Sizing a toroid core power transformer.
Moderators: pompeiisneaks, Colossal
Sizing a toroid core power transformer.
I have inherited a phaze amplifier that uses a 100VA 300V toroidal power transformer. Its quiet and runs cool. I've been looking at Antek products but their ratings seem different from what typical IE cores are rated. Robrob has a 50va 190volt that he rates at 130 ma. How does he get that figure ?
Heavens, an unused PI input !
Re: Sizing a toroid core power transformer.
190V x 130mA is 25VA, that leaves 25VA for heater power. Is it 6V3 or 12V6?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Sizing a toroid core power transformer.
The amount of power a transformer can pass through is limited by the temperature its wire and layer insulation can withstand. The iron and copper are good up to the melting point of the copper and/or the Curie point of the iron. But an internal short caused by melting/burning insulation can generate localized heating that can melt the copper wire or burn it through. The same iron and copper can pass more power if you can run it hotter.
That's why transformer wire and layer insulation is rated for specific temperature ranges. See https://en.wikipedia.org/wiki/Insulation_system
The lowest actual temperature range, Class A or Class 105 allows 105C for an internal hot spot. By coincidence, in an EI stacked transformer, an internal hot spot of 105C makes the exterior metal heat up to about 130F, and this is the temperature that most humans will not keep their index finger tip on.
Toroids are different, being much more "airy" - cooling air can get to the metal and insulation better. They run cooler for the same total power transfer; this is complicated by the heat source being nearer the surface, so the fingertip test is not as applicable. The outside surface gets hotter faster. But still, if you can touch and keep your fingertip on the surface, it's not too hot. There are other differences in the heat balance for toroids versus EI types, as well.
But if it's running cool to the touch, it can transfer more power without overheating.
That's why transformer wire and layer insulation is rated for specific temperature ranges. See https://en.wikipedia.org/wiki/Insulation_system
The lowest actual temperature range, Class A or Class 105 allows 105C for an internal hot spot. By coincidence, in an EI stacked transformer, an internal hot spot of 105C makes the exterior metal heat up to about 130F, and this is the temperature that most humans will not keep their index finger tip on.
Toroids are different, being much more "airy" - cooling air can get to the metal and insulation better. They run cooler for the same total power transfer; this is complicated by the heat source being nearer the surface, so the fingertip test is not as applicable. The outside surface gets hotter faster. But still, if you can touch and keep your fingertip on the surface, it's not too hot. There are other differences in the heat balance for toroids versus EI types, as well.
But if it's running cool to the touch, it can transfer more power without overheating.
Re: Sizing a toroid core power transformer.
130F = 54.4C in case any other centigraders out there were wondering 
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Re: Sizing a toroid core power transformer.
Good point. Thanks.
I find I've become a MKS system agnostic. I expect that whatever number is presented to me will be in the other measurement system and have memorized the conversions.
Not everyone does that, I guess.
I find I've become a MKS system agnostic. I expect that whatever number is presented to me will be in the other measurement system and have memorized the conversions.
Not everyone does that, I guess.
Re: Sizing a toroid core power transformer.
So less than 4A for the heater string and that's pressing it IME.
Tube junkie that aspires to become a tri-state bidirectional buss driver.