[Milsurplus] RF Ammeters

[Jim Whartenby]

OK, since the primary turn is in series with the load, it is a current
transformer. For whatever reason, it looks like link coupling to me but
perhaps they are in the end, one and the same.

The problem is still that if it is a 1:1 current transformer, the
thermocouple will see the primary current.  But the thermocouple is
rated at 0.75 amperes and it drives a meter with 10 amps full scale.

Because of the spacing between the primary and secondary turns, it is
at least a 10:1 turns ratio transformer, dontchathink?

Jim


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

>   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
> 
> 
>