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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 5th Oct 2019, 12:30 pm   #1
tritone
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Default 1:1 isolation xfrmr secondary V too high ?

Dear forum,

I have a 230V, 250VA isolation transformer that is supposedly to be a 1 for 1 ratio, @UK mains voltage here where I am, I have the mains 240v giong to the pri but I am getting 254v on the output secondary side.

Would there be any easy enough way that I could correct this somehow by myself, to get the secondary voltage down to 240v as per 1:1 without using a variac and stripping the transformer to the windings to modify it ?

With the middle jumper wire removed on the secondary side terminal block I get a nice 120v between the two outer terminals, please see the pic below for the taps.

I am assuming that the integrity of the actual copper windings are in good shape and not shorting out somewhere within to give a wrong 1:1 ratio, but I could be wrong.

Any ideas please, Thank you.
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Old 5th Oct 2019, 12:41 pm   #2
PJL
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Default Re: 1:1 isolation xfrmr secondary V too high ?

This will be to compensate for the voltage drop when under load. You can use a standard low voltage mains transformer to buck the input or output voltage. Search for 'bucking transformer'.
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Old 5th Oct 2019, 12:57 pm   #3
Trevor
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Default Re: 1:1 isolation xfrmr secondary V too high ?

Turn the transformer around use the primary as the secondary and see what you get
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Old 5th Oct 2019, 1:19 pm   #4
tritone
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Default Re: 1:1 isolation xfrmr secondary V too high ?

Thanks for your replies fellas,

OK I search for bucking transformer PJL.

Hello Trevor, I turned it around and I get 228v at the pri side ? What do you think ?
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Old 5th Oct 2019, 2:26 pm   #5
Radio Wrangler
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Default Re: 1:1 isolation xfrmr secondary V too high ?

It shows that the transformer designer made it a slight step up ratio not true 1:1 to compensate for copper losses at full load, as PJL said.

With it reversed, the ratio becomes a slight step down, as your measurements show.

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Old 5th Oct 2019, 4:06 pm   #6
TonyDuell
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Default Re: 1:1 isolation xfrmr secondary V too high ?

I think if there were any shorted turns in the windings then the transformer would run very hot and maybe blow the fuse on the input side. You don't mention anything about this, so I assume the transformer does not get hot (with no load).

If you have a lamp limiter, run the transformer off that. With no load on the secondary of the transformer, the lamp should be dark. Shorted turns would cause it to glow.

But I agree with the others, this is most likely a design feature of the transformer to compensate for losses when it is loaded.
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Old 5th Oct 2019, 4:32 pm   #7
Trevor
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Default Re: 1:1 isolation xfrmr secondary V too high ?

I agree re the ratio being out and possibly corrected for full load operation !
What do intend to use it for ?
I would tend to use it in the reducing mode as most equipment in europeis designed for 230 volt operation
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Old 5th Oct 2019, 5:09 pm   #8
tritone
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Default Re: 1:1 isolation xfrmr secondary V too high ?

Hiya guys,

Your replies has totally cleared up my concern and question, I understand now, and yes I remember now from way back in my electronic studies about transformer loading.
I think I need to dig out my old Hughes Electrical Technology book back out and re-read up on a lot of stuff again,

Hi TonyDuell, I'm going to give that lamp limiter test a go, thats a great piece of info you have shared with me.

Thank you to all of you. Excellent info fella's !
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