Rich Measures wrote:
>If the meter has 1095 ohms of R, yes. However, most 50uA meters have a
>bit more R. . I would use a 1.0 ohm shunt and put a series resistor
>between the shunt and the meter. At 300mA, there will be 300mV across
>the shunt. The meter needs 50uA x 1085 ohms = 54mV to read fs. The
>series resistor would be 300 -54 mV divided by 50uA - or 246mV/50uA =
>4.9k ohms. However, it is better to calibrate with a DMM by adjusting
>the series resistor to whatever is needed to produce a 300mA fs reading.
>- later, Sugi
That's very good advice. Using BOTH SHUNT AND SERIES resistors can give
much better protection to the meter.
The shunt resistor is in the main current flow, not across the meter.
The series resistor is in series ONLY with the meter.
If this is a grid current meter, follow the recommendation on Rich's web
pages, and use a BIG diode across the shunt resistor (anode to negative)
to hold down the voltage in a current surge. In normal operation the
diode is below its threshold voltage, but in a current surge the diode
will hold down the voltage across the resistor.
This protection works most effectively if the normal voltage drop across
the shunt resistor is close to 500mV. At voltages up to this level the
diode is not conducting and has no effect on calibration. Any excess
voltage drop makes the diode begin to conduct, shunting the excess
current through itself and protecting the resistors and the meter.
Note that the diode is connected across the shunt resistor, NOT the
meter. (You get very little protection if you simply shunt the meter
with a much lower-value resistor, because it takes a huge excess of
current before the diode starts to conduct. That's why Rich's
recommendation to use BOTH SHUNT AND SERIES resistors is a much better
way.)
The closer to 500mV you make the normal voltage drop, the better will be
the protection from the diode. Therefore it's just a leetle bit better
to use a 1.5 ohm shunt, which will drop 450mV at 300mA.
So now we need to make the 50uA meter read 450mV full-scale. The total
series R required is 450mV/50uA = 9000 ohms. Subtract the 1095 ohms that
is inside the meter, and the external series resistor needed is 7905
ohms. If you make that from a 6.8k fixed resistor plus a 2k trimpot and
you can adjust the full-scale to exactly 300mA.
Adjusting full-scale on a shunted meter should always be done "cold", ie
with no power on the amplifier. Use an external battery, an adjustable
series resistor and a DMM to set the correct current flowing through the
circuit (in this case 300mA). Then adjust the trimpot to read full-
scale, and fix it with a spot of paint - you'll never need to touch it
again.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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