Before taking a reading, you should select the highest range and if the needle does not
move up scale (to the right), you can select another range.
Always switch to the highest range before probing a circuit and keep your fingers away
from the component being tested.
If the meter is Digital, select the highest range or use the auto-ranging feature, by
selecting "V." The meter will automatically produce a result, even if the voltage is AC
or DC.
If the meter is not auto-ranging, you will have to select DC source or AC source.
DC means Direct Current and the voltage is coming from a battery or supply where the voltage is steady and not changing and AC means Alternating Current where the voltage is coming from a
voltage that is rising and falling.
You can measure the voltage at different points in a circuit by connecting the black
probe to chassis. This is the 0v reference and is commonly called "Chassis" or "Earth"
or "Ground" or "0v."
The red lead is called the "measuring lead" or "measuring probe" and it can measure
voltages at any point in a circuit. Sometimes there are "test points" on a circuit and
these are wires or loops designed to hold the tip of the red probe (or a red probe fitted
with a mini clip).
You can also measure voltages ACROSS A COMPONENT. In other words, the reading is
taken in PARALLEL with the component. It may be the voltage across a transistor,
resistor, capacitor, diode or coil. In most cases this voltage will be less than the supply
voltage.
If you are measuring the voltage in a circuit that has a HIGH IMPEDANCE, the reading
will be inaccurate, up to 90% !!!, if you use a cheap analogue meter.
Here's a simple case.
The circuit below consists of two 1M resistors in series. The voltage at the mid point
will be 5v when nothing is connected to the mid point. But if we use a cheap analogue
multimeter set to 10v, the resistance of the meter will be about 100k, if the meter has
a sensitivity of 10k/v and the reading will be incorrect.
Here how it works:
Every meter has a sensitivity. The sensitivity of the meter is the sensitivity of the
movement and is the amount of current required to deflect the needle FULL SCALE.
This current is very small, normally 1/10th of a milliamp and corresponds to a
sensitivity of 10k/volt (or 1/30th mA, for a sensitivity of 30k/v).
If an analogue meter is set to 10v, the internal resistance of the meter will be 100k for
a 10k/v movement.
If this multimeter is used to test the following circuit, the reading will be inaccurate.
The reading should be 5v as show in diagram A.
But the analogue multimeter has an internal resistance of 100k and it creates a circuit
shown in C.
The top 1M and 100k from the meter create a combined PARALLEL resistance of 90k.
This forms a series circuit with the lower 1M and the meter will read less than 1v
If we measure the voltage across the lower 1M, the 100k meter will form a value of
resistance with the lower 1M and it will read less than 1v
If the multimeter is 30k/v, the readings will be 2v. See how easy it is to get a totally
inaccurate reading.
This introduces two new terms:
HIGH IMPEDANCE CIRCUIT and "RESISTORS in SERIES and PARALLEL."
If the reading is taken with a Digital Meter, it will be more accurate as a DMM does not
take any current from the circuit (to activate the meter). In other words it has a very
HIGH input impedance. Most Digital Multimeters have a fixed input resistance
(impedance) of 10M - no matter what scale is selected. That's the reason for choosing
a DMM for high impedance circuits. It also gives a reading that is accurate to about
1%.
MEASURING VOLTAGES IN A CIRCUIT
You can take many voltage-measurements in a circuit. You can measure "across" a
component, or between any point in a circuit and either the positive rail or earth rail
(0v rail). In the following circuit, the 5 most important voltage-measurements are
shown. Voltage "A" is across the electret microphone. It should be between 20mV and
500mV. Voltage "B" should be about 0.6v. Voltage "C" should be about half-rail
voltage. This allows the transistor to amplify both the positive and negative parts of
the waveform. Voltage "D" should be about 1-3v. Voltage "E" should be the battery
voltage of 12v.
MEASURING VOLTAGES IN A CIRCUIT
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