[ARC5] Re: [Milsurplus] RF Ammeter Puzzle

[J Forster]

Sure, you can confirm the sensitivity of the meter itself, but if, as
you say, the transformer scales the current, the combination must be
measured at RF.

My reference about delicacy was not in reference to the meter movement
itself, but to the TC assembly.

TC RF Ammeters almost always have very low resistance (hence high
current) movements because the thermal EMF is millivolts. You need a low
movement resistance to get the number of ampere-turns needed to deflect
the pointer with the low voltage from he TC.

If the meter has an internal TC and the dial says 250 mA full scale, end
of discussion. ( I wonder how they made a TC assembly for that low
current.) Therefore transformer must scale 20:1 in current. As I said, I
don't have one.

The bottom line is the questioner needs to test his meter by applying DC
very gradually to find out what it really is. No shortcuts by IDing the
gear it came from.

Thanks,
-John



WA5CAB at cs.com wrote:

> John,
>
> Bruce was merely trying to say that (assuming that the thermocouple is
> still good) you could confirm the full scale sensitivity of the meter
> itself with DC just as well as with AC or RF.
>
> But no (I realized I wasn't certain of the correct spelling of "au
> contraire"), a 250 ma full scale meter is not very delicate.  It's
> quite robust.  20 ua movements are delicate.  And the fine print on
> the scales on almost a dozen RFA's of three or four different makes
> removed from ATC and AN/ART-13 transmitters that I have all clearly
> say "250 MA FS".
>
> The answer to the original question is yes.
>
> In a message dated 2/27/2008 11:09:01 PM Central Standard Time,
> jfor at quik.com writes:
>
>> (a) I don't have an ART-13 or even a meter, and (b) the DC won't go
>> through the
>> current transformer...  you'd have to have a high current RF source.
>>
>> Robert Downs opined a 20:1 ratio on the transformer ( it's wound on
>> a Steatite
>> bobbin, not a magnetic core), but I'm no so sure. A 250 ma FS TC RF
>> ammeter would
>> be very delicate.
>>
>> The original question was if the referenced meter needed an external
>> shunt and /
>> or current transformer and, to that end, I was trying to ID it's
>> original
>> purpose.
>>
>
> Robert Downs - Houston
> <http://www.wa5cab.com> (Web Store)
> MVPA 9480
> <wa5cab at cs.com> (Primary email)
> <wa5cab at comcast.net> (Backup email)