Learning Event 8:
ANALYZE METER OPERATIONS
The remaining major capability of the meter is, of course, the current
measurement function. When using the PSM-6, direct current up to 1 ampere may be
To do this, set the function switch to the DC MA position and set the
range switch at one of its positions.
Refer again to Figure 1-22, the basic meter movement. As discussed earlier,
the amount of current through the movable coil determines the amount of meter
deflection. In the case of PSM-6, 50 microamperes of current through the coil is
necessary to cause full-scale deflection.
What would happen if we put 100
microamperes through the coil?
This would damage or burn: out the coil.
resistance allows only 50 microamperes through the coil if the range switch is set
for higher than the total current in the circuit.
For example, when the range
switch is set at 50 and a current of 50 milliamperes are measured, 49.95
milliamperes flow through the resistor and 50 microamperes flow through the coil.
This condition produces a full-scale deflection for that range setting and reads 50
milliamperes on the scale. In the actual circuit of the meter, different values of
shunt resistors are used for each setting of the range switch.
Here, as with voltage measurements, be extremely careful to ensure that a
range setting of sufficient amplitude is obtained so that the meter needle will not
If the needle does not deflect far enough to obtain an accurate
reading, the range setting can always be decreased.
Learning Event 9:
MATCH OHMMETER READINGS TO CAPACITOR OR INDUCTOR CONDITIONS
Reactive components can become open or shorted. In either case the component
is useless because it cannot store energy.
Coils and capacitors can also become
only partially efficient because of partial shorting or leaking.
Capacitor troubles. A leaky capacitor is equivalent to a partial short. The
dielectric gradually loses its insulating properties under the stress of applied
voltages. A good capacitor has very high resistance (in the megohms). A shorted
capacitor shows zero resistance, while a leaky capacitor indicates less than normal
a. When the ohmmeter is initially connected, its battery charges the
Maximum current flows at the first instant of charge and the meter
indicates low resistance.
As the capacitor charge slows, less current flows, and
the meter indicates more resistance. When the capacitor has charged to the meter
potential, the charging current is zero, and the ohmmeter reads only a small