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Old 29th May 2020, 12:31 am   #1
flyingtech55
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Default RS 196-303 Mains Transformer

Hi all,


Further to my AL42 BlueTooth project, I am intending to use the above transformer to power the BlueTooth module. This transformer has two separate 12V 3VA windings. Is it all right to wire these in parallel or will this cause problems? If anyone has the RS data sheet does it mention whether it is permissible to parallel these windings?


Thanks in advance for your thoughts/information


TimR
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Old 29th May 2020, 5:38 am   #2
Diabolical Artificer
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Default Re: RS 196-303 Mains Transformer

If your tfmr is 0 12v 0 12v then yes, it's fine to parallel as long as you get them in phase, I have a similar 0 4.5v x 2 3VA RS tfmr I'm using at present in a project.

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Old 29th May 2020, 9:23 am   #3
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Default Re: RS 196-303 Mains Transformer

I have run many mains transformers (some from RS) with the secondary windings in parallel with no problems. I've also seen it done in commerical devices.

The only 'gotcha' is that you must connect the windings the right way round. It's AC, but you have to connect them in phase. For transformers with 4 tags in a row, you normally connect tags 1 and 3 together and tags 2 and 4 together. If you are not sure (or the transformer has flying leads), connect one end of one winding to one end of the other winding and measure the AC voltage between the other two connections (mains on the primary of course). If it's 0V then you got it right, just make the other connection. If it's twice the nominal secondary voltage you got it wrong, so reverse the connections to one of the windings.
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Old 29th May 2020, 10:04 am   #4
60 oldjohn
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Default Re: RS 196-303 Mains Transformer

Quote:
Originally Posted by TonyDuell View Post
I have run many mains transformers (some from RS) with the secondary windings in parallel with no problems. I've also seen it done in commerical devices.

The only 'gotcha' is that you must connect the windings the right way round. It's AC, but you have to connect them in phase. For transformers with 4 tags in a row, you normally connect tags 1 and 3 together and tags 2 and 4 together. If you are not sure (or the transformer has flying leads), connect one end of one winding to one end of the other winding and measure the AC voltage between the other two connections (mains on the primary of course). If it's 0V then you got it right, just make the other connection. If it's twice the nominal secondary voltage you got it wrong, so reverse the connections to one of the windings.
I had a vague memory that it was important which way around the two windings were connected, but not a clue how to ensure I get them right. What would be the result of getting the connection wrong? Reading the above it is brilliant.
Moderator's I think it should be made a "Sticky" if it is not already.


John.
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Old 29th May 2020, 10:06 am   #5
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Default Re: RS 196-303 Mains Transformer

I bought a few sub miniature tx's off Ebay there were two nine volt winding's these were paralleled to give enough current ,to supply a blue tooth module ,no problem Mick.
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Old 29th May 2020, 7:30 pm   #6
TonyDuell
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Default Re: RS 196-303 Mains Transformer

If you get the polarity of the secondaries wrong then effectively you end up wiring a 24V AC output transformer (in this case) with a shorted output. It will draw a high current from the mains and you hope the fuse blows before a winding burns out!

If you have a lamp limiter, connect the transformer to the mains via that. With no load on the secondary side the lamp should be dim or out. If it glows brightly then you've got the polarity wrong.

But the easiest way to get it right is to do what I suggested in an earlier post. I'll explain it in a bit more detail

Suppose we call the 2 secondary windings '1' and '2' with the ends called '1a', '1b' and '2a', '2b'.

Connect 1b to 2b. Connect an AC voltmeter (reading up to at least 24V in this case) between 1a and 2a. In other words a circuit with the 2 windings in series and a voltmeter across the ends.

Connect the primary winding to the mains and switch on.

If the voltmeter reads 0V then the 2 windings are correctly connected. Switch off and unplug the mains, disconnect the voltmeter and connect a wire between 1a and 2a (the terminals that the voltmeter was connected to).

If the voltmeter reads 24V then you have the windings incorrectly connected. Switch off, unplug the mains and reverse the connections to one of them. For example, connect 1b to 2a and the voltmeter between 1a and 2b. Plug in and switch on again. The voltage should be 0V now, but it is best to check. Assuming it is, switch off, unplug the mains, remove the voltmeter and connect 1a to 2b (the terminals the voltmeter was connected to).


In both cases, you now take the output from 1a and 1b (say).
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Old 30th May 2020, 2:22 pm   #7
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Default Re: RS 196-303 Mains Transformer

Yes, the two windings must be connected with the correct polarity, easily determined by trial and error and measurement as already described.

Incorrect connection is in effect short circuiting both windings and is almost certain to destroy the transformer. In theory the mains fuse might blow, but in practice the short circuit will destroy the transformer before the fuse operates.

Almost any small low voltage transformer with two similar windings may have these windings connected in series or in parallel.

In the case of higher voltage transformers as used for HT supplies, then two windings should NOT be parallel connected, unless the manufacturer specifically states that you can.
Firstly the two windings may have slightly different voltages, and circulating currents between the two windings will cause extra heating.
Secondly, the "end" of one winding may be very close to the "beginning" of the other winding, with minimal insulation between the two.
Parallel connection may result in insulation breakdown.

HT transformers are typically intended for use center tapped with a double diode rectifier, or for a 4 diode full wave bridge connected to both sections IN SERIES.
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Old 7th Jun 2020, 12:54 pm   #8
flyingtech55
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Default Re: RS 196-303 Mains Transformer

Dear all

I thought I would like to 'put this one to bed'.

Firstly, thanks for all your collective input. With regard to the phasing of the two windings...helpfully the transformer manufacturer clearly marked the start and finish each of the two windings so ensuring the correct phasing of the two shouldn't be a problem either series or parallel.

What I did wonder is that unless the two secondaries are wound bi-filar, there will be more copper on the 'outer' winding than the 'inner' one. If I parallel the windings I was concerned that this would cause a circulating current within the transformer.and cause overheating. The transformer is physically quite small. This is why I asked if anyone had the data sheet for the transformer to see if parallel connection was permissible.

However, in the end I didn't need to parallel the windings after all. Each secondary is rated at 12V 3VA. This equates to (approximately) 250mA per winding. The transformer is being used to power a small Bluetooth module via a 5V regulated power supply. The module consumes a surprisingly small amount of power, so a single winding proved to be more than enough to supply its needs. I've therefore left the second winding unconnected. I may well find a legitimate use for it at some time in the future.

Thanks again for all the helpful input. Lots of food for thought as usual.

TimR
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Old 7th Jun 2020, 9:06 pm   #9
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Default Re: RS 196-303 Mains Transformer

Quote:
Originally Posted by flyingtech55 View Post
...What I did wonder is that unless the two secondaries are wound bi-filar, there will be more copper on the 'outer' winding than the 'inner' one. If I parallel the windings I was concerned that this would cause a circulating current within the transformer and cause overheating.
As long as each secondary has exactly the same number of turns and sees the same magnetic flux it will have the same induced voltage irrespective of its length. They will share the current unequally because of their differing resistances though.
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