Multimeter 2007

Faculty of Engineering

Multimeter Name: Samia Ahmed Youssef Ahmed .Department: Electrical Engineering

2nd year

2009-12-05 Multimeter History

Scientists 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 shortcircuits, 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 measured :Contemporary 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 threequarters 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 lowimpedance 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 teraohm), 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 Current The switch is closed making a .complete circuit so current can flow

Voltage but No Current

No Voltage and No Current

The switch is open so the circuit is .broken and current cannot flow

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 Connecting a voltmeter in parallel 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•

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 supply dual the the is the the

Some circuits require a supply with three supply connections as shown in diagram. For these circuits zero volts reference point middle terminal between .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 .urrent is the rate of flow of charge urrent is not used up, what flows into a component

Connecting an ammeter in series

• C •C

.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

Voltage and Current for components in Series in

2V

.Voltages add up for components connected in series Currents are the same through all components connected .series In this circuit the 4V across the resistor and the 2V across the LED add up to the battery voltage: .+ 4V = 6V .The current through all parts (battery, resistor and LED) is 20mA

Voltage and Current for components in Parallel

6V 60mA

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

Meters Analogue display Analogue displays have a pointer which moves over a the graduated scale. They can be difficult to read because of 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 Correct Wrong the pointer is hidden behind the pointer itself. If you reflection hidden reflection visible .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 display Values 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

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•

Connecting a voltmeter in parallel

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 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

.mmeters measure current •A .urrent is measured in amps (amperes), A •C 1A is quite large, and so mA (milliamps) and µA (micro amp) are often used. 1000mA = 1A, .1000µA = 1mA, 1000000µA = 1A .mmeters are connected in series •A To connect in series you must break the circuit and put the ammeter across the gap, as shown in the .diagram .mmeters have a very low resistance •A The need to break the circuit to connect in series Connecting an ammeter 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 the

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 diagrams below. The photograph shows an educational 100µA galvanometer for which various multipliers and .shunts are available

Making a Voltmeter

Making an Ammeter

A galvanometer with a high resistance multiplier in series to make a .voltmeter

A galvanometer with a low resistance shunt in parallel to make an .ammeter

Galvanometer with multiplier and shunt .(Maximum meter current 100µA (or 20µA reverse This meter is unusual in allowing small .reverse readings to be shown

Ohmmeters to

An ohmmeter is used

ohms ( rarely meters

measure resistance in ). Ohmmeters are found as separate but all standard multimeters have an .ohmmeter setting is quite small so k 1 and M are often .used 1k = 1000 , 1M = . 1000k = 1000000 Analogue Multimeter Digital Multimeter

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

Multimeters

Multimeters 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 multimeter The 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

Digital multimeters All 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•

•R

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

Testing a diode with a DIGITAL multimeter

Diodes a = anode k = cathode

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

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 .Select a range with a maximum greater than you expect the reading to be.1 .Connect the meter, making sure the leads is the correct way round.2 .Digital meters can be safely connected in reverse, but an analogue meter may be damaged

.If the reading goes off the scale: immediately disconnect and select a higher range.3 :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

.Measuring voltage at a point

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

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 Analogue Multimeter Scales and choose an appropriate scale. For These can appear daunting at first but remember some ranges you may need to multiply !that you only need to read one scale at a time or divide by 10 or 100 as shown in the .The top scale is used when measuring resistance 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 .Set the meter to a resistance range greater than you expect the resistance to be.1 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 .Touch the meter probes together and check that the meter reads zero.2 .If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try again .Put the probes across the component.3 !Avoid touching more than one contact at a time or your resistance will upset the reading

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

.Set the meter to a suitable resistance range.1 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 .the × 1k

choose

range

old the meter probes together and adjust the control on the front of the

H .2

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 .ut 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

P .3

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

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 range: 260 1k

10 ×

range: 26k ×

Using a Multimeter

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•

Function

Range

Accuracy

DC Voltage

200mV-500V (2dgt+0.5%)±

AC Voltage

1V-500V (10dgt+1.2%)±

DC Current Resistance

2000µA-200mA

(2dgt+1%)±

200Ω-2000KΩ (2dgt+0.8%)±

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

Ounces 2.10

　

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•

Function Range Accuracy DC Voltage 200mV-1000V (2dgt+0.5%)± AC Voltage 1V-750V (10dgt+1.2%)± DC Current 200uA-10A (2dgt+1%)± Resistance 200Ω-2MΩ (2dgt+0.8%)± 　 Diode Test Yes 　 Continuity Buzzer Yes 　 Power Supply 9V Battery Included 　 Display Size 48mm×16 1999 　 Maximum Display 　 Products Size 5L×2-3/4W×1D Inches 　 Product Net Weight Ounces 5.25 Standard Accessories Test Lead, Battery, Manual, Rubber Holster

DT858L - 3-1/2 DIGIT MID SIZE METER W/ BACKLIGHT Digit Midsize Multimeter with Backlight 3.5 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• hour battery life 200• Diode Test• Transistor Test•

Function Range Accuracy DC Voltage 200mV-600V (2dgt+0.5%)± AC Voltage 1-600V (10dgt+1.2%)± DC Current 2mA-10A (2dgt+1%)± Resistance 200Ω-2MΩ (2dgt+0.8%)± Temperature 40°C ~ 750°C (2dgt+1%)± 　 Diode Test Yes 　 Transistor Test Yes 　 Continuity Buzzer Yes 　 Data Hold Yes 　 Back Light Yes 　 Power Supply (9V Battery (Included 1999 　 Maximum Display 　 Products Size 5-1/2L×2-3/4W×1-1/4D Inches 　 Product Net Weight Ounces 6 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• .Hour Battery Life 200• .500V Insulation Tester• .(Standard 9Volt Battery (included•

Function Range Accuracy DC Voltage 1000V (1dgt+0.5%)± AC Voltage 750V (4dgt+1%)± AC Current 200A-1000A (5dgt+2%)± Resistance 200Ω-2000MΩ (2dgt+2%)± Frequency 2KHz (1dgt+1%)± 　 Continuity Buzzer Yes 　 Display Hold Yes Diode Test Yes 　 Power Supply 9V Battery 6F22 1999 　 Maximum Display 　 Display Size 47mm×14 　 Product Net Weight Ounces 12 　 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 ohms/volts 20,000• Ranges 23• Meter ”1/2 3• 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