A multimeter is a measuring instrument that anyone involved in electronics will need to use at some stage. A multimeter can be used to measure voltage, current, resistance, continuity and other parameters.
Measuring DC Voltage and Resistance
The video below shows a multimeter being used to measure voltage from batteries (DC (direct current) voltage) and the resistance of some resistors.
Measuring DC Voltage
Selecting the DC Voltage Range
To measure DC voltage using a multimeter, simply turn the selector dial on the multimeter to the DC voltage setting. On an auto-ranging multimeter such as the one shown in the video, there will only be one DC voltage selection on the dial.
A multimeter that is not auto-ranging will have a selection of voltages on the dial that can be chosen, e.g. 2V, 20V, 200V, 1000V. On this type of multimeter, start by selecting the highest voltage on the dial and then turn down to a lower voltage if the voltage measured is seen to be low. If you are measuring a single battery cell and you know that it is a 1.2V or 1.5V cell, then you can start by setting the multimeter dial to 2V or 20V.
Taking a Measurement
After selecting DC voltage on the multimeter dial, use the two probes to measure across the battery terminals. The black probe should be used on the negative terminal of the battery and must be connected to the COM connection on the multimeter. The red probe should be used on the positive terminal of the battery and connected to the connection on the multimeter that is marked V. This connection may have other symbols marking it as well, such as the ohm symbol (Ω).
After applying the probes to the battery, the voltage of the battery will be shown on the multimeter display.
Polarity
If the red and black multimeter probes are connected the wrong way around to the battery (i.e. black on positive and red on negative) then a digital multimeter will show a negative sign next to the voltage reading on the display.
On a digital multimeter, it does not matter if the leads are reversed when measuring voltage. Getting the leads the right way around (correct polarity – red on positive and black on negative) does matter on the old analog multimeters (the type with the indicator needle). An analog (or analogue) multimeter can be damaged if the polarity is reversed on the leads.
Measurement Results
In the video, batteries made up of 1.2V rechargeable cells are used. The first one measured has six cells, so the voltage should be about 1.2V × 6 = 7.2V. The second battery measured contains two cells or 1.2V × 2 = 2.4V. A single 1.2V cell is measured last.
When 1.2V batteries are fully charged, they will have a voltage of slightly more than 1.2V. This can be seen by the measurements taken in the video.
Measuring Resistance
Selecting the Resistance Range (Ohms – Ω)
To measure resistance, using a multimeter, turn the multimeter dial to the ohm marking. As for measuring voltage on an auto-ranging multimeter, there will be only one dial setting for measuring resistance.
On a multimeter that is not auto-ranging, there will be a number of different ranges marked on the multimeter dial, e.g. 200, 2k, 20k, 200k, 2M, 20M. If the approximate range of the resistance to be measured is unknown, then always start measuring with the biggest range, e.g. 20M. If the value measured on this range is seen to be small, then the dial can be turned down to a smaller range.
Precaution
Never measure resistance on a live circuit – i.e. a circuit that has power switched on to it.
Taking the Resistance Measurement
Apply the tips of the multimeter probes across the resistance to be measured and read the resistance value on the multimeter display.
Measurement Results
The resistors measured in the video all have a 5% tolerance. This means that the values of the resistors could be between the 5% more or 5% less than the stated value of the resistor that you would read by decoding the resistor colour code
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