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In Reply to: RE: Transformer Inductance posted by hennfarm on October 13, 2016 at 12:13:45
When you guys measured the primaries, did you apply a DC current? Is it possible the numbers in practice might be worse than what you saw?
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Buy Chinese. Bury freedom.
Follow Ups:
My measurements are made with a variable DC current plus about 12vRMS at 60Hz.
FWIW, I figure that when the current is enough to reduce the inductance by 20% relative to zero DC current, that's a practical maximum-current specification for output transformers and plate chokes. I base this on my measurements of a fairly large number of commercially-available SET transformers and chokes.
Measured inductance will increase if a higher AC voltage is applied, or if a lower frequency is used, as long as the flux is below the saturation level. A higher frequency (such as 1000Hz, widely used in multimeters) or a smaller voltage will give a smaller inductance.
another important thing to note is that meters that sample at 1Khz often give incorrect results since the self resonant frequency of many of these devices is in the 1Khz range.
dave
Many thanks, Paul. You did talk about that in your earlier post. I got distracted at work and forgot I'd read that part before replying. Old age, I guess. Anyway, this might be a good time to use my CCS test method for checking the transformers with various DC currents. Current in the final circuit should always be much less than the SE stages these transformers were designed for. Thanks for the info, I'll report back with results in case anyone else is interested in trying this.
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Buy Chinese. Bury freedom.
I use an Amprobe Lcr55a (1khz) with no DC current through primary. From what I have read and what Mr. Joppa has already stated inductance falls with DC current through the winding. Every device will be different, some may have an impedance hump <100hz.
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