DC Fundamentals Unit 13 – Direct Current Meters
In an analog meter, the most common type of meter movement is the d'Arsonval meter
movement, which uses a moving coil. The d'Arsonval meter movement consists of a permanent
magnet, control springs, jewelled bearings, mounting hardware, and a moving element.
This moving element is wrapped by a coil of wire and has a pointer attached. In the d'Arsonval
meter, the deflection of the pointer is directly proportional to the amount of current flowing
through the coil.
The current required to deflect the pointer all the way to the right, to the last mark on the meter
scale, is called full-scale meter current (IM). The wire used in the meter coil has some
resistance. This resistance is the internal meter resistance (RM).
The specifications of the meter movement used in this unit are:
IM = 1 mA full-scale current
RM = 2300 ohms, ± 20%
Meter Type: moving magnet
Based on the specifications, the meter coil will drop 2.3 volts at full-scale deflection (1 mA x
2300 ohms).
A basic meter movement combined with other components can be made to indicate voltage,
current, or resistance.
When a meter movement is configured to read current, the external circuit generally provides a
parallel or shunt path to bypass excess current (greater than IM) around the meter coil.
When the meter movement is configured to read voltage or resistance, the external circuit
generally provides a series resistance to limit current to the value of IM (at full-scale deflection).
NEW TERMS AND WORDS
moving coil - The part of a meter having a pointer that deflects with current.
permanent magnet - A magnet that retains its magnetism after the magnetizing force has been
removed.
full-scale meter current (IM) - Specified current required for full-scale deflection.
meter resistance (RM) - The ohmic resistance of the wire making up the moving coil.
sensitivity - The ratio of the total resistance of an instrument to the full-scale meter deflection.