Hi > On Nov 28, 2014, at 2:27 PM, Richard (Rick) Karlquist <rich...@karlquist.com> > wrote: > > > > On 11/28/2014 10:08 AM, Dave M wrote: >> Rick, >> Thanks for the brief review of MiniCircuits stuff (I'm not connected >> with them in any way except as a customer). >> Since you've characterized some of their parts, perhaps you could help >> answer a question that someone else posted, and one that I would like to >> have answered as well. >> Have you measured the effects of DC current in the windings of an RF >> transformer, such as is seen if the transformer is in the collector >> circuit of an amplifier? If so, could you provide a generalization of >> the effects, such as changes in frequency response, losses, etc.? >> >> Many Thanks!, >> Dave M >> > > 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, but > 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.
If you do need to run substantial current through a choke core, the larger binocular cores with a half turn through them are a better choice. Still useless for 20A (or even 2A) though … Bob > > Rick Karlquist N6RK > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.