Isolation Transformer for DUT Recommendations

L0rdGwyn

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2019-12-14 6:20 pm
Cleveland, Ohio
Would someone be willing to recommend me an inexpensive isolation transformer for ground isolation / safety? I am putting together my first full-scale DIY amplifier building bench and I am taking all of the necessary safety precautions.

I have looked into several isolation transformers, but it seems that inevitably, each one has some fatal flaw (no pun intended), whether that be no true ground isolation, grounded neutral, etc. Any recommendations would be appreciated, thanks :)
 
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Most devices commercially available have their own isolation barrier, so you don't need extra isolation.

But if you still want it, you must add the power consumption of the total devices to be powered, add a safety margin for future expansions/modifications, and see in the booklets of the manufactures, one that satisfies this conditions.
 

L0rdGwyn

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2019-12-14 6:20 pm
Cleveland, Ohio
Excepting some old AC/DC radios and TV's, almost any modern device has a transformer for line isolations and voltage conversion. Your amp is specified to be AC powered? If this is the case, it has a transformer inside it. I discard SMPS because they are rare in industrial manufactured tube amps.

Yes, it will be AC powered. The chassis will be at earth ground without ground isolation, meaning I will be at the same potential. Perhaps I am misunderstanding.
 

L0rdGwyn

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2019-12-14 6:20 pm
Cleveland, Ohio
Inexpensive isn't going to happen, you are paying for a big chunk of copper. Unless you can find one using aluminium wire.

*Relatively inexpensive, to serve its intended purpose without being a premium product, whatever that cost may be, I am willing to pay it for the sake of safety. Although Osvaldo does not seem to think it is necessary. I am a novice, so maybe it isn't, but everything I have read to this point has recommended it as a safety precaution.
 
OK, yes, I am certain that it isn't needed for such a kind of devices, but feel free to do what you want. I give my own opinion as the Engineer I am, but it isn't determinant.

Redundant isolation are not needed for simple devices like an audio amplifier.

Perhaps if you can find a good CVT you will have three advantages: extra isolation, good sine wave for the amp., and current limiting. But I don't know if they are still being manufactured.
 

L0rdGwyn

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2019-12-14 6:20 pm
Cleveland, Ohio
OK, yes, I am certain that it isn't needed for such a kind of devices, but feel free to do what you want. I give my own opinion as the Engineer I am, but it isn't determinant.

Redundant isolation are not needed for simple devices like an audio amplifier.

Perhaps if you can find a good CVT you will have three advantages: extra isolation, good sine wave for the amp., and current limiting. But I don't know if they are still being manufactured.

Fair enough, I am not an engineer and I respect your expertise. If it is not needed for saftey, then I will not pursue it, and it will save me some cost. I do wonder then why it is so commonly recommended if this isolation is inherent to the device's mains transformer.
 
There are very few reasons you would need an isolation transformer, and even those situations typically need the output neutral connected to protective earth - but all situations need competant assessment due country codes and appreciating all the reasons. Some example situations are:

Some vintage equipment, such as where there is no internal isolation transformer, or some parts connect to mains side that have suspect isolation.

Some power supply toroids don't like low levels of dc.

Some equipment is susceptible to noisy neutral from other nearby equipment.

Some equipment is susceptible to conducted mains noise.

If you have a noisy earth distribution, or earth related hum/noise loops, then that is a different beast.
 
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L0rdGwyn

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2019-12-14 6:20 pm
Cleveland, Ohio
Thanks trobbins, my use case is not on your list. Having done much reading on safety precuations taken when building/testing a valve amplifiers, I often came across recommendations to float the mains ground via an isolation transformer. It seems that was not accurate, which is fine by me as I will be saving myself some time and money :) thank you for your input.
 
I've read a lot about the pros and cons of isolation transformers, and not being an EI, I have to rely on what seems to be a consensus of authorities. One factor that may be present in this country that may not be in others is that our neutral line is tied to ground at the breaker panel.

Everything I've read and been told about the use of isolation transformers is to understand when they are actually isolating ground and not also tied to it, and when they are appropriate to use for safety reasons.

Having been convinced of the occasional need for one, I bought a really nice 700VA hospital isolation transformer off of the big auction site for about $100.
 
Here in Buenos Aires, neutral is grounded at the line cable run, each about 1000mts.

But as lines are in some cases 80-100 years old, they burn frequently because of lack of maintenance, sometimes you have at house about 170V between neutral and earth, 220V across line and neutral.

Once tears ago, before a T storm that impinged a substation, I had at house an oscillating voltage between 20 and 150V during 10 days (yes, ten days), until the company solved the problem in 3 minutes. My god!
 
I have a VTA-ST120, I believe is floating ground, not earth ground.

I believe this due to the following:
- Two prong plug (though neutral is tied to earth at the electrical panel)
- With amp unplugged and ohm meter attached to neutral prong I have probed the chassis and several areas of the circuit, including the negative speaker jack and never received a reading.
- I have done the above test with meter connected to other prong just in case I have something REALLY WRONG, same result.

It would appear the power transformer is acting as an isolation transformer for the entire audio circuit.

I would like to perform some tests using an oscilloscope WHICH IS earth ground reference.

I fear I may have missed something in the above probing.
I also fear damaging my scope.

- If the amp circuit is floating ground would it be safe to use an oscilloscope that is earth ground reference?
- Would it be safer to get an isolation transformer where the output is not earth ground reference for my scope; scope and amp would both be floating ground?
- Am I being overly cautious or missed something VERY OBVIOUS?

The amp is functioning without issue. My desire is to run a few tests such as THD, output, view wave forms at the speaker jack, etc.
This is purely for learning purposes.
I do have a function generator and dummy loads.
 
What is a VTA-ST120 and did you buy it new with a manual?

Equipment can have a double insulated input mains supply rating, such as many plug-pack and laptop bricks that have no protective earth pin with their AC mains plug. With such equipment (like a laptop) you can connect it to accessory equipment that comes with a power supply that has a protective earth connection back to the mains AC supply.

If you didn't buy the VTA-ST120 new, then there is a risk that someone has modified it. Some countries force any secondhand electrical equipment to have a notice attached to advise the new user to get the equipment tested before connecting to the mains AC, or alternatively to have had the equipment 'tested and tagged' prior to sale.