[Milsurplus] RF ammeters

[Jim Whartenby]

J&J
Don't think you can say that there is a current transformer in the
BC-442.  Both the primary and secondary coils are one turn each on a
ceramic form with a ferrite core.  It is more likely a loosely coupled
RF transformer used to keep the current below the 0.75 ampere maximum
of the thermocouple.  It serves the same purpose as a shunt in DC and
low frequency AC measurement.

Since heat is what is actually measured, frequency is not important.

Accuracy is what you are willing to pay for.  I doubt that the BC-442
is better then 20%.  It is such a simple machine.  It is just an
indicator to let the operator know that something is really happening!
Regards,
Jim

--- jfor <jfor at quik.com> wrote:

> > A question about the typical thermocouple
> > RF ammeters used in a lot of WWII transmitters.
> >
> > How accurate are they at 60 to 1000 cycles?
> >
> >
> > Jack
> 
> Since they are based on heating effect, they should be pretty good as
> long as the periods of the applied waveform is significantly less
> than
> the time constant of the thermal element. Thermal true RMS converters
> used to be common.
> 
> Be aware that some of them (like those in the ARC-5 and BC-442) have
> current transformers in the circuit which are useless at audio
> frequencies.
> 
> -John
> 
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