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Old 24th Sep 2020, 1:52 pm   #1
MartinMarris
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Default Isolation Transformer

For most 405 TVs from the 50s, is a 250VA isolation transformer sufficient or would it be safer to go for 400VA or 500VA? I am leaning toward the latter, just to give a nice margin. Of course the sets almost always have a power rating on the back panel.

Am I right that if a set is operated through the transformer, the secondary of the transformer has no polarity, i.e. there is no longer such a thing as "neutral" and "live"?
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Old 24th Sep 2020, 2:12 pm   #2
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Default Re: Isolation Transformer

Hi Martin,

You are correct in assuming that there is no "neutral" or "line" as neither side of the transformer secondary should be connected to earth in any way.

The majority of B&W sets will consume around 140 - 180 watts so your 250VA transformer will be fine. (My own one is less than this and I work on colour sets with it! Decca Bradford etc.)

Cheers
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Old 24th Sep 2020, 3:18 pm   #3
ajgriff
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Default Re: Isolation Transformer

It's really important to understand the 'safety versus risk' implications of using an isolation transformer and the reasons for doing so. There are many past threads on the subject and it's well worth searching for them and studying them carefully.

Alan
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Old 24th Sep 2020, 4:00 pm   #4
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Default Re: Isolation Transformer

Go for the big transformer. There is a reason that's often forgotten about.

The 250W transformer is rated to be OK delivering 250W into a RESISTIVE load that takes a nice sinewave current.

TVs are not nice resistive loads. They don't take current spread across the full waveform, their rectifiers take it in shorter, harder pulses when the mains voltage goes above the reservoir voltage. The heating in the transformer and the voltage drop due to resistance are both exaggerated by this pulse-iness.

Then there is the really nasty bit.

Older tellies (valved ones) used half-wave rectifiers and so took current on only one half-cycle of the mains waveform... essentially the DC taken by the set flows in the transformer supplying the set. This was a bad enough problem for substation transformer designers with loads of sets ganging up on it, but by playing tricks with 3-phase systems they could cancel some of it out. With your single load and single transformer you can't play these games.

You need a good enough transformer to still work properly and not go part way into saturation despite the DC from such a set. Few isolating transformers actually have any spec or even any mention of DC components in the load current. Mine's rated at 600VA.

The safety aspects of using an isolating transformer were done to death in a recent thread, so no need to reopen that can of worms again. So I've only commented on transformer rating and problematical load characteristics.

David
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Old 24th Sep 2020, 4:11 pm   #5
MartinMarris
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Default Re: Isolation Transformer

Quote:
Originally Posted by ajgriff View Post
It's really important to understand the 'safety versus risk' implications of using an isolation transformer and the reasons for doing so. There are many past threads on the subject and it's well worth searching for them and studying them carefully.
Alan
Quote:
Originally Posted by Radio Wrangler View Post
The safety aspects of using an isolating transformer were done to death in a recent thread, so no need to reopen that can of worms again. So I've only commented on transformer rating and problematical load characteristics.
David
Thank you both for bringing my attention to that. I do not think of the isolation transformer as a panacea, and would not want to shirk any of the usual safety procedures. I do use vintage test equipment (scope, VTVM and signal generator, all from the 1950s) that is not earthed, but I am still wary of any inadvertent earthing while connecting test equipment to a live-chassis TV.

I have read some of the threads on that topic and will read some more!
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