Multimeter
How to use a multimeter?
Working with electrical or electronic equipment without a multimeter is like reading without light, or exploring without a compass. The multimeter is, without doubt the most useful instrument you can have. So before you get serious about electronics, there are several things you’ll need to know about using your multimeter.
Unpacking and setting up
When you purchase a multimeter, it is seldom ready for use. Most often, the batteries must be fitted. Make sure you follow the manufacturer’s instructions carefully, and remember, even if the batteries are already inside the compartment, you may still need to remove some plastic wrap, or clip on a battery clip.
Included with your multimeter is a set of test leads. Sometimes these are permanently attached to the meter, but often they need to be plugged into the meter before use.
There are two test leads, one red and one black. If your multimeter only has two sockets, they are usually colour coded to show which lead plugs into which socket. If there are more than two sockets, find the one marked ‘COM’ and plug the black lead into this. Now look for another socket marked with ‘V-?’ (which is usually coloured red) and plug in the red lead.
Switch the power switch ‘on’ (if it has one), select one of the ‘OHMS’ ranges using the main selector switch (usually a very large dial on the front of the meter) and then connect the ends of the test leads or ‘probes’ together. The display should read something close to zero. If you‘ve come this far ... you‘re ready for work.
What can you measure?
Most multimeters give you at least four measurement options: DC voltage; DC current; AC voltage and Resistance (ohms). More expensive meters often provide other options like AC current, capacitance, frequency, etc.
Each measurement option is divided into ranges to give more flexibility. A range is named after the maximum reading possible for that given range. For example, if your meter has a 10 volt DC range, trying to measure more than 10 volts when this range is selected will cause a digital meter to indicate ‘overload’. Generally, the more ranges there are, the better, because you can obtain a more precise reading from your meter.
Always play it safe!
Your multimeter is a valuable test instrument which should be used with care to protect both the operator (you), and the instrument. Before making any measurement, check that the meter is on a suitable range and the test leads are plugged into the right sockets. Most meters have some built-in protection if you make a mistake but you can´t always rely on this.
Most meters do not have fuse protection for higher current ranges. If, for example, you tried to measure the voltage of a 12 volt car battery and accidentally plugged the probe into the ‘10 amp’ socket instead of the ‘V-?’ socket, you would effectively be shorting out the battery. The likely result would be personal injury due to burns, a possible exploding battery and of course, a wrecked meter. An even greater danger applies when measuring high voltages, such as the mains.
CAUTION: Most meters are designed for measuring voltages and currents in low power electronic circuits. Measurements should not be made on high power circuits unless the meter is suitably rated and extreme care is taken.
Measuring DC current
The important thing to remember when measuring current is, that the meter must be placed in SERIES with the circuit. This means that the circuit must be open, with the meter leads connecting the two sections together so all the current flows through the meter.
Normally, a circuit is opened by disconnecting a wire or component but unfortunately, this is not always possible. For instance, if you want to know the current flowing through a certain PCB track, you must break the track to include the meter in series. This can usually be done by carefully cutting the track with a very sharp blade. After you have taken the measurement, flow solder over the cut to restore the connection.
Remember, when measuring current, start with your meter on the highest range and work down. It is also important that the test leads are plugged into the correct sockets on the meter.
If in doubt, check the manual
Finally, the probes should be connected to the circuit with the correct polarity. The red probe is connected to the more positive side of the circuit; the black probe is connected to the more negative side. On a digital meter, you still get the right reading, but with a ‘minus’ sign in front of the value.
Measuring DC voltage
Measuring voltage is the exact opposite to measuring current: the meter is placed in PARALLEL with the circuit or component being measured and you measure the potential difference (voltage) between the two points. Once again, the test leads must be in the right meter sockets, polarity must be observed, and you should start with the highest range and work down.
Measuring AC voltage
This is the same as measuring DC voltages, just follow the same steps and remember that the meter must be placed in PARALLEL with the circuit or component being measured.
A word of explanation about what the meter actually reads. During one cycle an AC wave starts at zero, rises to a peak, falls back to zero and then does the same thing in the other direction.
The majority of meters indicate what is called the ‘RMS’ value of the voltage. This value (which stands for Root Mean Square) is a type of average, that takes into account the fact that the voltage is always changing.
The ‘RMS’ voltage has exactly the same ‘work value’ as a DC voltage of the same magnitude. In other words, if you supplied an electric heater with 240 volts RMS, and then 240 volts DC, in both cases the heater would give out the same heat.
Multimeters fall into two classes when it comes to measuring AC voltage. Most meters can only indicate the correct ‘RMS’ voltage when the AC signal is a sine wave. More expensive meters can indicate the ‘RMS’ value for a variety of wave shapes; these meters are called ‘true RMS’ meters.
Fortunately the majority of AC voltages that you will want to measure are in the form of a sine wave, so your meter will generally indicate the correct value.
Measuring resistance
When measuring resistance, it is important that the component you are reading is not affected by other components in the circuit. There is no point measuring a resistor when there is another resistor in parallel which is interfering with the reading. Therefore, you should remove one end of the component from the circuit, to avoid any possible influence when measuring. Always make sure that the power is turned off before making any measurement or disconnecting any components.
Select the lowest resistance range and place the probes across the resistor (or component you are testing). If the meter does not give a sensible reading, switch up to the next range.
The best range to choose is the one where the meter gives the greatest number of digits after the decimal point.
Points to remember
- Meter in Series for current.
- Meter in Parallel for voltage.
- Start on the highest range and work down for voltage and current.
- Isolate component when measuring resistance.
- Take care of your meter.
