Measuring dc Voltages
You set the multimeter to operate as a dc voltmeter by placing the function switch in either of two
positions: +DC or -DC. The meter leads, as in the case of the ohmmeter function, must be connected to
the proper meter jacks. When you measure dc voltages, be sure the red lead is the positive lead and the
black lead is the negative, or common, lead. View A of figure 4-4 is a functional block diagram of dc
voltage circuits in a multimeter. View B shows the jacks and switch positions for measuring dc voltages.
Figure 4-4.Functional block diagram of dc voltage circuits.
When the meter is connected in a circuit, it becomes a circuit component. Because all meters have
some resistance, they alter the circuit by changing the current. The resistance presented by the voltmeter
depends on the amount of voltage being measured and the position of the function switch.
Some multimeters use a 20,000 ohms-per-volt meter sensitivity for measuring dc voltage and a 5,000
ohms-per-volt sensitivity for measuring ac voltage. The higher the meter resistance, the less it will load
the circuit. The idea is to keep circuit loading to an absolute minimum so that the circuit under test is
unaffected by the meter. In this way, you can get a clearer picture of what the circuit malfunction is, not
the effect of the meter on the circuit.
Again, refer to figure 4-4. With the function switch set to either +DC or -DC, lets consider the
effect of the range switch on the meter scale to be used. When measuring dc voltages, you have eight
voltage ranges available: .25V, 2.5V, 10V, 50V, 250V, and 500V (1- and 1,000-volt special application
plug-ins are also available). The setting of the range switch determines the maximum value represented
on the meter. When measuring dc voltages, use the scale marked DC (figure 4-3). The last number at the
extreme right side of the DC scale indicates the maximum value of the range being used. When the range
switch is in the 2.5V position, the scale represents a maximum of 2.5 volts.
To simplify the relationship between the digits on the meter scale and the setting of the range switch,
always use the multiple of the full-scale-deflection digits on the meter face that correspond to the numbers
of the range switch. For example, use the 250 scale for the 250MV jack, 2.5V, and 250V ranges; the 50
on the scale for 50V and 500V ranges; and the 10 on the scale for the 10V and 1,000V ranges.