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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 12th Oct 2021, 4:19 pm   #1
radiomobile
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Default Determining the ratio of transformers

As a relative newcomer to this site I apologize if this tip has been posted before
To determine the ratio of an output transformer, inter-stage transformer or indeed practically any transformer, simply feed 1 volt AC into the secondary and the measure the voltage across the primary. This gives the ratio. Where do I get 1 volt AC I hear you cry? One can simply add a few turns to an old mains transformer. Choose one where there is a gap between the laminations and the outside of the windings, this avoids the need to dismantle the transformer Around 8 turns is a good starting point, adjusting the number of turns until 1 volt is obtained. If you have a transformer with 4v and 5v secondaries they can be wired is series opposition to give 1 volt. Elstone made mains transformers with 0-4-5-6.3v as well as ht windings for universal replacement purposes. The foregoing only applies to mains and audio transformers of course not high frequency types, switch mode etc.
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Old 12th Oct 2021, 7:35 pm   #2
kalee20
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Default Re: Determining the ratio of transformers

It's a good and simple method.

Though, you really need to measure the two voltages at the same time - your '1V' may sag a bit when loaded with a transformer - and also the mains may vary up and down a bit. When faced with something like this, I usually use two meters, take a photo, and then take the readings in the photo.
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Old 13th Oct 2021, 2:35 am   #3
Refugee
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Default Re: Determining the ratio of transformers

I normally use a scope and signal generator.
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Old 13th Oct 2021, 8:07 am   #4
GrimJosef
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Default Re: Determining the ratio of transformers

This approach should get you close. But if there is significant current flowing in either winding, and if the winding has significant resistance, then you need to allow for the ohmic voltage losses too, as these will be part of the voltages that you measure at the transformer terminals. If there are significant phase shifts between the primary and the secondary then things can get even more complicated. Ref's scope and sig gen approach can give insights in that case.

Cheers,

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Old 13th Oct 2021, 2:54 pm   #5
radiomobile
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Default Re: Determining the ratio of transformers

The simple system as described works well and experience shows that results correlate well with transformers with known ratios.
There is negligible current flowing in either winding unless the transformer is faulty, eg shorted turns, so there is significant 'sag' of the 1 volt supply, and if greater accuracy is required simply divide the output voltage by the input voltage (nominal 1 Volt) as measured on a meter. As a meter draws negligible current a single meter will suffice, similarly as no current is being drawn the resistance of the windings, (ohmic losses or copper losses ) can be ignored. At a frequency of 50c/s phase shift errors can also be ignored. Although the comments are technically correct there is a great danger of over-thinking what is basically a simple and effective procedure
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