348 Section 3 Fundamentals of Electricity and Electronics
Voltage Measurements
Voltage measurements are also useful in deter-
mining the condition of a circuit or components. By
checking the voltage, you can gather more infor-
mation about the electrical problem. For example,
by measuring battery voltage, you can check the
charge or condition of a vehicle’s battery. A fully
charged 12-volt battery should have 12.6 volts.
If your voltmeter reads only 11 volts, the battery
either needs to be recharged, or it has an inter-
nal problem, such as low electrolyte, deteriorated
plates, or shorts in cells.
As another example, imagine your engine cranks
very slowly and will not start. You might measure
the voltage across your positive and negative bat-
tery terminals to measure the battery’s state of
charge. A fully charged 12-volt battery will have 12.6
volts. As it is drained of electrical energy, this volt-
age goes down to 11.5 volts, 10 volts, until the bat-
tery is “dead” at zero volts.
Voltage Drop Measurements
A voltage drop measurement determines the
internal resistance of a wire or component. Instead
of disconnecting the power source and using an
ohmmeter, you can check voltage drops to fi nd resis-
tance for each circuit section without disconnecting
the circuit. This is a much faster way of checking a
circuit for high resistance.
For example, suppose you suspect a switch of hav-
ing a poor internal connection and high resistance.
To measure the voltage drop, connect the voltmeter
across the input and output leads of the switch, as
shown in Figure 22-24. If the voltage drop across the
closed switch is too high (more than about one volt
for most switches), then the switch has a poor inter-
nal connection. A good switch, when closed, has
little or no voltage drop.
Figure 22-25 further illustrates the principle of
voltage drop measurements. If the load resistances
are equal, the voltage drop is equal across these
components. As you may recall from Chapter 3, the
sum of all voltage drops equals the supply voltage
in a series circuit.
Figure 22-26 shows how voltage drops vary with
resistance. In this example, if a blower switch is set
on low, a small amount of current passes through the
resistor pack to the blower motor and the motor runs
slowly. This shows up as a high voltage drop at the
low-speed (high-resistance) terminal of the switch.
With the switch on medium, less resistance allows
more current to the motor, resulting in higher blower
rpm. With the switch on high, the voltage drop is zero
because there is no resistance across the resistor pack.
Resistance Measurements
Resistance measurements are commonly taken
when the circuit or component is disconnected or
when very accurate resistance readings are needed.
Figure 22-27 illustrates some examples of resistance
measurements.
Figure 22-17. Study the basic parts of this digital-analog
multimeter.
Figure 22-18. Auto ranging meters automatically adjust to the
correct value range. This value is being displayed in kilohms (kΩ).
Digital
display
Control
buttons
Test lead
sockets
Analog
bar graph
Function
knob
Value is displayed in Kilohms