I have a project that needs 30-0-30 Vac, at about 100VA.

I have several 15-0-15 Vac toroidal transformers, rated at about 50VA by the size of them (no markings at all).

Can I series connect the full secondaries of two transformers, as shown? I'm pretty certain I'll get 30-0-30V, but will the power handling double from 50VA to the 100VA I will need ?

 

I'm pretty certain I'll get 30-0-30V

As long as the outputs are in phase.
If you don't get 30V then just reverse one of the transformer's windings.

will the power handling double from 50VA to the 100VA

Yes.

 

As long as the outputs are in phase.
If you don't get 30V then just reverse one of the transformer's windings.
Yes.

That's great, thanks, thought it would, doubling the hardware, but wanted to make certain....

As for the phasing, If I use the existing colour-coding of the secondaries (the transformers are identical) we should be good to go ....

 

will the power handling double from 50VA to the 100VA I will need ?

Yes and no. If the load is from Green 30VAC to Black 30vac then 100 watts.
If the load is from Green 30VAC to GND then the load is 50 watts max.
You can have two 50 watts load if each one is on a different transformer.

 

Yes and no. If the load is from Green 30VAC to Black 30vac then 100 watts. YES
If the load is from Green 30VAC to GND then the load is 50 watts max. NO
You can have two 50 watts load if each one is on a different transformer.

 

Note that if the load is a rectifier-capacitor to generate a DC output, you need to derate to about 60% of its rating or 60VA due to the high peak RMS currents such a load generates.

 

Note that if the load is a rectifier-capacitor to generate a DC output, you need to derate to about 60% of its rating or 60VA due to the high peak RMS currents such a load generates.

Yes the load is full-wave rectified, and it's an amplifier module that supposedly delivers 100W. I do not know on what basis they say 100W, could be peak, could be continuous, could be RMS, could be bananas, just not specified.

But the amp modules are cheap enough to experiment with, and see how it delivers.

https://www.ebay.co.uk/itm/395250819610?_trkparms=amclksrc=ITM&aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=20231107084023&meid=bc379e70e45a421bad3e3d48675bd17c&pid=101875&rk=4&rkt=4&sd=196085452083&itm=395250819610&pmt=1&noa=0&pg=4429486&algv=SimplAMLv11WebTrimmedV3MskuWithLambda85KnnRecallV1V2V4ItemNrtInQueryAndCassiniVisualRankerAndBertRecallWithVMEV3CPCAutoWithCassiniEmbRecallManual&brand=Unbranded&_trksid=p4429486.c101875.m1851&itmprp=cksum:395250819610bc379e70e45a421bad3e3d48675bd17c|enc:AQAJAAABcLSVkHmTL63bebovD9RfpraPWbY9f4anjbziTIaEIOVXg2DWhVLks3Lfqff9vxxfjIh4XEXusIehlT2iEH8%2BCxXNySMxlDIQJ3g86%2Bx3AAIAq9gznqdGSR72ksP41kjzPN1reUbVsJuld0S4i4qXlQfTmkMeIJTCXTM8mLpr7ju2fMvruDu87ZX%2B4bShOvD%2B4ksn353vgpE22RwZbpmGYnUQIMH9QhNu9vOUMbl%2BliGlKve3L84PIKbGqPfxGWmIvGr4Zn7Vwex9TIWO1Sc%2BRQpTGk1wIWrwbFS%2Ffk0LxHfSRBJUJ6FPU4YmRWM67CKkcrVsptFDlzMu6cfrwjgLYqkowUpxxxe3ogXOMhpxPTiyRhoMw8Iz2XrYDBicmeuWI2CoBNtXY5o05zWuS%2FajyDa24WtpLVIchALdtdkyxq%2F4wzVLQcsV8wmbcL9yORbk0O6U3ZY8k8cWhBLkge6BWoKkKaTHaz0GM9jtMXNi%2FNgk|ampidL_CLK|clp:4429486&itmmeta=01HTYZSKZKD17V98MT3T8JT8NC

It's like the KAM PA amplifier/speaker I have that is sold as an "800W" power, but the IC amplifier inside is a TDA7277, and I'll leave it to you to discover the maximum output from that amp is 37W !!

 

the load is full-wave rectified, and it's an amplifier module that supposedly delivers 100W.

Should not be a problem then since, unless you are doing an audio test, the average music power is much less than 100W, even if the peaks are that high.

 

Isnt there a problem with circular flows if in series and transformers arent exactly the same?

 

Isnt there a problem with circular flows if in series and transformers arent exactly the same?

What do you mean by circular flows? If there is no shared magnetic flux between transformer cores, they are isolated energy sources in a serial circuit.

 

Isnt there a problem with circular flows if in series and transformers arent exactly the same?

The transformers are toroidal, and they'll be mounted to the chassis separately, not on top of each other.

I don't know how much magnetic flux "leaks" out of a toroidal transformer, I'm guessing "not a lot" (how's that for a technical term !!). If I mount the transformers a couple of inches apart, I doubt there will be enough interaction to cause a problem.

 

...
I don't know how much magnetic flux "leaks" out of a toroidal transformer, I'm guessing "not a lot" (how's that for a technical term !!). If I mount the transformers a couple of inches apart, I doubt there will be enough interaction to cause a problem.

View attachment 319502

The whole purpose of toroidal construction is to minimize the amount of flux outside the core.

 

The transformers are toroidal, and they'll be mounted to the chassis separately, not on top of each other.
I don't know how much magnetic flux "leaks" out of a toroidal transformer, I'm guessing "not a lot" (how's that for a technical term !!). If I mount the transformers a couple of inches apart, I doubt there will be enough interaction to cause a problem.

One thing I have seen on occasion is where a transformer cover/mounting clamp has been used from the centre bolt to the transformer exterior to the frame.
This results in a Shorted Turn creating a LOT of heat. !

 

One thing I have seen on occasion is where a transformer cover/mounting clamp has been used from the centre bolt to the transformer exterior to the frame.
This results in a Shorted Turn creating a LOT of heat. !

Ok, so that's not a mounting issue, that's physical damage to the windings causing shorts, which is of course is a mounting issue in the first place!

Rest assured these transformers have rubber-coated mounting "cups", almost guaranteeing no damage to the windings. Seemed good enough for where they came from ....

 

Ok, so that's not a mounting issue, that's physical damage to the windings causing shorts, which is of course is a mounting issue in the first place!

Rest assured these transformers have rubber-coated mounting "cups", almost guaranteeing no damage to the windings. Seemed good enough for where they came from ....

Not talking about anything coming in contact with the windings causing damage, but those that have placed a retainer or cover that extends down to the bottom metal chassis effectively causing a shorted turn.

 

Not talking about anything coming in contact with the windings causing damage, but those that have placed a retainer or cover that extends down to the bottom metal chassis effectively causing a shorted turn.

It sounds to me like you are suggesting that a LOT of flux escapes the toroidal core, and I can't see that happening as the conductor is wound around the ferrite ring.

I can't see it happening myself, especially as the standard mounting of toroidal transformers is two metal "cups" and a metal bolt through the centre to anchor it to the chassis. Don't think that would be normal practice if the bolt could act as a short in the windings. And many transformers, not just toroidal, have a shielding can.

 

It sounds to me like you are suggesting that a LOT of flux escapes the toroidal core, and I can't see that happening as the conductor is wound around the ferrite ring.

I can't see it happening myself, especially as the standard mounting of toroidal transformers is two metal "cups" and a metal bolt through the centre to anchor it to the chassis. Don't think that would be normal practice if the bolt could act as a short in the windings. And many transformers, not just toroidal, have a shielding can.

The cover would follow the same path as the windings themselves.
Usually the mounting is a single bolt through the centre with a metal retaining disc on top,