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Old 28th Dec 2018, 10:27 pm   #1
Cathode Ray
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Default Variac first or Isolation first ?

Folks,
I've managed to source a 2KVA variac and a 500VA isolation transformer.

What's the recommendation connection sequence?
I've seen people do it both ways.
Some have variac connected to the mains followed by isolation trafo on its output.
Others have the reverse, i.e. isolation connnected to mains.

I'm worried the large 10A variac could trip my mains fuse on power up (remenance effect), so would putting the 500VA (2 amp) isolation first minimise this risk ?

Thanks
Ray
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Old 28th Dec 2018, 11:12 pm   #2
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Default Re: Variac first or Isolation first ?

Isolation transformer first every time.
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Old 29th Dec 2018, 1:16 am   #3
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Default Re: Variac first or Isolation first ?

Originally my combination of isolation transformer followed by variac would trip the main B20A breaker feeding the socket, and for a while I fed the transformer off the variac to lessen the surge.

Then I fitted an NTC surgeguard to the transformer and it's cured that, so that is plugged in first.
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Old 29th Dec 2018, 9:45 am   #4
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Default Re: Variac first or Isolation first ?

Thanks guys,
I agree the isolation should be first for safety reasons.
But I've seen people like Mr Carlson's and some others put is second, perhaps they have their reasons.
I was considering a soft start using a relay but the NTC is a better idea, especially since my isolation trafo is only 500VA, I think I've already got a 3A NTC from a switchig PSU which will do nicely.

Cheers
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Old 29th Dec 2018, 10:33 am   #5
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Default Re: Variac first or Isolation first ?

I have has a 1000VA isolation transformer that ate the switch.
They do make the lights flicker some times at switch on.
They need a much higher rated switch. The 13 amp fuse appears to be fine.
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Old 29th Dec 2018, 11:20 am   #6
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Default Re: Variac first or Isolation first ?

Quote:
Originally Posted by Refugee View Post
They do make the lights flicker some times at switch on.
This may explain why its only 'sometimes' ...
https://electronics.stackexchange.co...use-at-no-load

It depends on where in the cycle the power is applied.

A correctly rated NTC Thermistor or soft-start circuit should sort it out.

See diagram, this is a soft-start circuit from an Ameritron HF amp.
D1, R3 and C3 delay the closing of K2, allowing R4 and R5 to limit the surge for a few cycles.

Ray
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Old 29th Dec 2018, 12:54 pm   #7
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Default Re: Variac first or Isolation first ?

It's not clear-cut.

Suppose you go TX -> Variac

On the assumption you are using the isolation TX in the usual workshop way - that is, to allow the connection of earthed test equipment to the "live" parts of a DUT - then you will not be connecting through the earth connection. The reasons for that have been covered here many times in the past.

But as a result, the metal body of the variac will now be floating. Or will it? What happens when you connect your earthed test equipment to some point of the DUT? Or suppose there is a fault in the variac or the DUT?

It all feels rather risky to me. When I do this - which I admit to doing fairly often - I'm always very aware that something could go wrong with this, and I avoid touching the metalwork of my variac. Easy to do, as it's mounted on a lump of timber. Or I actively ground it when I know that the DUT is definitely Class 2.

OK, suppose you go Variac -> TX

This is better because it provides soft-start for the TX and allows the body of the variac to be correctly earthed, but what happens if you wind the variac up past 230V? The core of the transformer will saturate, causing buzzing and overheating or worse. Unless you pay extra for a low-flux design, all standard-grade transformers made today are designed to be on the limit in normal operation - copper and iron has never been more expensive - and don't like being over-run.

Anyway, I've arrived at the conclusion that there is no 100% safe way to reliably run a variac and isolation transformer as two separate boxes, and have been planning to rebuild my isolation transformer and variac into a nice metal case that combines both functions safely. I have the box and everything else I need (including some nice metering) - I just haven't had the time to do it.
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Old 29th Dec 2018, 2:30 pm   #8
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Default Re: Variac first or Isolation first ?

Interesting points you make there.
My isolation transformer hums anyway, and draws about 500ma with no load.
It's a potted hospital type.
So I definitely won't be runing it up to 265v !

And my variac is an older type with a floating case, photo below.
The entire case is metal and floating.

So you overall suggestion to house both is probably the safest way to go.

Thanks for that.
Ray
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Old 29th Dec 2018, 6:10 pm   #9
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Default Re: Variac first or Isolation first ?

You say it's a 500VA isolation transformer and a 2kVA Variac.

For maximum usefulness put the isolation transformer FIRST. Here's why:

The isolation transformer, rated at 500VA, will be able to cope with a load of just over 2A, without the windings burning out - whatever the voltage - because that's what the wire gauge will have been chosen to handle.

The Variac brush (usually the limiting factor for a Variac) will be able to cope with just over 8A, as it's a 2kVA Variac.

So if you have Variac first, transformer second, you'll only ever be able to pull 2A from the combination, whatever the Variac setting. But if you have transformer first, Variac second, then if the Variac is set to give 250V you can pull 2A; at 200V 2.5A; at 100V 5A; at 62.5V 8A, (all these being limited by the 500VA rating of the transformer), and then anything below 62.5V you can draw up to 8A (the Variac limit) without overloading anything.

Which, I think you'll agree, is more useful and flexible!
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Old 29th Dec 2018, 6:21 pm   #10
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Default Re: Variac first or Isolation first ?

Quite a few threads on this topic. One in September threw in a Lamp Limiter for good measure [perhaps] as well.

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Old 29th Dec 2018, 6:57 pm   #11
Cathode Ray
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Default Re: Variac first or Isolation first ?

So a little research and it turns out the trafo is drawing 330mA without any output load because it's a pure inductive load.
A Power Factor Correction capacitor can cure it.

Anyone care to estimate a capacitor value which may work ?
The trafo is rated at 500VA

Thanks
Ray
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Old 29th Dec 2018, 7:18 pm   #12
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Default Re: Variac first or Isolation first ?

As you have a variac, why don't you try seeing how the transformer current varies with applied voltage? Give it a go; you might be surprised

Hint #1: there might be something else happening here.

Hint #2: I've already mentioned it.

Hint #3: The solution is to replace the transformer.
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Old 29th Dec 2018, 7:30 pm   #13
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Default Re: Variac first or Isolation first ?

I think your hinting that the transformer is saturated , which explains the hum and current drawn.
So the transformer is low efficiency, and I should probably get a better one.

How close am I ?

Would a PFC cap help reduce be the current in some respects, but not fully due to the inefficient core?
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Old 29th Dec 2018, 7:47 pm   #14
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Default Re: Variac first or Isolation first ?

Well, don't take my word for it; do the experiment

I've got a couple of 500VA transformers here that take about the same current. I think that's normal for the breed, sadly. As they are only used intermittently*, it's not a big problem. However, I wouldn't want to use them to run isolated bench outputs on a full-time basis (something that some favour) because that would just waste energy and the noise would drive me mad.

*: As isolation transformers are only needed to allow the safe connection of earthed test gear to the "live" parts of a DUT, it follows that most of the time, they are unnecessary - so only use them them absolutely required, and rely on the RCD for the rest of the time.


Here's another question:

Have you figured out how many radios you can safely power from your 500VA isolation transformer?
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Old 29th Dec 2018, 9:39 pm   #15
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Default Re: Variac first or Isolation first ?

I'll have a go at that one...

The trafo is about 85% efficient, it draws 75w on no load, and it's a 500VA rated trafo, so 75/500=15% loss
Assuming 500VA is the continioulsy rated value that leaves 425 watts to play with. My Philips BX548 draws 65 Watts, so I think it could power 6 radios.
Do I get a ?

But like you said, I only want this for periodic testing. It's not intended for continuous bench.

Going back to the shed now to see how the current changes with voltage, I expect it's non-linear.
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Old 29th Dec 2018, 10:35 pm   #16
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Default Re: Variac first or Isolation first ?

A valiant effort

However, I'm afraid it was a trick question. Not intended to be cruel, but to help make a really important point:

Only ONE device may be connected to an isolating transformer at once.

This has nothing to do with power ratings, which is the usual thing that springs to mind. Rather, it is related to safety.

The devices might well be isolated with respect to mains earth, but as soon as you connect a piece of earthed test equipment to one of them, then that device is no longer floating. But, all the other devices stop floating at the same time. And depending on the exact circumstances, this could create all sorts of unexpected hazards.

The worst way this could manifest itself was in the old TV workshops that had a massive isolation transformer that powered many outlets simultaneously, with several engineers working on different sets all at once. All was fine until Engineer #1 needed to hook up a 'scope to observe a waveform on a set - this means that all the other sockets are now referenced to mains earth in some way. If you were unlucky enough to touch Live or Neutral, then you'd be in with a chance of getting a shock - that, of course, is the risk that many people believe the isolation transformer is meant to eliminate (never make that assumption!).

Moreover, suppose Engineer #2 needs to connect his 'scope to the chassis of the set in front of him. This could be lethal - if set #1 uses a full-wave rectifier and set #2 uses a half-wave arrangement, then the fault current flowing through the earth path (via the ground clips of the 'scope probes) could be massive - assuming you manage to make that connection without getting a serious shock in the process.

It's a miracle that more people didn't die back then. It's also amazing just how much misunderstanding existed back then, and persists today.

A document that I frequently refer to is attached to this post: https://www.vintage-radio.net/forum/...8&postcount=18 - it covers isolation transformers in workshop scenarios in decent detail. Well worth a read if you haven't already seen it

Thinking about your workings, I'm no expert on transformer ratings, but I believe that a 500VA transformer should be able to provide 500VA at the output, so the "waste" energy should be added to the primary rather than subtracted from the throughput - so 575VA in, 500VA out. In practice, the losses will rise because of I-squared-R losses in the windings, amongst other things. And if the load is anything other than a resistive load, there will be de-rating to consider. Luckily, as the failure mode is thermal, they are fairly tolerant to short periods of abuse.

Hope this helps,

Mark
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Old 29th Dec 2018, 10:59 pm   #17
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Default Re: Variac first or Isolation first ?

Good stuff Mark, and I can't disagree with any of it.
You go me alright, I went head first into it!

Oh and managed to take some current readings with different Primary voltages.
60v = 60mA
100v = 110mA
150v = 180mA
200v = 270mA
230v = 350mA
A non-linear load as expected.
I also found this useful video,
https://www.youtube.com/watch?v=NLYbzrRna7Q&t=5s
This chap deserves more subscribers.
Using his forumla I caculate a 47uF cap is needed to make the trafo a 'real' load.
I may just try it and repeat the current test above.

Cheers
Ray
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Old 29th Dec 2018, 11:06 pm   #18
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Default Re: Variac first or Isolation first ?

300mA off load primary current won't be 75W, it'll be 75VA, of which only some will be watts. Mostly the watts will be the I^2(R) losses in the primary winding resistance.

The TX may only be 85% efficient at full load, but not all those losses will be primary side- there's secondary side losses and magnetic circuit losses as well. At no load, the TX will be 0% efficient- nothing out but some power in.

With the transformer first, you still can't have 500VA out- the no load VA of the variac will come off the total output from the TX.

If there's a worry about overvolting the TX when fed from the variac, most variacs I've met have an option to retap for 0-100% output rather than 0-110%.
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Old 29th Dec 2018, 11:42 pm   #19
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Default Re: Variac first or Isolation first ?

Quote:
Originally Posted by Raymo99 View Post
Good stuff Mark, and I can't disagree with any of it.
You go me alright, I went head first into it!

Oh and managed to take some current readings with different Primary voltages.
60v = 60mA
100v = 110mA
150v = 180mA
200v = 270mA
230v = 350mA
A non-linear load as expected.
I also found this useful video,
https://www.youtube.com/watch?v=NLYbzrRna7Q&t=5s
This chap deserves more subscribers.
Using his forumla I caculate a 47uF cap is needed to make the trafo a 'real' load.
I may just try it and repeat the current test above.

Cheers
Ray
Hi Ray,

Your results don't show a distinct "knee" from saturation, but I should explain that my experience here comes from toroidal transformers, which tend to be more abrupt in their saturation behaviour. This looks more like hysteresis and eddy current loss rather than saturation.

I'd measure my 500VA types that I mentioned earlier (those are conventional types) but my variac is on duty in the lounge (for the incandescent Christmas tree lights, believe it or not! ).

I'm not sure I'd bother to add any sort of PFC cap - what will it achieve? The transformer will still run hot and buzz. If the transformer follows the variac, I don't think it'll make any material difference to the variac. Plus, a 47uF cap that is rated for mains duty is likely to be an expensive beast.

Cheers,

Mark
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Old 30th Dec 2018, 12:16 am   #20
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Default Re: Variac first or Isolation first ?

Thanks Chris and Mark,
I'm happier now that I've managed to run it up and make some measurements.
I was fearful it would cause mains trips, but none so far.
The variac also makes a slight hum, but not as loud as the isolation trafo.
I won't bother with PFC, it's fine for the odd chassis test, that's all it's for.

Chris, you points about my unloaded measurements are well made.
I'll just accept that it's not super efficient, but still gets the job done.

Photo of my efforts below , it won't win any prizes but it works.

Cheers

Ray
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