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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 13th Nov 2017, 11:47 pm   #1
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Default 'Alternative' isolation transformer

Purely because I have a few of them lying around.... there any particular reason why I can't wire two transformers (each of which are 240V:24V) back-to-back to create a 'cheapskates' isolation transformer?

i.e. 240:24 - 24:240

Obviously there will be double the usual losses (heat) but is there anything else to be wary of?

Otherwise the transformers are just sharp edged door stops.
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Old 13th Nov 2017, 11:55 pm   #2
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Thumbs up Re: 'Alternative' isolation transformer

No reasons to be concerned at all - except that make sure the VA of each is about the same and that the VA will be adequate for the task envisioned.

Many years ago, I did like-wise: two transformers, each 230v. / 115v.

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Old 14th Nov 2017, 1:30 am   #3
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Default Re: 'Alternative' isolation transformer

The output voltage will be a bit lower than expected as the turns ratio doesn't compensate for losses but otherwise it will work just fine.
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Old 14th Nov 2017, 2:32 pm   #4
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Default Re: 'Alternative' isolation transformer

It'll work just fine, but will, as others have noted, be a bit more lossy than a single transformer.

In a previous existence I used 2 back-to-back transformers this way to power a US-made battery-valve portable radio: the 2 transformers I used both had 9V secondaries and I was happy to 'steal' 50mA from the back-to-back secondaries to be rectified and provide the floating 7.5V heater supply [five 1.4V valves in series].

The second transformer had a 9v primary and US-style 110-120V secondary which again rectified nicely to provide the 120V B+ supply.
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Old 14th Nov 2017, 8:35 pm   #5
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Default Re: 'Alternative' isolation transformer

Thanks to all for the confidence in your replies.
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Old 15th Nov 2017, 12:38 am   #6
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Default Re: 'Alternative' isolation transformer

I use two identical transformers both 12 VAC with a center tap, back to back for some of my low power experiments at 120 VAC. The transformers are only rated at 12 VA, but it works for me.
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Old 15th Nov 2017, 11:46 am   #7
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Default Re: 'Alternative' isolation transformer

A nice, cheap way to help ensure your safety etc.
Assuming you use identical, or very similar transformers, the only practical problem (and in the application you are using it for it may be of little importance) is that the regulation will be noticeablely poorer than a single transormer.
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Old 15th Nov 2017, 3:35 pm   #8
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Default Re: 'Alternative' isolation transformer

Don't forget about magnetising power. The first transformer will have to supply the magnetising power to the second transformer, so this needs to be subtracted from the first one's VA rating. Bigger transformers often need a lot of magnetising power, for example, 30VA for a 50VA transformer I just measured! This means that if you used two of these 50VA transformers back to back you would in fact only have 20VA of useful power available! It's therefore a good idea to stick to a smallish (12VA to 15VA say) transformer for the second one, and derate the first transformer by at least 5VA.

Last edited by daviddeakin; 15th Nov 2017 at 3:43 pm.
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Old 15th Nov 2017, 4:27 pm   #9
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Default Re: 'Alternative' isolation transformer

The off load VA isn't magnetising power, real watts are, nearer 5W max I guess.
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Old 15th Nov 2017, 5:12 pm   #10
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Default Re: 'Alternative' isolation transformer

Some confusion over VA and watts going on here.

Magnetizing current is mostly wattless, except for the losses from winding resistance and in the imperfections of the magnetic core- hence 30VA of winding volts * winding current may only be 5 Watts of losses.

However, the first TX will still have to deliver the current involved in the wattless VA of the second TX so yes, the total power available from the second TX will be less to allow for this without overloading the first TX.

With the setup running, check that none of the winding currents in the various transformers exceeds its VA rating when multiplied by the voltage across the winding in question. Nothing will heat up fast enough to damage anything during the "on" time it takes to make the measurements.
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