I have a big toroidal audio power transformer that can be wired for 120 or 220. It's from a home theater amp so it has six secondaries with around 32-0-32 originally running class AB with a published 120 watts per channel. At the IEC the rear panel says 1200 watts maximum. I wired the primary for 220, fed it with USA line voltage (117) and paralleled three secondaries for each channel of my DIY Aleph J, and it runs fine at +/- 19.5 volts .85 amps per irfp240. Two questions: any downside to doing this? ; and, what does halving the voltage do to current capacity on the secondaries? (I believe running three secondaries in parallel triples current capacity, but what about simply halving the voltage for each secondary: is the secondary capacity a watts / VA calculation or is it just a matter of the magnet wire gauge [which obviously is unchanged]?)
Reducing the voltage is fine - increasing it could lead to saturation and disaster, however.
The current handling is basically the same, although IR voltage drops will be proportionally twice as big. VA limits for transformers are really current limits in disguise I suggest, ie I-squared-R losses. You can't trade voltage for current or vice versa, but decreasing one of them is fine, so quoting it as VA is somewhat confusing (its done because you can configure widings in series or parallel, and then the overall VA product is conserved, although each winding's V and A limits are unchanged)
The current handling is basically the same, although IR voltage drops will be proportionally twice as big. VA limits for transformers are really current limits in disguise I suggest, ie I-squared-R losses. You can't trade voltage for current or vice versa, but decreasing one of them is fine, so quoting it as VA is somewhat confusing (its done because you can configure widings in series or parallel, and then the overall VA product is conserved, although each winding's V and A limits are unchanged)
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Slightly off topic, but it i amassing how the line voltage has crept up in the United States since I was born in the 1950's. It started out at 110/220, then went to 115/230, 117/234 and now is 120/240 +-6%.
Isn't it considered good practice to add a small series resistor when paralleling windings? -- something like 2 to 5% of the DCR of one secondary?
If not, it won't be the first time I'm completely full of $&@*. 😕
Cheers
If not, it won't be the first time I'm completely full of $&@*. 😕
Cheers
series resistor on paralleled secondaries
thanks for the replies. The 'can't trade volts for amps' was a big part of my concern. But the idea of series resistors on the windings was another. 2-5% of a couple ohms seems like it would not do much. Is use of a series resistor beneficial or not?
thanks for the replies. The 'can't trade volts for amps' was a big part of my concern. But the idea of series resistors on the windings was another. 2-5% of a couple ohms seems like it would not do much. Is use of a series resistor beneficial or not?
What I meant to propose was one resistor per paralleled secondary -- each with a value of a few percent of *its* secondary. Don't remember where I saw it, but it was offered as an alternative to improve the equality of sharing, where separate bridge rectifiers were not desired or feasible.
Of course, as always, I could be roundly mistaken .. 😱
And it will cost that small percentage of additional voltage drop under load.
Regards
Of course, as always, I could be roundly mistaken .. 😱
And it will cost that small percentage of additional voltage drop under load.
Regards
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