# [time-nuts] Minicircuits 10% discount in December

Bob Camp kb8tq at n1k.org
Fri Nov 28 16:04:38 EST 2014

```Hi
> On Nov 28, 2014, at 2:27 PM, Richard (Rick) Karlquist <richard at 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 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
>
> 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
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```