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Old 11th July 2006, 10:45 AM   #1
Klimon is offline Klimon  Belgium
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Default Toroidal core saturation


I'm looking for a little advice on filament supply: I need to feed two rectifier tubes with 5V/2,3A each and have a small toroidal trafo that's specified for 2* 9V/1,7A

Can I use this trafo without overloading it (using resistors to reduce the voltage if still higher than 5V even with the bigger current draw)? (slightly) Overloading a block-core trafo isn't that dramatical experience tells me but I remember from electronics classes that toroidal trafo's saturate very suddenly = might be another cup of tea... Better to ask first than to be sorry later


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Old 11th July 2006, 11:31 AM   #2
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I need to feed two rectifier tubes with 5V/2,3A each and have a small toroidal trafo that's specified for 2* 9V/1,7A
Don't do it - the key thing is the current. You need 4.6A and you have only 3.4A available. That's 35% overload. Even if your trafo doesn't saturate, it will get hot.
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Old 11th July 2006, 01:38 PM   #3
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You could run the rectifier heaters in series... 4.5V each, maybe a bit more, since it's below the rated load.

Saturation is from overvoltage, not overcurrent. A small mount of DC will saturate a toroid - never use one with a half-wave rfectifier.
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Old 11th July 2006, 01:59 PM   #4
AndrewT is offline AndrewT  Scotland
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parallel the two secondaries to give 9V 3.4A.
Adjust the output voltage by adding a few extra turns to each secondary to give nearer the 10V the two, series connected, filaments require.

You need only 2.3A so you are well in and the transformer will run cool.
regards Andrew T.
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