuring the primary voltage between active & earth,as in most cases the voltage is almost the same as between active & neutral.(If you use an analog VOM that way,however,the RCD(earth leakage detector) will drop out because the VOM draws enough current to trip it! )

With a normal Live Earth Neutral 120volt & 220/240volt system this will be almost the same as the voltage across the transformer primary,BUT--with an American 240v system,you would see 120 volts  between earth & both ends of the primary,so the primary voltage would be  120v+120v=240v.

He got hopelessly lost on the secondary side,as if you could still read volts between one side of the secondary & earth,it wouldn't be an isolation transformer!
VK6ZGO