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TEST INSTRUMENTS

V. Ryan 2002 - 2009

 

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Voltage, current and resistance can easily be measured by using a multimeter. However, an ammeter measures current, a voltmeter measures the potential difference (voltage) between two points, and an ohmmeter measures resistance. However, a multimeter measures all these. There are two types, analogue and digital. The multimeter is the most important electronic test instrument. Two wires are normally used along with the multimeter (called probes) and they are colour coded - black and red.

   

     

A DIGITAL Multimeter is highly accurate and easier to read than an analogue type. It is best used for finding the precise value of a voltage, current or resistance.

An ANALOGUE Multimeter is less expensive and less precise than a digital type. Often it will be used for measuring a slowly changing voltage, current or resistance.

 

The probes (seen with the digital metre) are connected to the meter. They can be disconnected and so it is important to ensure that they are attached to the correct sockets when in use. Also, some meters have four possible sockets which means you must follow the manufacturers instructions carefully when attaching the two probes.

   

 

   

Either meter has a variety of settings depending on whether you need to measure resistance, current or voltage. Normally the function selector has a setting for each of these and there is also a setting for the range that you are trying to read.
For example you may need to measure the value of a resistor in ohms when a colour chart is not be available.

   
In order to do this you would follow these steps:
1. Set the function selector to ohms and to the range where you expect to find a reading. There may be a range for zero to 1K OR 1K and over etc....
2. Ensure that the probes are attached correctly and touch them across the component you are measuring, in this case a resistor.
3. The results can be read on the scale. If there is no reading, try another range setting.

To measure DC voltage follow these steps:
1. Set the function selector to one of the DCV positions.
2. Connect the test probes to the circuit and read the voltage on the scale.
   

EXAMPLE OF DIGITAL METER

EXAMPLE OF ANALOGUE METER

 
 
 

QUESTIONS:

1. Draw simplified diagrams of both types of meters.

2. Explain how digital and analogue meters differ.

 

 

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