probably open" not "DC voltmeter bad." By being specific they can assist repair
personnel and insure rapid return of the equipment to a serviceable condition.
f. Figure 10 shows the simplified diagram of a typical DC ammeter. The ammeter
circuit must shunt most of the current being measured away from the meter movement.
On the 10 milliampere (10mA) range, the meter movement resistance of 2K ohm is in
series with the 3K ohm resistance of R11. R17, R18, R19 and R22 are in series with
each other and are in parallel with R11 and the meter movement. Resistive values
have been selected so that 50A will flow through the meter and R11 and 9.950mA
will flow through R22, R17, R19 and R18 when 10mA is being measured. The truth of
this statement can be proved using Ohm's law. Let "x" be equal to the amount of
current flowing through the meter when there is a 10mA current input to the
circuit. The voltage across both parallel sections of the circuit must be equal so:
(1) x(2K 3K) = (.01 -X)(.025 .475 2 22.5)
(2) x(5K) = (.01 -X)(25)
(3) 5000X = .25 - 25X
(4) 5025X = .25
(6) x = .0000497 ampere or 49.7A
Ohm's law proves the validity of the circuit; 49.7A is close enough to 50A to
produce the required accuracy.
h. When the circuit selector switch is moved from the 10mA to the 100mA
position, R18 is placed in series with R11 and the meter movement. This circuit
change will allow a full scale deflection on the meter to be representative of a
100mA input current. Using Ohm's law, calculate the meter current at full scale
under these circuit conditions. Again, your answer should be 49.7A.
OS 010, 7-P13