Isolating Transformer

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IslandPink
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#31 Re: Isolating Transformer

Post by IslandPink »

Nick wrote: Sat Aug 04, 2018 8:48 pm Well, assuming the casework was not earthed, then with a single device (and ignoring leakage current) there is no way the case could go "live" as there is no current path from the secondary live or neutral to ground. Its why a site isolation transformer is used. You should be able to jump into a swimming pool holding a power tool and the transformer will keep you safe.
Yeh but with most power tools the casework is plastic and the electrical bits are buried inside.
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#32 Re: Isolating Transformer

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IslandPink wrote: Sat Aug 04, 2018 10:40 pm
Nick wrote: Sat Aug 04, 2018 8:48 pm Well, assuming the casework was not earthed, then with a single device (and ignoring leakage current) there is no way the case could go "live" as there is no current path from the secondary live or neutral to ground. Its why a site isolation transformer is used. You should be able to jump into a swimming pool holding a power tool and the transformer will keep you safe.
Yeh but with most power tools the casework is plastic and the electrical bits are buried inside.
But not normally water tight.
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#33 Re: Isolating Transformer

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Indeed and I can see that in that case the power coming out of the transformer secondary is dissipated in the water between the anything connected to the + and - sides of the input. But if you have an amplifier case made partially or entirely from metal and a wire comes loose - what then ?

I agree btw about the inter-winding capacitance being crucial to any human contact - I suppose the impedance of the interlayers at 50Hz needs to be a lot more than eg. 200k which is a typical human resistance with dry hands.
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#34 Re: Isolating Transformer

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Ok, worst case, you do what Steve's diagram shows. You stand on a earthed metal plate in bare feet, with one hand on a water pipe. And the other hand touches one of the secondary wires. What happens?
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#35 Re: Isolating Transformer

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Quick calc about the intertwining capacitance. I think 13nf would give you 1ma with 240v 50Hz.
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#36 Re: Isolating Transformer

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Nick wrote: Sun Aug 05, 2018 12:09 am Ok, worst case, you do what Steve's diagram shows. You stand on a earthed metal plate in bare feet, with one hand on a water pipe. And the other hand touches one of the secondary wires. What happens?
Secondary winding slides up or down in potential on the core, the end you touch goes to zero volts as long as the inter-winding capacitance is very low - eg. with split bobbins.

So, you connect a power amp to this, signal circuit floating and case not grounded. If you connect a DAC or phono amp with a signal (DC) ground into the power amp, this pins the whole power amp circuit to ground on one side and the interconnects and signal wires now become the route to earth. Which is 'far from ideal' as Harry Hill would say.
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#37 Re: Isolating Transformer

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But I don't see how the non fault case is any different to normal. Live and neutral on the power input of any mains powered hifi kit will be isolated from the rest of the system and if used case ground by it's transformer.
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#38 Re: Isolating Transformer

Post by IslandPink »

I think I understand that sentence ....
Reading through in detail again, Steve did what I would have wanted with the mains earth, and so I'm happy and it looks like he's happy too - result :D
Last edited by IslandPink on Sun Aug 05, 2018 3:45 pm, edited 1 time in total.
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#39 Re: Isolating Transformer

Post by Nick »

Just measured the capacitance between the primary and secondary of a 50va toroid at just over 500pf
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#40 Re: Isolating Transformer

Post by Cressy Snr »

The truth is that I built the transformer project with the earth bridged through in the first place. Then I read that the earth should not be connected through, so disconnected it, got worried, got confused, so kept looking for information until I found the dire warning about multiple devices. IOW I did what worried people do and that is to search and search until I found something eventually, that confirmed/reinforced my fears. The prompt to search for info was a direct result of the post below by Mark:
IslandPink wrote: Sat Aug 04, 2018 5:58 pm I'm just trying to understand how it's safe - if you touch something that's HT in the amp, does it just go to 0V and hence the isolator secondary just flaps up and down in potential at 50Hz ?
Turns out (doing more research) that whether you connect the earth through to the other side is dependent on what you want your transformer to do.

You can ground one side of the secondary and re-create a neutral, to make an entirely new isolated mains, keep the secondary isolated but connect earth through and power a load of devices whilst filtering out hash, or not bridge the earth and power a *single* device and keep safe outdoors, or keep safe when doing workbench diagnostics on a faulty device.

There is a story I found that two techs were working next to each other off one isolation TX, on a faulty radio set each and everything was fine until one guy passed the other a scewdriver, both were touching the chassis of their radio. Both were electrocuted. How true this is I don't know, but it was enough to make me shut the project down until I could get that earth path reconnected.

Anyway, it all works now.
You'd have thought I'd have learned by now, but it's all very well having unlimited access to information, but it's knowing which information applies to your situation and which does not that requires the skill.
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#41 Re: Isolating Transformer

Post by Cressy Snr »

Finally found something that is clear, straightforward and unambiguous re isolating transformers.

http://www.reo.co.uk/files/safety_8_-_i ... ormers.pdf

Looking at the last para in that document, it would seem that Phil got a great buy with those 1KVA trannies.
Incoming HF hash ought to be kept well out of the picture by those babies. :)
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#42 Re: Isolating Transformer

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I'm pleased that those who took advantage of my "great buy" are happy with their purchase. :D
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#43 Re: Isolating Transformer

Post by Ray P »

I've got one from Phil's purchase hereabouts that I bought off Greg, must treat this as a reminder to do something with it.

Steve, are you just using the transformer or do you have a DC blocker/RF filter/whatever on the 'input' side?
Sorry, I couldn't resist!
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#44 Re: Isolating Transformer

Post by Cressy Snr »

Ray P wrote: Sun Aug 05, 2018 6:34 pm
Steve, are you just using the transformer or do you have a DC blocker/RF filter/whatever on the 'input' side?
I'm just using the transfomer.
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#45 Re: Isolating Transformer

Post by Ray P »

Cheers Steve.

When I got the transformer from Greg it had an article on building a DC blocker and RF filter on the front end - anyone got any views on whether its a worthwhile exercise?
Sorry, I couldn't resist!
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