How to use a Multimeter

Descrition of a Multimeter
Abbreviations and Symbols
Metric Multipliers
Sensitivity
Voltage Measurements (AC & DC)
Current Measurements (AC & DC)
Resistance Measurements
Continuity Tests
Temperature Measurements
Diode Testing
Transistor Testing
Relative Zero
Frequency Measurements
Capacitance Measurements
Important Points to Remember

Description of a Multimeter

A basic multimeter measures Volts (AC & DC), Resistance, and DC current. More expensive ones also measure AC Current and have a continuity buzzer (beeper). Increasingly common also are: Temperature, Frequency and Capacitance measurement capabilities and Auto-Ranging. When most of these functions are combined in one DMM costing around $100, then it becomes very good value.
The two pencil-like probes have metal tips for making electrical contact. One is held in each hand. The red one is for connecting to positive while the black one - which always stays in the "COM socket (jack) - is for negative. These test leads are also available separately with alligator clips for hands-free operation. (See your multimeter supplier)
Unless the DMM is auto-ranging, the number at each position of the function/range switch is the maximum possible value of measurement for that range. "1" on the left side of the display (or "OL") means that the measurement value is above the upper limit for that range position (i.e. "Over range"), so simply choose a higher range. When changing the range, usually you are simply adjusting the resolution by moving the decimal point. If you prefer to disable the "Auto range" function, (on some DMMs), simply press repeatedly the "RH" (Range Hold) button to select a suitable range. (Press and hold it down to return to "Auto range").

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Abbreviations and Symbols

A

Ampere (unit of current)

DH or H

Data Hold (ie display freeze)

~

Alternating Current (AC)

- - -

Direct Current (DC)

Dwell

Duty per Cycle (shown as degrees)

%Duty

Duty per Cycle (shown as a percentage)

Ohm (unit of resistance)

Hz%

*Hertz or *Duty Cycle percentage

V

Volts

Hz

Hertz (unit of frequency)

H

Henry (unit of inductance) see “DH”

F

Farad (unit of capacitance)

Lx

Inductance

Cx

Capacitance - see “F”

hFE

DC current Gain (of transistor)

 

*Audible Continuity (buzzer)

*Diode

RH

RH (Range Hold)

* You may need to toggle between two symbols with one button

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Metric Multipliers

Description

Multiplier

Exponent
Form

Prefix

Symbol

 

1 000 000 000

109

giga

G

Millions of

1 000 000

106

mega

M

Thousands of

1 000

103

kilo

k

 

100

102

hecto

h

 

10

101

deca

da

 

0.1

10-1

deci

d

 

0.01

10-2

centi

c

Thousandths of

0.001

10-3

milli

m

Millionths of

0.000 001

10-6

micro

u

 

0.000 000 001

10-9

nano

n

 

0.000 000 000 001

10-12

pico

p

If you are confused by a display reading in the 100s or 1,000s then that range is most likely measuring thousandths (not thousands) of a unit.
For example: thosandths of volts (mV); or it might be thousandths of kΩ which are simply equal to single ohms.

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Sensitivity

If you do not know what the measurement reading should be, set the rotary switch to the highest range first. Then work down the ranges to increase the number of decimal places and hence improve sensitivity. Even the highest ranges measure values right from zero up to their maximum. But the lowest range possible will give you the best resolution and hence the most accurate reading.

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Voltage Measurements (AC &DC)

Set the range switch to the next voltage range higher than the voltage you want to measure. For example, for 12V, set the switch to 20V, for 24V set the switch to 200V and for 1.5V, set the range switch to 2V or 2000 mV (millivolts). If millivolts are confusing (i.e. no decimal point shown), try the next whole volts range where the decimal point is returned. Even though you lose one decimal place this way, accuracy will not suffer much.
If you don't know what the voltage should be, set the switch to the highest range. Then if you only get zeros or a very small reading, you can go the next lowest range and so on. If you still only get a small reading, you may need to press the AC/DC (or "Function") button.
When using a multimeter in place of a test light, always select Volts. In other words, to test if any power (voltage) is present always use voltage ranges with the meter across the power source. (Fig. 1)
If you connect the test probes the wrong way around (wrong polarity), the measurement will still be accurate, but you will get a minus sign shown. Sparks at the probes mean that something is wrong.
Do not attempt to measure voltage higher than the maximum printed on each probe. Do not assume that unmarked probes are capable of safe insulation at the maximum voltage measurement capability of the multimeter. Do not attempt to test electric fence voltages -which are 1,000s of volts - or you will damage your multimeter.
It is best to test Battery Voltage with a load across it (in parallel). For example, have the battery "in-situ" and with the equipment (e.g. headlights) switched on. For a 1.5 Volt battery, use a load of 10 to 15 ohms. For a 9 Volt battery, use a load of 500 to 1000 ohms.
Voltage measurements of Car Batteries are best interpreted by your Auto Electrician. But if you record -for future reference- your car battery Voltage -with and without headlights on and/or alternator charging- when everything is okay, then you may be able to isolate future alternator and battery faults. The same principle applies to other rechargeable batteries.

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Fig 1

Fig 2

Current Measurements (AC & DC)

When measuring Current (amps) the multimeter must be placed in series with the circuit. This means that the circuit must be broken (Fig. 2) at the point you wish to measure. The circuit is then closed by both multimeter test leads connecting the two sections together, with the multimeter in the middle. Don't connect the DMM (ampmeter) across a voltage source (i.e. in parallel). Of course this procedure does not apply when using a clamp meter, which simply clamps around the wire in which current is flowing.
If you are not sure whether AC or DC is flowing in the circuit, don't worry. The multimeter will give a negligible or zero reading if you have chosen the wrong one.
For measuring h igh current (eg 10 Amp or 20 Amp ranges), most multimeters have a separate jack (socket) for the red test lead only. The maximum current allowable for each jack is printed beside it. DO NOT USE-THIS JACK FOR VOLTAGE TESTING or you will notice large sparks and will damage your meter.
If an amp range stops working and reads zero all the time, it is likely that you have blown an internal fuse - (see the operator's manual). Many 10A and 20A jacks don't have fuse protection. This is clearly shown on the front of the DMM

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Resistance Measurements

The multimeter's internal battery supplies all the power for resistance measurements. (This includes continuity and diode tests). So when checking in-circuit resistance, be sure the circuit has all power removed and any capacitors are fully discharged. Otherwise you will damage the multimeter or at least get a false reading.
Disconnecting one end of the component under test will make sure that any adjacent component won't interfere with the reading. (Fig.3)

Fig 3

Continuity Tests

The continuity buzzer/beeper will only sound when power (current)_from the DMM's internal battery is flowing out one probe and in the other. By connecting both test probes to a component or a circuit (one each end), the buzzer confirms that current flows. Continuity effectively means: 'conducts current', or that there is 'no break in the (part of) circuit or wire' under test. (i.e. between the test probes). (Fig.3)
Since the Continuity Buzzer function is actually another resistance test, the instructions for resistance measurements above also apply to continuity testing. NEVER EVER use the continuity function to test for power in the circuit as you would use a test light! Use voltage ranges instead for this.

Here are some practical uses for continuity testing:

  • To check for a blown fuse or globe filament.
  • To determine the on/off positions of a switch - by touching the probes on the switch terminals while throwing the toggle.
  • When a short circuit to earth keeps blowing fuses:- Connect one probe to earth and the other to the cold side of the fuse holder (fuse removed). The buzzer will stop sounding when you isolate and remove the short.
  • A rough comparison of capacitors can be made by noting the length of time the buzzer sounds while the capacitor is charging up.

If the Continuity Buzzer refuses to sound when you touch both the probes together, then try pressing either the , or "Function" button (while checking the symbol displayed).

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Temperature Measurements

The temperature range capabilities of meters usually far exceed that of the thermocouples supplied. The maximum temperature possible is limited by the thermocouples' insulation material (e.g. fibreglass/Teflon - approx. 400o C; plastic - approx. 250oC ). However, thermocouples which can withstand higher temperature are available.

Diode Testing

A diode should conduct (continuity) in one direction only. (Fig. 3) This is called the "forward" direction (i.e. from the anode to the cathode of the diode). The DMM will give an approximate forward voltage drop reading. (Switch position is to be used).
Often only a "go/no go" test is required on a diode. In practice this means that a very low reading for forward, and the overrange symbol (1) when the probes are reversed confirms that the diode is okay. If the same reading is obtained for both directions then the diode is faulty.
The instructions for 'Resistance Measurements' also apply to diode testing.
 

Transistor Testing

Before testing a transistor, you must determine which leads/connections are the Emitter, Collector and Base (E, C, B) as well as whether the transistor is PNP or NPN. Most DMM's have a spare emitter socket.
Select the "hFE" position on the range switch. The "hFE" of a transistor is the ratio of current change at the collector to the smaller current change at the base. In other words it is the Amplification or Gain of the transistor.
To find the expected gain (hFe), refer to the transistor supplier's data sheet. Transistor data books and pinout (lead connections) diagrams are available from multimeter suppliers. Alternatively you could compare the hFE reading with another similar transistor which is okay.

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Relative Zero

The "Relative Zero" button will erase the current measurement shown on the display screen while still actually measuring it internally. This feature is handy for subtracting the DMM's internal value displayed (e.g. mV, mA ). Until you cancel the "Relative Zero" (by pressing it again) the DMM assumes that you want that measurement to be displayed as "zero" on the display. Hence any subsequent measurement shown is really the difference from (or relative to) that artificial zero point.

Frequency Measurements

Waves on the beach or sound waves (vibrations) have '*frequency" (i.e. how often each wave is). One Hertz (Hz) equals one cycle (high/on and low/off once) per second. DMMs can only measure vibrations or pulses of voltage, (e.g. the audio signal -in the wires- to a speaker, or the alternating speed of AC Voltage).
Each complete wave cycle (i.e. 100% of the cycle) has a "high" or "on" -duration- portion. While measuring frequency, the "% Duty", "Hz%" or "Hz Duty" button is pressed to obtain the percentage of this -duration- portion ("duty cycle") relative to the full (100%) wave.

Capacitance Measurements

Often the initial reading is incorrect, since it can take up to 16 seconds for the reading displayed to stabilize.

Important Points to Remember

  • If the Display has frozen on a reading (or no reading) try releasing the "Hold" (H) or “Display Hold" (DH) Button.
  • The black test lead always goes in the "COM” (common) jack.
  • Check the position of the Range selection switch before each measurement.
  • Always be careful when working with voltages above 60V DC or 30V AC. Keep fingers behind the probe barriers while measuring.
  • Connect the meter in series for Current (Amps) and (Fig. 2) connect the meter in parallelf or Voltage. NEVER vice versa (Fig. 1)
  • If the probes are reverse polarity, a minus sign will appear and no damage will result. (i.e. red and black swapped).
  • Always check that the red (+) test lead is in the correct jack for that range. (It is easy to forget). If you test voltage with the " 10A” or "20A" jack (socket) you will either blow the fuse or permanently damage your DMM.
  • Before rotating the range selector to change functions, always disconnect the test leads from the circuit under test - especially if voltage is present.
  • Never exceed the protection limits for each range.
  • When the meter is linked to circuit do not touch unused terminals.
  • When the value scale to be measured is unknown, set the range selector at the highest position. (With "Autorange", the DMM will decide).
  • Never perform resistance measurements on live circuits. (Fig. 3)
  • Never use the meter unless the back cover is in place and properly fastened.
  • Switch the meter off when not in use. (Some DMMs will automatically switch off).
  • Read the meter's instruction manual thoroughly.

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