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Digital multimeters

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All digital meters contain a battery so they don’t use power from the circuit under test. This means that on their DC voltage ranges they have a very high resistance (usually called input impedance) of 1MΩ or more (usually 10MΩ), and they do not affect the circuit under test.

Typical ranges for digital multimeters:
(the values given are the maximum reading on each range)

· DC Voltage: 200mV, 2000mV, 20V, 200V, 600V.

· AC Voltage: 200V, 600V.

· DC Current: 200µA, 2000µA, 20mA, 200mA, 10A.

· Resistance: 200Ω, 2000Ω, 20kΩ, 200kΩ, 2000kΩ.

Analogue multimeters

Analogue meters take a little power from the circuit under test to operate their pointer. They must have a high sensitivity of at least 20kΩ/V otherwise they give an incorrect reading.

Batteries inside the meter give power for measurements in the resistance ranges. Don’t leave the meter set to a resistance range. The leads can touch accidentally and run the battery flat.

Typical ranges for analogue multimeters are:
(the voltage and current values given are the maximum reading on each range)

· DC Voltage: 0.5V, 2.5V, 10V, 50V, 250V, 1000V.

· AC Voltage: 10V, 50V, 250V, 1000V.

· DC Current: 50µA, 2.5mA, 25mA, 250mA.

· Resistance: 20Ω, 200 Ω, 2kΩ, 20kΩ, 200kΩ

 

Analogue Multimeter Scales
The top scale is used when measuring resistance.

 

Greek letter Ω means ohms

Greek letter µ means micro

Latin letter m means milli

 

Measuring voltage and current with a multimeter

1. Select a range with a maximum greater than you expect the reading to be.

2. Connect the meter, making sure the leads are the correct way round.
Digital meters can be safely connected in reverse, but an analogue meter may be damaged.

3. If the reading goes off the scale: immediately disconnect and select a higher range.

Multimeters are easily damaged by careless use so:

· Always disconnect the multimeter before adjusting the range switch.

· Always check the the range switch before you connect to a circuit.

· Never leave a multimeter set to a current range (except when taking a reading).

 

Text 2

Cables

A wire is a single conductor which may have insulation (usually plastic).

A cable is a group of one or more flexible conductors (wires).


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