b. Organizational maintenance personnel should understand basic monitoring
systems in the 500-ampere test stand. Meter movements by themselves are unable to
of the current to be measured is shunted around the meter movement; when used in
voltmeter circuits, the current is kept at a low value by placing a high resistance
in series with the meter movement.
c. A 2,000-ohm (2K), 50 microampere (50A) meter movement is being used in
the circuits illustrated in figures 9 and 10. The 2K states the internal
resistance of the movement and the 50MA states the amount of current required to
deflect the movement's needle to the full scale mark. By using Ohm s law the
voltage across the meter, when the meter is indicating full scale, can be
determined:
Example:
(1) E = I x R
(2) E = (50 x 10-6) (2 x 103)
(3) E = 100 x 10-3V
(4) E = .1V
To use the meter movement by itself we would have to be satisfied with an ampere
range of 0-50A or a voltage range of 0-.1V. These ranges are unsatisfactory for
most applications.
d. Figure 9 shows the simplified schematic diagram of a typical DC
voltmeter. The meter movement and associated circuit provides voltage measurement
ranges of 0-2.5VDC, 0-10VDC, 0-50VDC and 0-1,000VDC. When 2.5VDC is correctly
applied between the negative meter input and the 2.5V Jack, 50A will flow in the
circuit and the meter will deflect to full scale. The current is limited to 50A
by the combination of the 48K resistance in the circuit and the 2K resistance of
the meter movement.
Example
(1) E
IxR
(50 x 10-6) (48 x 103 2 x 103)
(2) E
(3) E -(50 x 10-6) (50 x 103)
(4) E = 2,500 x 10-3
(5) E = 2.5VDC
The 48K resistor which was selected to be placed in series with the meter
movement's resistance of 2K established the voltage which full scale deflection
represents. Use Ohm's law to analyze the 10V, 50V, 250V and 1,000V ranges. In
each case, 50A will flow through the meter when .the maximum voltage for the range
is applied. The scales on the meter face are marked off in appropriate increments.
When 2VDC is applied on the 2.5V range, the meter movement's pointer will move from
zero to 4/5 of full scale. Therefore, the scale graduation at this point would
indicate a 2VDC input.
e. Organizational maintenance personnel should be able to isolate the cause
of simple meter malfunctions. The meter shown in figure 9 responds to voltages
applied on the 2.5VDC, 10VDC and 50VDC ranges, but the pointer remains at zero when
voltage is applied to the 250VDC or the 1,000VDC ranges. What is wrong? The answer
to this question should be obvious to organizational maintenance personnel. When
requesting repair they would state "R14
OS 010, 7-P12
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