[Boatanchors] Milliamp meter conversion

[J. Forster]

The current transformer is needed because the RF and output terminals of the
'thermal converter' are electrically connected. W/o the transformer, the guts of
the meter will essentially be connected to the Tx antenna terminal, and be 'hot'
with RF. In a high VSWR situation, this RF voltage could well be considerable.

The BC-442 is 10 A FS and the current transformer looks like 1 T primary and 1T
secondary on some kind of dust or ferrite core inside a ceramic form.

Note also that the meters used with the thermal converters were pretty low
resistance.  Mine is a GE, but has no electrical information on it, just a part
number. The moving coil does, however, look to be wound with pretty heavy wire for
a meter. It MIGHT be something like 10 ma FS, but that's a total guess. The
thermal converter likely only puts out a few to a few tens of millivolts. I don't
have the loose parts available for electrical testing right now, and don't want to
unsolder connections in my BC-442.

Also, a note on current transformers. The secondary MUST be connected to a load
resistor. If it is not, the voltages can get very large. If you are going to try
the diode scheme, I'd use something like a 1:1 transformer and a 1 ohm load (or
smaller)  resistor for a 1 amp RF current.  If the anticipated RF current is
higher, I'd be inclined to use something like a 1:5 or 1:10 Current Transformer.

FWIW,
-John



WA5CAB at cs.com wrote:

> Since Bill will probably buy one and try it, I thought I'd better mention
> that in the RE-2/ARC-5 there is a 1:1 (single turn in primary and secondary)
> transformer.  The thermocouple is connected to the secondary, with it's output
> connected to the meter.  The transformer primary is connected between the
> transmitter and antenna (through the T/R relay in the box).
>
> Probably John's suggestion of a diode bridge would work with this transformer
> as well.