[time-nuts] Minicircuits 10% discount in December
dgminala at mediacombb.net
Fri Nov 28 17:42:22 EST 2014
> The very tiny cores on MiniCircuits transformers will start to
> saturate at hundreds of mA. The effect is that the magnetizing inductance
> drops, which matters more at low frequencies than high frequencies. I try
> to avoid feeding DC to an amplifier through a transformer winding.
> Instead I use a separate RF choke for that. However, it would
> probably work OK for, say, up to 25 mADC for a small signal transistor,
> why take a chance.
> If you are using a DC feed through a transformer winding, be careful
> not to accidently short circuit it, causing the full available current
> from the power supply to flow through the transformer. This can
> actually magnetize the core and permanently damage it. Saturation
> via DC is much more deleterious than saturation via AC.
> It is easy to calculate the flux density using Ampere's law, which
> is one of the four Maxwell's equations. H = I/(2piR). Since R
> (radius) is in the denominator, cores saturate from the inside
> first before the whole core is saturated. Multiply H by mu,
> (as any time nut knows) to get B. If R is 1 mm, and I is 628 mA,
> then H = 10 ampere turns per meter. If mu-relative is 1000, then
> B = 4piX10^-7 X 1000 X 10 = 125 mT. That is a hefty 1250 Gauss.
> Some materials may be affected at 1/10 this flux density.
> Now a days, a lot of RF is differential, in which case you are
> free to feed DC through the output transformer without worrying
> about this issue.
> I worked for several companies where those 6 hole cylindrical chokes
> were ubiquitous. I specifically characterized those and established
> a maximum current rating of only 100 mA. Of course, many production
> designs exceeded this limit and "worked" anyway. I actually observed
> someone try to put 20A through one of these. The tantalum capacitors
> on the "cold" side of the bead actually exploded due to RF current.
> Rick Karlquist N6RK
Thanks for the insight, Rick. You confirmed many of my own assumptions
about RF transformers and cores. I hadn't thought about permanently
magnetizing a core with excessive DC current, but it makes sense. Same
theory applies to line frequency power transformers and inductors; if the
core saturates, inductance takes a nose-dive and current goes wild.
A couple weeks ago, I sent an email to the Minicircuits technical support
folks in hopes of getting this, or similar, info about a couple of their
transformer models (specifically, T1-1 and T4-1-KK81), but so far, I'm still
waiting. Guess I should give them a call.. got great technical advice from
them when I called for help some time ago.
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