[Milsurplus] RF Ammeters

[jfor]

  J&J
  Don't think you can say that there is a current transformer in the
  BC-442.

And why not??

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.

Not so, unless you claim the ferrite is designed to saturate at a
current below which would damage the thermal element, which I don't
believe.. The transformer is to isolate the thermal element and meter
from being hot at RF, since the unit is inserted in the transmitter's RF
output line. The current in the secondary is equal to the current in the
primary, because the turns ratio is 1:1. It's a 1:1 current transformer.
[In fact, if you wanted to change the range, you would only need to
rewind the current transformer] However the secondary is near ground
while the primary is hot with RF. The thermal element can likely sustain
little voltage between the RF and DC sides (The RF and DC sides could
well be connected....  I've not measured one)

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

Within the limits in my original reply.
-John

  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


  > > 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