[Amps] Meter Shunts

Rich 2@vc.net
Sun, 8 Dec 2002 17:36:02 -0800


>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.