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Old 10th May 2006, 11:10 PM   #1
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Default Does 300VA + 300VA = 600VA?

If you wire up 2 300VA in either of these configurations, are yuo effectively getting a 600VA transformer? Or are you getting a very expensive 150VA? Or a good way of melting copper? or are the figures incorrect and I'm getting an 80V (or a 20V) 300VA?

If this is a valid way of wiring 2 transformers, should they physically be mounted on top of each other, next to each other (flat), or some other orientation (strung from the top of the casing with organic cotton)?

The reason, of course, is money - 300VA (40-0-40) on special for $40; the cheapest 500VA (40-0-40) I can find is $86.15

Raka asked something like this in thread Parallel transformers and bridge, but it didn't specifically answer my question - and I'm looking at an unregulated supply.
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Old 10th May 2006, 11:45 PM   #2
lndm is offline lndm  Australia
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AFAIK, this is valid in theory. As far as physically mounting is concerned, I would keep the xfmrs apart like any others, over the concern of inadvertent magnetic bucking.
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Old 11th May 2006, 12:03 AM   #3
Eva is offline Eva  Spain
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The resulting VA rating from several transformers connected together is the sum of the ratings of each one as long as the resulting complex connection still involves using all the primaries at their full voltage rating (and obviously all the secondaries).

Secondary windings from different transformers can be only paralleled when the same voltage is granted to appear across them (ie: when both transformers have exactly the same turn ratios and their primaries are fed with the same voltage or when primaries are connected in series thus balancing mutually).
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Old 11th May 2006, 12:37 AM   #4
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Quote:
Originally posted by lndm
AFAIK, this is valid in theory. As far as physically mounting is concerned, I would keep the xfmrs apart like any others, over the concern of inadvertent magnetic bucking.
I'll probably go that way, as I was also keen to avoid 'crushing' the 2 sets of windings together anyway (if they were on top of each other). I'm not sure if there would be "magnetic bucking" (did you mis-spell that word?) with them epoxied together - but I'm open to being persuaded. I'd be able to have 2 power supplies in the one case if I could put them on top of each other - otherwise I'll have to use 2 cases (the caps are 150x75mm ea). I guess this is OK as 2U rack mounting boxes are also on special

Quote:
Originally posted by Eva
Secondary windings from different transformers can be only paralleled when the same voltage is granted to appear across them (ie: when both transformers have exactly the same turn ratios and their primaries are fed with the same voltage or when primaries are connected in series thus balancing mutually).
The transformers will all be from the same batch and they'll all be being fed from the same source. Looks like I might be getting a cheap 600VA trannie (well, 4, actually).

BTW, thanks for the responses. I wonder if anyone has actually done this?
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Old 11th May 2006, 12:43 AM   #5
lndm is offline lndm  Australia
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Quote:
Originally posted by Cloth Ears
"magnetic bucking" (did you mis-spell that word?)
m-a-g-n-e-t-i-c, no I think that's how it's spelled

The transformers only need to be connected electrically to do what you want. Magnetic interference will only hinder them. When mounting them together (which ordinarily I would never recommend), the results wouln't be that predictable.
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Old 11th May 2006, 12:44 AM   #6
Eva is offline Eva  Spain
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I routinely use the secondaries of small transformers scrapped from other equipment connected in series-parallel (with primaries in parallel) in order to obtain a low-cost isolated 230V mains source for SMPS work. Note that winding polarity matters.

BTW: You can mount several power toroids in any way you want, stray magnetic fields are low enough to yield any mutual interaction negligible. That does not apply for SMPS transformers, tube output transformers or signal transformers, though.
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Old 11th May 2006, 01:17 AM   #7
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Quote:
Originally posted by lndm
m-a-g-n-e-t-i-c,



Quote:
Originally posted by Eva
BTW: You can mount several power toroids in any way you want, stray magnetic fields are low enough to yield any mutual interaction negligible. That does not apply for SMPS transformers, tube output transformers or signal transformers, though.
I can theoretically fit 2 amp boards, 2 soft-start circuits, 4 trannies and filter caps in a 2U rack mount box, with a bit of space to spare. The trannies won't fit on top of each other anyway (114mm), so I won't worriy about that. I'll just get the bits home and see if they fit 'properly'. It doesn't matter if I have to get another couple of cases to get them to fit 'nicely' (and it makes them easier to work in).

Gee, that was an easy decision - monoblocks!
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Old 11th May 2006, 04:27 AM   #8
kartino is offline kartino  Indonesia
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Paralleing transformer shall consider that the transformer are have identically characteristic, such as voltage level, phase error angle, polarity, etc.

For big (utility power company) they must consider harmonics when energizing parallel Transformer. For small transformer like amps power supply, I have no problem experience so far. On text book the primary cable shall have same length, but it may neglected anyway.

When we have no problem then we got 300VA+300VA=600VA
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Old 11th May 2006, 04:40 AM   #9
kartino is offline kartino  Indonesia
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Add: the safest way is connected the DC output, ie, both transformer rectified.
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Old 11th May 2006, 05:28 AM   #10
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Quote:
Originally posted by kartino
Add: the safest way is connected the DC output, ie, both transformer rectified.
From the picture in the first post, 40V outputs go to bridge rectifier, 0V to ground.

Any preference in wiring on secondary side (picture on left or right)? Or are both the same?

Thanks for post
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