[AMPS] Chokes

[Tom Rauch]

Hi Marv,

>    Yes, which certainly disqualifies them for a hi power resonant
> circuit.  The article described a loosely coupled non-resonant
> application. 

Resonant or not, the choke current is high and it varies at a RF 
rate. I'd be really careful what ferrite I used around the choke, no 
matter how "loosely coupled" the "choke designer" claims it is. 
 
>   I would have posted it, if either the article or the Lafayette catalog
> disclosed it...the Lafayette number was 32C6102 -- 1969 catalog p. 246.
>   Do you want to hazard a guess that it was type 61, since all three
> parts listed in that section have "61" in the number??
 
Guessing isn't a good idea, but what the heck.

61 material has a low flux density rating, and provides a Q of less 
than 2 at 30 MHz. If they had the material in the axis of the coil, no 
matter what the air gap between the rod and the winding, coupling 
would be very high.

Q of that material, or inductors wound on that material, is ten at 10 
MHz.

Whatever value of "inductance" was measured, you can bet the 
parallel resistance was only about ten times higher at 10 MHz, and 
nine times higher at 2 MHz...and of course around twice the 
reactance value at 30 MHz.

All of that loss heats the core material. A 1/2 inch by 5 inch rod in 
free air at normal ambient would reach curie temperature of the 
material (350 C) in about ten minutes with 25 watts dissipation.

I don't know the impedances, but I suspect that would be a hot 
selling option.
 
>   I don't have any formulas for flux density in rods... do you have a
> reference?

Flux density is not the problem, since saturation isn't the issue 
unless the duty cycle is so short heating is not a limitation. 
Saturation causes the core to suddenly behave like air, because it 
is "full" of magnetic flux. The core simply can't carry any more flux, 
so additional magnetizing forces can only increase flux outside the 
core material, it's like the core is partly air and partly a soft iron. 

Heating is primarily caused by eddy currents in the resistive core 
material. Loss caused by eddy currents are the primary concern, 
and they vary with frequency. If we know the Q and reactance, and 
the voltage across the winding we can calculate heating. 
 
I think most chokes in this application should have a Q in the 
hundreds, not single digits. That would require mounting the 61 
ferrite off to one side on the outside of the choke, and a few winding 
diameters away. Maybe on the cabinet wall.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/ampsfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm