MEASURING VOLTAGE

SECTION TWO

MEASURING VOLTAGE

Most of the readings taken with a multimeter will be VOLTAGE readings.

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 ACor DC.

If the meter is not auto-ranging, you will have to select if the volt age is from a DC source or if the voltage is from an 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 midpoint. 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.

A

The reading should be 5v as show in diagram.

But the analogue multimeter has an internal resistance of 100k and it creates a circuit C shown in.

The top 1M and 100k from the meter create a combined PARALLEL resistance of 90k.

This forms a series circuit wit h 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 1vIf 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 about1%.

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 and500mV. 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 VOLTAGE IN A CIRCUIT