[Boatanchors] [ARC5] RF Ammeter Design and Construction

[J. Forster]

Hi Ray,

Thanks very much. It confirms what I suspected.

I am still surprised to see it in a commercial unit.

As has been pointed out, it is easy to calibrate a TC RF Ammeter at line
frequency, where the DC component will not influence the readings, and at
a low enough frequency where current can be measured without worrying
about power factor/VSWR.

The capacitive discharge welder you built is very similar to Unitek and
Raytheon ones available pretty cheaply in the US. They were widely used in
the aerospace business for making welded cordwood modules. The one
difference is the commercial units have a step-down pulse transformer
between the capos and SCR and the work electrodes.

Thanks,

-John

============

> Hi John,
>
> There is some info on RF ammeters here....
> http://www.tuberadio.com/robinson/Thermocouples/
> If the weld is not at exactly at one spot,
> then the DC current direction will give different results.
> DC calibration is normally fine.
>
> Regards
> Ray
>
>
> Background: A question was posted about an RF Ammeter and I suggested some
> DC tests, which produced odd results...   if you applied a DC current in
> one direction the meter read one value; if the current was reversed, a
> dramatically different value.
>
> In thinking about this, the meter design must have been different from
> conventional.
>
> A conventional design looks sort of like this:
>
> O--------o-------O    Main RF path
>         / \
>         | |    Thermocouple leads
>        meter
>
> The ones I've seen have the two Thermocouple leads welded together and
> then attached to the main resistance wire with a small drop of cement.
> This design should be completely independant of the current flow in the
> main path.
>
> Another design?
>
> Based on the measurements, about the only plausible design fitting the
> data is one like this:
>
> O------o---o-----O    Main RF path
>       /     \
>      |       |    Thermocouple leads
>       meter
>
> It may look funky, but should work, and explain the measurements. From the
> meter circuit's point of view, the two series couples (left TC to main &
> main to TC right) appears as just a single couple, if the main line is
> uniform. The middle section of dissimilar metals cancels out.
>
> However, if there is a DC component in the main line, ther IR drop between
> the couple points will add or subtract from the thermoelectrically
> generated voltage, giving different readings depending on the direction of
> the DC current.
>
> This is strange to me, and I've never seen it discussed anywhere.
>
> Does anybody know of references. It is important because it is hard to
> calibrate RF Ammeters at RF, and this rules out the use of a thermocouple
> as a transfer standard between AC and DC.
>
> Best,
>
> -John
>
> ======================
>
>
>
>
>
>
>
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