>I want to get away from the low value hand-made meter shunts. I plan on
>using the meter (a 0-1 ma) as a voltmeter to measure the drop across
>more standard values of resistance. Knowing that my Ohms Law is a little
> rusty, I am looking for some re-assurance that I'm right. The
>metering circuit is in the B- CT return and the B- is floating 10 ohms
>from chassis ground. If I change the meter to a 1 volt voltmeter, do I
>then need a 1 ohm resistor to measure 1 amp FS of Plate current and a
>3.3 resistor to measure 300 ma FS of Grid Current ?. Of course there
>will be a 1k resistor in series with the meter. Is this corrext ??.
>
To begin to calculate metering circuitry, one needs to measure the
full-scale mV-drop across the meter movement as well as the full-scale
current. For example, a 1mA meter indicates 57mV and 1.04mA at
full-scale.
- movement-R = 57mV/1,04mA = 54.8-ohms
To make this meter read 1A full-scale:
- With a 1-ohm shunt at 1A, there would be 1V drop -- but the meter
wants only 57mV to indicate full-scale. Therefore, the series R between
the shunt and the movement needs to drop 943mV at 1.04mA, since R=E/I, R
= 943mV/1.04m-ohm = 906.7-ohms.
>Next question is exactly where and what is needed for diodes to protect
>the meter ? I have seen so many different commbinations and placements
>that I'm totally confused. What is the current consensus ??.
>
2, 3A-rms/200a-peak diodes, cross-polarized across the meter movement
will do.
>Thanks...es 73
>
>Ed W3NR
- Rich..., 805.386.3734, www.vcnet.com/measures.
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