Transformers

[Cursitor Doom]

Gentlemen,

So in my capacious junkbox I have some 230V to 18VAC miniature mains transformers. However, the secondaries have twin 18V output windings. There are two input pins for the primary mains voltage and four output pins for the 2 x 18V secondary windings. Now, I'm going to need every VA this thing can provide for the load I have in mind, so I'd like to parallel-up the secondaries for maximum current. Anyone tried this before and does one need to factor in the phase relationship between the secondaries in any way? The total VA will be less than 5, BTW, so we're not talking huge power by any means.

Cheers,

CD.

[Paul]

Must know the "dots" on the transformer, for phasing.

Rectifier config info.

Sample transformer. Wire colours on datasheet, gives "dot" info.

"Size (VA) 38 VA

Secondary Series 12.6V @ 3A Secondary Parallel 6.3V @ 6A "

Paul

[Rod Speed]

Cursitor Doom [e-mail] wrote

None of those here, just us animals.

Yes, plenty have.

Yep, the windings do need to have the same end if each wiring connected. That's normally show with a dot on the terminal of each winding.

[Ian Jackson]

In message [e-mail], Rod Speed [e-mail] writes

To physically verify the phasing of the two 18V windings, first join one end of each winding to the other. Now measure the voltage between the two open ends.

If it is around 0V, the two windings are in phase, so it is safe to join the two open ends, and you are now good to go.

If it is 36V, the two windings are in antiphase, so disconnect the joined ends, and reverse one of the windings.

Finally, to be sure to be sure, repeat the above.

[jon]

Nearly right.

[nothanks]

If the secondary pins aren't marked to show which way round to connect them then look for which way the wires are wound on the bobbins.

A "secondary" comment - if the traffo doesn't have a screen to earth then it would be wise to earth one side of the secondary windings to protect against insulation breakdown (arguably only necessary if the windings are overwound, rather than being on separate bobbins).

[Brian Gaff]

Depends very much how they are wound and if the windings are separate or just tapped in the middle. Yes you need them in phase obviously, but the other solution iis two bridge rectifiers and then parallel up the dc outputs. Brian

[Ian Jackson]

I have to admit I did have second thoughts about my terminology. However, if one end of each of the two 18V windings are connected together, and there is no voltage between the two other ends, it must be safe to connect those two ends together, thus giving you two 18V windings in parallel.

[Ian Jackson]

In message <u7s11a$3bb02$ [e-mail], Brian Gaff [e-mail]> writes

As the two 18V windings have four pins, I assumed they were separate.

[Cursitor Doom]

The tranny in question is potted, so no inspection possible, I'm afraid.

Yes, I'm going to earth one secondary in any event.

[Cursitor Doom]

I would think that's correct. BTW, there are no dots on this tranny, so I've been finding the comments about deducing the internal arrangement of the windings useful and interesting. :-)

Bit of background. I have a vintage Hacker radio which takes two PP9 batteries. These are quite sizeable lumps and don't come cheap these days (if they ever did) and given that this radio chews 'em up on a six monthly basis, I thought I'd wire in a mains supply. It's going to be a tight fit with this tranny as this radio is designed to work from

18V and gobbles up quite a bit of current even on low volume settings. Distortion doesn't become noticeable until the supply falls below 12V, so there's some elbow room there at least. I'm hoping to be able use this small tranny with a doubled-up secondary via a bridge rectifier into a linear rectifier to get me close to the 18V target. I don't have any 18V regs, but do have some adjustable ones that would cover it. However, I might try to come up with some ingenious configuration to get two 9V linear regs to work together to reach the target (though I haven't thought that through fully as yet). Those Hackers can be beasts; real current hogs.

[Animal]

it's standard practice

of course, otherwise you short the output. If it's the standard rectangular block with 4 secondary connections on one side, the order of conenctions is: wind 1 start, wind 1 end, wind 2 start, wind 2 end

[Animal]

It was a terrible idea to make them without transforrmers.

18v windings will get you a peak of about 27v, minus 2 diode drops of about 1v each = 25v. A 15v reg can cut that to 15v, but you'd do better with a lower voltage trafo. A 12v trafo would give you about the right voltage, no reg required. ust a nice fat CRC filter.

[Cursitor Doom]

Yes indeed. I didn't phrase that quite right. What I actually was asking was if anyone *here* on this group had tried this. As per your other suggestion, I would rather use a 12v tranny instead if I can as obviously the linear regulator for the voltage drop from the 18V tranny represents quite a considerable loss of efficiency going up in heat. I'll see if I have any 12V trannies of a suitable size and VA rating for the job.

[mm0fmf]

Can you still say that in today's woke world?

[Cursitor Doom]

Say whatever you like. All that nonsense about appropriate labels is for the Lefties to beat themselves up about, so unless you're one of them (in which case the restrictions apply to you) feel free to express yourself as you please.

[nothanks]

You could connect them in series to see which way gives double the voltage, then you will know how to wire them in parallel.>

[Animal]

countless times.

No regulator means even less heat waste

[Cursitor Doom]

Ian's already explained how to do this further up the thread.

[Cursitor Doom]

Indeed. However, I'm just a bit concerned the VA of the tranny might not be sufficient to supply current peaks when required at higher volumes resulting in distortion. Hopefully my fear will prove unfounded.......