Faculty of Engineering

Multimeter

Name: Samia Ahmed Youssef Ahmed Department: Electrical Engineering.

2nd year

2009-12-05

Multimeter

HistoryScientists originally used galvanometers to measure current. A galvanometer may be wired to measure resistance (given a known voltage source) or voltage (given a fixed resistance). While appropriate for primitive lab use, switching from one setup to another is inconvenient in the field.

Multimeters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. The invention of the first multimeter is attributed to Post Office engineer Donald Macadie, who became dissatisfied with having to carry many separate instruments required for the maintenance of the telecommunication circuits. Macadie invented an instrument which could measure amps, volts and ohms, so the multifunctional meter was then named Avometer. The meter comprised a galvanometer, voltage and resistance references, and a switch to select the appropriate circuit for the input under test.

Macadie took his idea to the Automatic Coil Winder and Electrical Equipment Company (ACWEEC, founded probably in 1923). The first AVO was put on sale in 1923, and although it was initially a DC-only instrument many of its features remained almost unaltered right through to the last Model 8.

As modern systems become more complicated, the multimeter is becoming more complex or may be supplemented by more specialized equipment in a technician's toolkit. For example, where a general-purpose multimeter might only test for short-circuits, conductor resistance and some coarse measure of insulation quality, a modern technician may use a hand-held analyzer to test several parameters in order to validate the performance of a network cable.

Quantities measuredContemporary multimeters can measure many quantities. The common ones are:

Voltage in volts. Current in amperes. Resistance in ohms.

Additionally, multimeters may also measure:

Capacitance in farads. Conductance in Siemens. Decibels . Duty cycle as a percentage. Frequency in hertz Inductance in henrys

Temperature in degrees Celsius or Fahrenheit.

Digital multimeters may also include circuits for:

Continuity that beeps when a circuit conducts. Diodes and Transistors

Various sensors can be attached to multimeters to take measurements such as:

Light level Acidity/Alkalinity(pH) Wind speed Relative humidity

Resolution

Digital

The resolution of a multimeter is often specified in "digits" of resolution. For example, the term 5½ digit refers to the number of digits displayed on the readout of a multimeter.

By convention, a half digit can display either a zero or a one, while a three-quarters digit can display a numeral higher than a one but not nine. Commonly, a three-quarters digit refers to a maximum value of 3 or 5. The fractional digit is always the most significant digit in the displayed value. A 5½ digit multimeter would have five full digits that display values from 0 to 9 and one half digits that could only display 0 or 1. Such a meter could show positive or negative values from 0 to 199,999. A 3¾ digit meter can display a quantity from 0 to 3,999 or 5,999, depending on the manufacturer.

While a digital display can easily be extended in precision, the extra digits are of no value if not accompanied by care in the design and calibration of the analog portions of the multimeter. Meaningful high-resolution measurements require a good understanding of the instrument specifications, good control of the measurement conditions, and traceability of the calibration of the instrument.

Specifying "display counts" is another way to specify the resolution. Display counts give the largest number, or the largest number plus one (so the count number looks nicer) the multimeter' display can show, ignoring a decimal separator. For example, a 5½ digit multimeter can also be specified as a 199999 display count or 200000 display count multimeter.

Often the display count is just called the count in multimeter specifications. In some designs the underlying analog-to-digital converter mechanism may have more or less digits of precision than displayed.

Analog

Resolution of analog multimeters is limited by the width of the scale pointer, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display. Accuracy of readings obtained is also often compromised by miscounting division markings, errors in mental arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales and larger meter movements are used to improve resolution; two and a half to three digits equivalent resolution is usual (and may be adequate for the limited precision actually necessary for most measurements).

Resistance measurements, in particular, are of low precision due to the typical resistance measurement circuit which compresses the scale heavily at the higher resistance values.

Accuracy

Digital multimeters generally take measurements with accuracy superior to their analog counterparts. Analog multimeters typically measure with about three percent accuracy. Standard portable digital multimeters claim to be capable of taking measurements with an accuracy of 0.5% on the DC voltage ranges. Mainstream bench-top multimeters make claims to have an accuracy of better than ±0.01%. Laboratory grade instruments can have accuracies in the parts per million figures.

A multimeter's quoted accuracy is specified as being that of the lower (mV) DC range, and is known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current, resistance, AC and other ranges will usually have a lower accuracy than the basic DC volts figure.

Manufacturers can provide calibration services so that new meters may be purchased with a certificate of calibration indicating the meter has been adjusted to standards traceable to the National Institute of Standards and Technology. Such manufacturers usually provide calibration services after sales, as well, so that older equipment may be recertified. Multimeters used for critical measurements may be part of a metrology program to assure calibration.

Sensitivity and input impedance

The current load or how much current is drawn from the circuit being tested may affect a multimeter's accuracy. A smaller current draw usually will result in more precise measurements. With improper usage or too much current load, a multimeter may be damaged therefore rendering its measurements unreliable and substandard.

Meters with electronic amplifiers in them, such as all digital multimeters and analog meters using a transistor for amplification, have input impedance that is usually considered high enough not to disturb the circuit tested. This is often one million ohms, or ten million ohms. The standard input impedance allows use of external probes to extend the direct-current measuring range up to tens of thousands of volts.

Most analog multimeters of the moving pointer type are unbuffered, and draw current from the circuit under test to deflect the meter pointer. The impedance of the meter varies depending on the basic sensitivity of the meter movement and the range which is selected. For example, a meter with a typical 20,000 ohms/volt sensitivity will have an input resistance of two million ohms on the 100 volt range (100 V * 20,000 ohms/volt = 2,000,000 ohms). Lower sensitivity meters are useful for general purpose testing especially in power circuits, where source impedances are low compared to the meter impedance. Some measurements in signal circuits require higher sensitivity so as not to load down the circuit under test with the meter impedance.

Sometime sensitivity is confused with resolution of a meter, which is defined as measure of the lowest voltage, current or resistance that can change measurement reading. For general-purpose digital multimeters, a full-scale range of several hundred mill volts AC or DC is common, but the minimum full-scale current range may be several hundred milliamps. Since general-purpose multimeters have only two-wire resistance measurements, which do not compensate for the effect of the lead wire resistance, measurements below a few tens of ohms will be of low accuracy. The upper end of multimeter measurement ranges varies considerably by manufacturer; generally measurements over 1000 volts, over 10 amperes, or over 100 mega ohms would require a specialized test instrument, as would accurate measurement of currents on the order of 1 micro amp or less.

Burden Voltage

On both DC and AC current ranges a multimeter will cause voltage drop in the circuit under test. This is primarily due to the current shunt resistor used for measurement. This voltage drop is known as the burden voltage, specified in

volts per amp. The value can change depending on the range the meter selects, since different ranges usually use different shunt resistors.

Alternating current sensing

Since the basic indicator system in either an analog or digital meter responds to DC only, a multimeter includes an AC to DC conversion circuit for making alternating current measurements. Basic multimeters may utilize a rectifier circuit, calibrated to evaluate the average value of a rectified sine wave. User guides for such meters will give correction factors for some simple waveforms, to allow the correct root mean square (RMS) equivalent value to be calculated for the average-responding meter. More expensive multimeters will include an AC to DC converter that responds to the RMS value of the waveform for a wide range of possible waveforms; the user manual for the meter will indicate the limits of the crest factor and frequency for which the meter calibration is valid. RMS sensing is necessary for measurement s of non-sinusoidal quantities, such as found in audio signals, or in variable-frequency drives.

Digital Multimeters (DMM or DVOM)

Modern multimeters are often digital due to their accuracy, durability and extra features.

In a Digital Multimeter the signal under test is converted to a voltage and an amplifier with an electronically controlled gain preconditions the signal.

A Digital Multimeter displays the quantity measured as a number, which prevents parallax errors.

The inclusion of solid state electronics, from a control circuit to small embedded computers, has provided a wealth of convenience features in modern digital meters. Commonly available measurement enhancements include:

Auto-ranging, which selects the correct range for the quantity under test so that the most significant digits are shown. For example, a four-digit multimeter would automatically select an appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging meters usually include a facility to 'freeze' the meter to a particular range, because a measurement that causes frequent range changes is distracting to the user.

Auto-polarity for direct-current readings, shows if the applied voltage is positive (agrees with meter lead labels) or negative (opposite polarity to meter leads).

Sample and hold , which will latch the most recent reading for examination after the instrument is removed from the circuit under test.

Current-limited tests for voltage drop across semiconductor junctions. While not a replacement for a transistor tester, this facilitates testing diodes and a variety of transistor types. A graphic representation of the quantity under test, as a bar graph. This makes go/no-go testing easy, and also allows spotting of fast-moving trends.

A low-bandwidth oscilloscope. Automotive circuit testers, including tests for automotive timing and dwell

signals.

Simple data acquisition features to record maximum and minimum readings over a given period, or to take a number of samples at fixed intervals.

Integration with tweezers for surface-mount technology. A combined LCR meter for small-size SMD and through-hole components. Modern meters may be interfaced with a personal computer by IrDA links, RS-

232 connections, USB, or an instrument bus such as IEEE-488. The interface allows the computer to record measurements as they are made. Some DMM's can store measurements and upload them to a computer.

The first digital multimeter was manufactured in 1955 by Non Linear Systems.

Analog Multimeters

A multimeter may be implemented with a galvanometer meter movement, or with a bar-graph or simulated pointer such as an LCD or vacuum fluorescent display. Analog multimeters are common, although a quality analog instrument will be about the same cost as a digital multimeter. Analog multimeters have the precision and reading accuracy limitations described above, and so is not built to provide the same accuracy as digital instruments.

Analog meters are sometimes considered better for detecting the rate of change of a reading; some digital multimeters include a fast-responding bar-graph display for this purpose. The ARRL handbook suggests that analog multimeters are often less susceptible to radio frequency interference.

The meter movement in a moving pointer analog multimeter is practically always a moving-coil galvanometer of the d'Arsonval type, using either jeweled pivots or taut bands to support the moving coil. In a basic analog multimeter the current to deflect the coil and pointer is drawn from the circuit being measured; it is usually an advantage to minimize the current drawn from the circuit. The sensitivity of an analog multimeter is given in units of ohms per volt. For example, an inexpensive multimeter would have a sensitivity of 1000 ohms per volt and would draw 1 mill ampere from a circuit at the full scale measured voltage. More expensive, (and more delicate) multimeters would have sensitivities of 20,000 ohms per volt or higher, with a 50,000 ohms per volt meter (drawing 20 microamperes at full scale) being about the upper limit for a portable, general purpose, non-amplified analog multimeter.

To avoid the loading of the measured circuit by the current drawn by the meter movement, later analog multimeters use an amplifier inserted between the measured circuit and the meter movement. While this increased the expense and complexity of the meter and required a power supply to operate the amplifier, by use of vacuum tubes or field effect transistors the input resistance can be made very high and independent of the current required to operate the meter movement coil. Such amplified multimeters are called VTVM (vacuum tube voltmeters) TVM (transistor volt meter), FET-VOM, and similar names.

Probes

A multimeter can utilize a variety of test probes to connect to the circuit or device under test. Crocodile clips, retractable hook clips, and pointed probes are the three most common attachments. The connectors are attached to flexible, thickly-insulated leads that are terminated with connectors appropriate for the meter. Handheld meters typically use shrouded or recessed banana jacks, while bench top meters may use banana jacks or BNC connectors. 2mm plugs and binding posts have also been used at times, but are not so common today.

Meters which measure high voltages or current may use non-contact attachment mechanism to trade accuracy for safety. Clamp meters provide a coil that clamps around a conductor in order to measure the current flowing through it.

Safety

Some multimeters include a fuse, which will sometimes prevent damage to the multimeter if it is overloaded. However the fuse often only protects the highest current range on the multimeter. A common error when operating a multimeter is to set the meter to measure resistance or current and then connect it directly to a low-impedance voltage source; meters without protection are quickly damaged by such errors, and can sometimes explode causing injury to the operator.

On meters that allow interfacing with computers, optical isolation protects the computer and operator from high voltage measurants.

Digital meters are category rated based on their intended application, as set forth by the CEN EN61010 standard. There are four categories:

Category I: used where current levels are low. Category II: used on residential branch circuits. Category III: used on permanently installed loads such as distribution panels,

motors, and appliance outlets. Category IV: used on locations where current levels are high, such as service

entrances, main panels, and house meters.

Each category also specifies maximum transient voltages for selected measuring ranges in the meter. Category-rated meters also feature protections from over-current faults.

DMM Alternatives

A DMM is generally considered adequate for measurements at signal levels greater than one microvolt or one micro amp or below one giga-ohm — levels far from the theoretical limits of sensitivity. Other instruments can be employed in applications in which a DMM is not considered the optimal choice. These include nanovoltmeters, electrometers and picoammeters. If the application demands greater voltage sensitivity and the source resistance is low, a nanovoltmeters is capable of measuring at levels much closer to the theoretical limits of measurement.

In measuring voltages with very high source resistance values (such as one tera-ohm), a DMM’s input resistance of ten mega-ohms to ten giga-ohms is several orders of magnitude lower than the source resistance, which can lead to severe input loading errors. Additionally, DMM input currents are typically in the range of many microamperes. That creates large voltage offsets. An electrometer, then, is generally the best option for measurement. Low-level

current measurements pose a similar challenge for DMMs because they typically have a high input voltage drop (input burden), which affects low-level current measurements and makes DMM resolution generally no better than one nanoamp. Given their much lower input burden and better sensitivity, either an electrometer or picoammeters is optimal.

Voltage and Current

Voltage is the Cause, Current is the Effect

Voltage attempts to make a current flow, and current will flow if the circuit is complete. Voltage is sometimes described as the 'push' or 'force' of the electricity, it isn't really a force but this may help you to imagine what is happening. It is possible to have voltage without current, but current cannot flow without voltage.

Voltage and CurrentThe switch is closed making a complete circuit so current can

flow.

Voltage but No CurrentThe switch is open so the circuit

is broken and current cannot flow.

No Voltage and No Current

Without the cell there is no source of voltage so current

cannot flow.

Voltage, V

Voltage is a measure of the energy carried by the charge. Strictly: voltage is the "energy per unit charge".

The proper name for voltage is potential difference or p.d. for short, but this term is rarely used in electronics.

Voltage is supplied by the battery (or power supply). Voltage is used up in components, but not in wires. We say voltage across a component. Voltage is measured in volts, V. Voltage is measured with a voltmeter, connected in parallel. The symbol V is used for voltage in equations.

Connecting a voltmeter in parallel

Voltage at a point and 0V (zero volts)Voltage is a difference between two points, but in electronics we often refer to voltage at a point meaning the voltage difference between that point and a reference point of 0V (zero volts).

Zero volts could be any point in the circuit, but to be consistent it is normally the negative terminal of the battery or power supply. You will often see circuit diagrams labeled with 0V as a reminder.

Zero volts for circuits with a dual supplySome circuits require a dual supply with three supply connections as shown in the diagram. For these circuits the zero volts reference point is the middle terminal between the two parts of the supply.

On complex circuit diagrams using a dual supply the earth symbol is often used to indicate a connection to 0V, this helps to reduce the number of wires drawn on the diagram.

The diagram shows a ±9V dual supply, the positive terminal is +9V, the negative terminal is -9V and the middle terminal is 0V.

Current, I

Current is the rate of flow of charge. Current is not used up, what flows into a component must flow out. We say current through a component. Current is measured in amps (amperes), A. Current is measured with an ammeter, connected in series.

To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram.

1A (1 amp) is quite a large current for electronics, so mA (milliamps) are often used. M (mille) means "thousandth":

1mA = 0.001A, or 1000mA = 1A

The need to break the circuit to connect in series means that ammeters are difficult to use on soldered circuits. Most testing in electronics is done with voltmeters which can be easily connected without disturbing circuits.

Connecting an ammeter in series

Voltage and Current for components in Series

Voltages add up for components connected in series.Currents are the same through all components connected in series.

In this circuit the 4V across the resistor and the 2V across the LED add up to the battery voltage: 2V + 4V = 6V.

The current through all parts (battery, resistor and LED) is 20mA.

Voltage and Current for components in Parallel

Voltages are the same across all components connected in parallel.Currents add up for components connected in parallel.

In this circuit the battery, resistor and lamp all have 6V across them.

The 30mA current through the resistor and the 60mA current through the lamp add up to the 90mA current through the battery.

MetersAnalogue display

Analogue displays have a pointer which moves over a graduated scale. They can be difficult to read because of the need to work out the value of the smallest scale division. For example the scale in the picture has 10 small divisions between 0 and 1 so each small division represents 0.1. The reading is therefore 1.25V (the pointer is estimated to be half way between 1.2 and 1.3).

The maximum reading of an analogue meter is called full-scale deflection or FSD (it is 5V in the example shown).

Analogue meters must be connected the correct way round to prevent them being damaged when the pointer tries to move in the wrong direction. They are useful for monitoring continuously changing values (such as the voltage across a capacitor discharging) and they can be good for quick rough readings because the movement of the pointer can be seen without looking away from the circuit under test.

Taking accurate readings

To take an accurate reading from an analogue scale you must have your eye in line with the pointer. Avoid looking at an angle from the left or right because you will see a reading which is a little too high or too low. Many analogue meters have a small strip of mirror along the scale to help you. When your eye is in the correct position the reflection of the pointer is hidden behind the pointer itself. If you can see the reflection you are looking at an angle.

Instead of a mirror, some meters have a twisted pointer to aid accurate readings. The end of the pointer is turned through 90° so it appears very thin when viewed correctly. The meter shown in the galvanometers section has a twisted pointer although it is too small to see in the picture.

Digital displayValues can be read directly from digital displays so they are easy to read accurately. It is normal for the least significant digit (on the right) to continually change between two or three values, this is a feature of the way digital meters work, not an error! Normally you will not need great precision and the least significant digit can be ignored or rounded up.

Digital meters may be connected either way round without damage; they will show a minus sign (-) when connected in reverse. If you exceed the maximum reading most digital meters show an almost blank display with just a 1 on the left-hand side.

All digital meters contain a battery to power the display so they use virtually no power from the circuit under test. This means that digital voltmeters have a very high resistance (usually called input impedance) of 1M or more, usually 10M, and they are very unlikely to affect the circuit under test.

For general use digital meters are the best type. They are easy to read, they may be connected in reverse and they are unlikely to affect the circuit under test.

Correctreflection hidden

Wrongreflection visible

Connecting meters

It is important to connect meters the correct way round: The positive terminal of the meter, marked + or colored red should be

connected nearest to + on the battery or power supply. The negative terminal of the meter, marked - or colored black should be

connected nearest to - on the battery or power supply. Voltmeters

Voltmeters measure voltage. Voltage is measured in volts, V. Voltmeters are connected in parallel across components. Voltmeters have a very high resistance.

Measuring voltage at a point

When testing circuits you often need to find the voltages at various points, for example the voltage at pin 2 of a 555 timer IC. This can seem confusing - where should you connect the second voltmeter lead?

Connect the black (negative -) voltmeter lead to 0V, normally the negative terminal of the battery or power supply.

Connect the red (positive +) voltmeter lead to the point you where you need to measure the voltage.

The black lead can be left permanently connected to 0V while you use the red lead as a probe to measure voltages at various points.

You may wish to use a crocodile clip on the black lead to hold it in place. Voltage at a point really means the voltage difference between that point and

0V (zero volts) which is normally the negative terminal of the battery or power supply. Usually 0V will be labeled on the circuit diagram as a reminder.

Analogue meters take a little power from the circuit under test to operate their pointer. This may upset the circuit and give an incorrect reading. To avoid this voltmeters should have a resistance of at least 10 times the circuit

Connecting a voltmeter in parallel

resistance (take this to be the highest resistor value near where the meter is connected).

Most analogue voltmeters used in school science are not suitable for electronics because their resistance is too low, typically a few k . 100k or more is required for most electronics circuits.

Ammeters

Ammeters measure current. Current is measured in amps (amperes), A.

1A is quite large, and so mA (milliamps) and µA (micro amp) are often used. 1000mA = 1A, 1000µA = 1mA, 1000000µA = 1A.

Ammeters are connected in series. To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram.

Ammeters have a very low resistance.

The need to break the circuit to connect in series means that ammeters are difficult to use on soldered circuits. Most testing in electronics is done with voltmeters which can be easily connected without disturbing circuits.

Galvanometers

Galvanometers are very sensitive meters which are used to measure tiny currents, usually 1mA or less. They are used to make all types of analogue meters by adding suitable resistors as shown in the diagrams below. The photograph shows an educational 100µA galvanometer for which various multipliers and shunts are available.

Making a VoltmeterA galvanometer with a high resistance multiplier in series to

Making an AmmeterA galvanometer with a low resistance shunt in parallel to make an

Galvanometer with multiplier and shuntMaximum meter current 100µA (or 20µA reverse). This meter is unusual in allowing small

Connecting an ammeter in series

make a voltmeter.ammeter.reverse readings to be shown.

Ohmmeters

An ohmmeter is used to measure resistance in ohms ( ). Ohmmeters are rarely found as separate meters but all standard multimeters have an ohmmeter setting.1 is quite small so k and M are often used.

1k = 1000 , 1M = 1000k

**Analogue multimeters consist of a galvanometer with various resistors which can be switched in as multipliers (voltmeter ranges) and shunts (ammeter ranges).

MultimetersMultimeters are very useful test instruments. By operating a multi-position switch on the meter they can be quickly and easily set to be a voltmeter, an ammeter or an ohmmeter. They have several settings (called 'ranges') for each type of meter and the choice of AC or DC. Some multimeters have additional features such as transistor testing and ranges for measuring capacitance and frequency.

Choosing a multimeterThe photographs below show modestly priced multimeters which are suitable for general electronics use, you should be able to buy meters like these for less than £15. A digital multimeter is the best choice for your first multimeter, even the cheapest will be suitable for testing simple projects.

If you are buying an analogue multimeter make sure it has a high sensitivity of 20k /V or greater on DC voltage ranges, anything less is not suitable for electronics. The sensitivity is normally marked in a corner of the scale, ignore the lower AC value (sensitivity on AC ranges is less important), the higher DC value is the critical one. Beware of cheap analogue multimeters sold for electrical work on cars because their sensitivity is likely to be too low.

Analogue Multimeter Digital Multimeter

Digital multimetersAll digital meters contain a battery to power the display so they use virtually no 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 1M or more, usually 10M , and they are very unlikely to affect the circuit under test.

Typical ranges for digital multimeters like the one illustrated: (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*.

*The 10A range is usually unused and connected via a special socket. AC Current: None. (You are unlikely to need to measure this). Resistance: 200 , 2000 , 20k , 200k , 2000k , Diode Test.

Digital meters have a special diode test setting because their resistance ranges cannot be used to test diodes and other semiconductors.

Testing a diode with a multimeterThe techniques used for each type of meter are very different so they are treated separately:

Testing a diode with a DIGITAL multimeter

Digital multimeters have a special setting for testing a diode, usually labeled with the diode symbol.

Connect the red (+) lead to the anode and the black (-) to the cathode. The diode should conduct and the meter will display a value (usually the voltage across the diode in mV, 1000mV = 1V).

Reverse the connections. The diode should NOT conduct this way so the meter will display "off the scale" (usually blank except for a 1 on the left).

Testing a diode with an ANALOGUE multimeter

Set the analogue multimeter to a low value resistance range such as × 10. It is essential to note that the polarity of analogue multimeter leads is reversed on the

resistance ranges, so the black lead is positive (+) and the red lead is negative (-)! This is unfortunate, but it is due to the way the meter works.

Connect the black (+) lead to anode and the red (-) to the cathode. The diode should conduct and the meter will display a low resistance (the exact value is not relevant).

Reverse the connections. The diode should NOT conduct this way so the meter will show infinite resistance (on the left of the scale).

Diodes

a = anodek = cathode

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 20k /V or they may upset the circuit under test and give an incorrect reading. See the section below on sensitivity for more details.

Batteries inside the meter provide power for the resistance ranges, they will last several years but you should avoid leaving the meter set to a resistance range in case the leads touch accidentally and run the battery flat.

Typical ranges for analogue multimeters like the one illustrated: (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.

A high current range is often missing from this type of meter. AC Current: None. (You are unlikely to need to measure this). Resistance: 20 , 200 , 2k , 20k , 200k .

These resistance values are in the middle of the scale for each range. It is a good idea to leave an analogue multimeter set to a DC voltage range such as 10V when not in use. It is less likely to be damaged by careless use on this range, and there is a good chance that it will be the range you need to use next anyway!

Sensitivity of an analogue multimeter

Multimeters must have a high sensitivity of at least 20k /V otherwise their resistance on DC voltage ranges may be too low to avoid upsetting the circuit under test and giving an incorrect reading. To obtain valid readings the meter resistance should be at least 10 times the circuit resistance (take this to be the highest resistor value near where the meter is connected). You can increase the meter resistance by selecting a higher voltage range, but this may give a reading which is too small to read accurately!

On any DC voltage range:     Analogue Meter Resistance = Sensitivity × Max. reading of range e.g. a meter with 20k /V sensitivity on its 10V range has a resistance of 20k /V × 10V = 200k .

By contrast, digital multimeters have a constant resistance of at least 1M (often 10M ) on all their DC voltage ranges. This is more than enough for almost all circuits.

Analogue Multimeter

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 is 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 please take these precautions:

Always disconnect the multimeter before adjusting the range switch. Always check the setting of the range switch before you connect to a circuit. Never leave a multimeter set to a current range (except when actually taking a reading).

The greatest risk of damage is on the current ranges because the meter has a low resistance.

Measuring voltage at a point

When testing circuits you often need to find the voltages at various points, for example the voltage at pin 2 of a 555 timer IC. This can seem confusing - where should you connect the second multimeter lead?

Connect the black (negative -) lead to 0V, normally the negative terminal of the battery or power supply.

Connect the red (positive +) lead to the point you where you need to measure the voltage.

The black lead can be left permanently connected to 0V while you use the red lead as a probe to measure voltages at various points.

You may wish to fit a crocodile clip to the black lead of your multimeter to hold it in place while doing testing like this.

Measuring voltage at a point.

Voltage at a point really means the voltage difference between that point and 0V (zero volts) which is normally the negative terminal of the battery or power supply. Usually 0V will be labeled on the circuit diagram as a reminder.

Reading analogue scales

Check the setting of the range switch and choose an appropriate scale. For some ranges you may need to multiply or divide by 10 or 100 as shown in the sample readings below. For AC voltage ranges use the red markings because the calibration of the scale is slightly different.

Sample readings on the scales shown: DC 10V range: 4.4V (read 0-10 scale directly) DC 50V range: 22V (read 0-50 scale directly) DC 25mA range: 11mA (read 0-250 and divide by 10) AC 10V range: 4.45V (use the red scale, reading 0-10)

Measuring resistance with a multimeter

To measure the resistance of a component it must not be connected in a circuit. If you try to measure resistance of components in a circuit you will obtain false readings (even if the supply is disconnected) and you may damage the multimeter.

The techniques used for each type of meter are very different so they are treated separately:

Measuring resistance with a DIGITAL multimeter

1. Set the meter to a resistance range greater than you expect the resistance to be. Notice that the meter display shows "off the scale" (usually blank except for a 1 on the left). Don't worry, this is not a fault, it is correct - the resistance of air is very high!

2. Touch the meter probes together and check that the meter reads zero. If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try again.

3. Put the probes across the component. Avoid touching more than one contact at a time or your resistance will upset the reading!

Analogue Multimeter Scales These can appear daunting at first but remember that you only need to read one scale at a time!

The top scale is used when measuring resistance.

Measuring resistance with an ANALOGUE multimeter

The resistance scale on an analogue meter is normally at the top, it is an unusual scale because it reads backwards and is not linear (evenly spaced). This is unfortunate, but it is due to the way the meter works.

1. Set the meter to a suitable resistance range. Choose a range so that the resistance you expect will be near the middle of the scale. For example: with the scale shown below and an expected resistance of about 50k choose the × 1k range.

2. Hold the meter probes together and adjust the control on the front of the meter which is usually labeled "0  ADJ" until the pointer reads zero (on the RIGHT remember!). If you can't adjust it to read zero, the battery inside the meter needs replacing.

3. Put the probes across the component. Avoid touching more than one contact at a time or your resistance will upset the reading!

Reading analogue resistance scales

For resistance use the upper scale, noting that it reads backwards and is not linear (evenly spaced).

Check the setting of the range switch so that you know by how much to multiply the reading.

Sample readings on the scales shown: × 10 range: 260 × 1k range: 26k

Using a Multimeter

Analogue Multimeter Scales The resistance scale is at the top, note that it reads backwards and is not linear (evenly spaced).

Input Jacks

The black lead is always plugged into the common terminal. The red lead is plugged into the 10 A jack when measuring currents greater than 300 mA, the 300 mA jack when measuring currents less than 300 mA, and the remaining jack (V-ohms-diode) for all other measurements.

Range

The meter defaults to auto range when first turned on. You can choose a manual range in V AC, V DC, A AC, and A DC by pressing the button in the middle of the rotary dial. To return to auto range, press the button for one second.

Automatic Touch Hold Mode The Touch Hold mode automatically captures and displays stable readings. Press the button in the center of the dial for 2 seconds while turning the meter on. When the meter captures a new input, it beeps and a new reading is displayed. To manually force a new measurement to be held, press the center button. To exit the Touch Hold mode, turn the meter off.

Note: stray voltages can produce a new reading.

Warning: To avoid electric shock, do not use the Touch Hold to determine if a circuit with high voltage is dead. The Touch Hold mode will not capture unstable or noisy

readings.

AC and DC Voltage

Resistance

Turn off the power and discharge all capacitors. An external voltage across a component will give invalid resistance readings.

Diode Test

Continuity Test

This mode is used to check if two points are electrically connected. It is often used to verify connectors. If continuity exists (resistance less than 210 ohms), the beeper sounds continuously. The meter beeps twice if it is in the Touch Hold mode.

Current Warning: To avoid injury, do not attempt a current measurement if the open circuit voltage is above the rated voltage of the meter.

To avoid blowing an input fuse, use the 10 a jack until you are sure that the current is less than 300 mA.

Turn off power to the circuit. Break the circuit. (For circuits of more than 10 amps, use a current clamp.) Put the meter in series with the circuit as shown and turn power on.

Various forms of Multimeter

DT182 - 3 1/2 DIGIT MINI MULTIMETER

Mini Multimeter

The DT182 is a compact, pocket-sized, 3.5 digit Multimeter that is ideal for use in the field, hobby, home and DIY applications.  This meter is extremely convenient for the on the go professional that only needs to do basic measurements.  

Features:

Audible Continuity test. Transistor test. Over Range indication. Auto Zero adjustment. Low Battery Indication. Small Size.

FunctionRangeAccuracy

 DC Voltage 200mV-500V ±(0.5%+2dgt)

 AC Voltage 1V-500V ±(1.2%+10dgt)

 DC Current 2000µA-200mA ±(1%+2dgt)

 Resistance 200Ω-2000KΩ ±(0.8%+2dgt)

 Diode Test Yes  Transistor Test Yes  Battery Capacity Check

 1.5V(AA) and 9V

 Continuity Test Audible 

 Power Supply 12V Battery (included)

 Maximum Display 1999  Products Size 4L×2W×1D Inches  Product Net Weight 2.10 Ounces  Standard Accessories

 Test Lead, Battery, Manual

DT830D - LOW COST 3 1/2 DIGIT MULTIMETER

Low Cost 3.5 Digit Digital Multimeter

The DT83 Series is a compact, pocket sized / hand held, 3.5 digit Multimeter.  This series of Multi meters has an incredible amount of features for the size and price.  Ideal for Laboratory, Workshop, Dyers and Home Applications.

Features:

Low Battery Indicator. Full Range over Load Protection. Diode Transistor Testing. Continuity Buzzer. Test Signal Generator.

FunctionRangeAccuracy DC Voltage 200mV-1000V ±(0.5%+2dgt)

 AC Voltage 1V-750V ±(1.2%+10dgt)

 DC Current 200uA-10A ±(1%+2dgt) Resistance 200Ω-2MΩ ±(0.8%+2dgt) Diode Test Yes  Continuity Buzzer Yes  Power Supply 9V Battery Included  Display Size 16×48mm  Maximum Display 1999  Products Size 5L×2-3/4W×1D Inches  Product Net Weight

 5.25 Ounces

 Standard Accessories

 Test Lead, Battery, Manual, Rubber Holster

DT858L - 3-1/2 DIGIT MID SIZE METER W/ BACKLIGHT

3.5 Digit Midsize Multimeter with Backlight

This mid sized handheld meter is really packed with functions at an great price.  Among all the other features listed below this little gem has a backlight and data hold option.

Features:

Data Hold Back Light Audible Continuity Test Temperature Meter 200 hour battery life Diode Test Transistor Test

FunctionRangeAccuracy DC Voltage 200mV-600V ±(0.5%+2dgt) AC Voltage 1-600V ±(1.2%+10dgt) DC Current 2mA-10A ±(1%+2dgt) Resistance 200Ω-2MΩ ±(0.8%+2dgt) Temperature -40°C ~ 750°C ±(1%+2dgt) Diode Test Yes  Transistor Test Yes  Continuity Buzzer Yes  Data Hold Yes  Back Light Yes  Power Supply 9V Battery (Included)  Maximum Display 1999  Products Size 5-1/2L×2-3/4W×1-1/4D Inches  Product Net Weight

 6 Ounces

 Standard Accessories

 Test Lead, Battery, Manual, Holster, K type thermocouple

DT266F - 3-1/2 DIGIT CLAMP MULTIMETER

Digital Clamp Meter

The DT266F is a rugged clamp meter designed for use by electricians, technicians, servicemen, and hobbyists who require an instrument that is accurate, reliable, and always ready for use.  It can also be used as a regular Multimeter with the included test leads.

Features:

Data Hold Button. Audible continuity test. 200 Hour Battery Life. 500V Insulation Tester. Standard 9Volt Battery (included).

FunctionRangeAccuracy

 DC Voltage 1000V ±(0.5%+1dgt)

 AC Voltage 750V ±(1%+4dgt)

 AC Current 200A-1000A ±(2%+5dgt)

 Resistance 200Ω-2000MΩ ±(2%+2dgt)

 Frequency  2KHz ±(1%+1dgt)

 Continuity Buzzer Yes  Display Hold Yes  Diode Test  Yes  Power Supply 9V Battery 6F22  Maximum Display 1999  Display Size  14×47mm  Product Net Weight

 12 Ounces

 Products Size 9-1/3L x 3-3/4W x 1.5D inches

 Standard Accessories

 Test Lead, Battery, Manual, Zipper Case

M1250 - ANALOG METER  M1250 Analog Multimeter - Kit or

Assembled

FEATURES:

20,000 ohms/volts 23 Ranges 3 1/2” Meter Mirrored scale Audible continuity Checks transistors/diodes 10A DC current dB Scale Battery test 

SPECS. AND RANGES:

Size: 3.7" L x 5.9" W x 1.25" D

Resistance:  X1, X10, X100, X1k, X10k 

DC Volts: .1, .5, 2.5, 10, 50, 250, 1000, 3% FSR

AC Volts: 10, 50, 250, 1kV, 4% FSR

DC Current: 50μA, 2.5mA, 25mA, 250mA, 10A 3% FDR

Transistor hFE 0-1000 ICEO 150μA, 1.5mA, 15mA, 150mA

Accessories: Test leads, Manual, 9V & (2) 1.5V AA batts.