Using A Multimeter

The multimeter is probably the first item of test equipment acquired by anyone with an interest in electronics, who does a little DIY at home, or who does some of their own car repairs. Their cost can range from the bargain items of 5 up to 200 or more for the better quality/more accurate/higher spec. items. It goes without saying that most modern ones will be digital, although analogue still has its uses. For our sort of use, one of the cheaper ones at around 20 to 40 will prove adequate for the job, but make sure it is one intended for general electronics use, rather than some more specialised use.

This amount should be able to buy a reasonable quality one with a good range of features. I would be looking for one with at least 4 DC volts, 4 AC volts ranges; 4 or 5 ranges of current AC & DC (including 10amps); 6 ranges of resistance.

In addition to this, a built-in continuity buzzer is useful, but ensure there is no delay at all in the sounding of this when the probes are shorted. A few milliseconds of delay, can cause some awful confusion during repetitive continuity testing. A diode test facility is useful, as is socket on the unit for the testing of transistors. Some include a range for the testing of capacitors, this again can be useful, especially if you have not yet gained some understanding of the printed codes used for values on these devices. A fold out from the back stand, makes the instrument a little more useable as a bench instrument and if you are forgetful, it is worth finding one with an auto shut off to save the battery when you leave it on. Some meters have an auto ranging feature, but a meter should be chosen where this is an optional feature rather than the default.

Modern digital instruments are a world away from their analogue counterparts of a few years ago in cost, accuracy and input impedance. The modern digital meter on any of the voltage measurement scales presents a load of 10M Ohm to the circuit under test. This should always be kept in mind, when conducting any tests on circuits, particularly circuits involving small signals. It should also be kept in mind when checking current flow, that the current consumption will be affected by the small amount of internal resistance of the meter once put into the circuit.

Any measurements made will always affect to some extent, the operating conditions within the circuit whilst ever the instruments probes are connected. Measure the voltage, and the measured voltage will be somewhat less, as a result of the measurement process itself.

It is good practise if the range selection knob does not include the on/off switch, to get into an habit of 'parking' it on the highest AC range available. This helps to avoid damage, should you unthinkingly connect the probes to a circuit without first selecting the correct range first.

For electronics work, the first thing I do with a new meter is replace the black lead with one with a crock clip on the end. With the black connected all the time to ground, it can save a great deal of effort trying to align two probes at the same time, whilst trying to check a series of voltages around the circuit. The majority of a service manuals indicated test points for a circuit, will give a voltage reading measured with respect to ground or the -ve power rail. Holding one probe on the ground with one hand, whilst trying to successfully manoeuvre the other positive probe is not often not easy.

Now to actually put the instrument to some basic use.....

The first thing you will likely gain amusement from is the continuity buzzer or sounder. These are very useful as a quick way to trace circuit tracks on a PCB. You should always ensure any power supplies and batteries are turned off before doing any of these continuity tests. Put one probe on one part of a circuit and the continuity sounder will sound when you touch the second probe anywhere on the PCB which is connected directly with copper track to the first point. Wires in a loom without any identification can also be tested using the continuity buzzer, it is also a quick way of testing fuses out of circuit. Two points to be aware of whilst testing in this way, continuity can be falsely indicated through a capacitor until it charges up and continuity can be expected through coils, chokes plus other similar windings. Most continuity buzzers are set up to sound on any impedance less than 30 to 60 Ohms.

Whilst using the voltage or current ranges, if the approximate value is not already known, then you should start with the higher ranges and progress down in range, the meter should be disconnected from the circuit before changing range and reapplied after. Actual polarity is not such a great problem as with older analogue instruments, as the display will show a minus sign (-) if the probes are connected in reverse. The method of testing voltage, is to first work out whether you are measuring an AC or DC value, or perhaps a combination of both. The probes are placed across the points to be measured, in parallel with the voltage test points for voltage measurements. Both the AC and DC components can be measured in a circuit, but you should be aware that AC ranges of meters are calibrated for sinusoidal waveforms and indicate the RMS voltage (average), not the peak voltage.... Fine for measuring the AC voltage output of the secondary of a mains transformer, or the mains ripple on a DC supply. For all other AC waveforms (none sinusoidal) the oscilloscope is the instrument of choice.

For normal current measurements, the meter is placed in series with one of the supply leads. To measure the current drawn by battery powered equipment, a useful to have gadget is small piece of double sided PCB with a short wire soldered to each side, to which the probes can be attached. This, inserted between a pair of cells, allows the current to be more quickly and easily measured.

Another way to measure current in a small section of a circuit, is to measure the voltage developed across a known value of resistor which is already part of the circuit. Using Ohms law the current flowing through the resistor can then be calculated.

The voltage used by a meter on the resistance ranges is not usually enough to forward bias a semiconductor junction, so in circuit tests can be often be conducted, providing care is taken to make allowance for other components which might be part of the circuit. The diode testing function is done at a slightly higher voltage, enough to forward bias a junction and thus provide a reading, The reading is in voltage. When the junction is conducting it will read somewhere between 0.5 to 0.7v and not conducting through the junction the open circuit voltage. The later depends on the individual instrument, but is usually around or below 1v.

The transistor test, requires the emitter, base and the collector pins of the device to be inserted into the correct sockets, then you select the type of device (PNP or NPN). It will then indicate a rough figure of gain, no gain indicates a faulty device. It is only really a rough and ready test of go/no go, the device may still be out of spec. or break down under voltage or current load conditions.

Digital multimeters are generally fine for most uses, however where a voltage is varying rapidly or where something needs to be peaked up, an old style analogue instrument excels. Digital meters require a finite amount of time of steady input, to be able to take a valid reading. If the value being measured varies rapidly enough, then the meter will become confused and provide false indications.

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