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Circuit Diagrams

Circuit diagrams show how electronic components are connected together. Each component is represented by a symbol and a few are shown here, for other symbols please see the Circuit Symbols page.

Circuit diagrams and component layouts

Circuit diagrams show the connections as clearly as possible with all wires drawn neatly as straight lines. The actual layout of the components is usually quite different from the circuit diagram and this can be confusing for the beginner. The secret is to concentrate on the connections, not the actual positions of components.

The circuit diagram and stripboard layout for the Adjustable Timer project are shown here so you can see the difference.

A circuit diagram is useful when testing a circuit and for understanding how it works. This is why the instructions for projects include a circuit diagram as well as the stripboard or printed circuit board layout which you need to build the circuit.

Drawing circuit diagrams

Drawing circuit diagrams is not difficult but it takes a little practice to draw neat, clear diagrams. This is a useful skill for science as well as for electronics. You will certainly need to draw circuit diagrams if you design your own circuits.

Follow these tips for best results:

• Make sure you use the correct symbol for each component.
• Draw connecting wires as straight lines (use a ruler).
• Put a 'blob' () at each junction between wires.
• Label components such as resistors and capacitors with their values.
• The positive (+) supply should be at the top and the negative (-) supply at the bottom. The negative supply is usually labelled 0V, zero volts.
  If you are drawing the circuit diagram for science please see the section about drawing diagrams the 'electronics way'.

If the circuit is complex:

• Try to arrange the diagram so that signals flow from left to right: inputs and controls should be on the left, outputs on the right.
• You may omit the battery or power supply symbols, but you must include (and label) the supply lines at the top and bottom.

Drawing circuit diagrams the 'electronics way'

Circuit diagrams for electronics are drawn with the positive (+) supply at the top and the negative (-) supply at the bottom. This can be helpful in understanding the operation of the circuit because the voltage decreases as you move down the circuit diagram.

Circuit diagrams for science are traditionally drawn with the battery or power supply at the top. This is not wrong, but there is usually no advantage in drawing them this way and I think it is less helpful for understanding the circuit.

I suggest that you always draw your circuit diagrams the 'electronics way', even for science!

[I hope your science teacher won't mind too much!]

Note that the negative supply is usually called 0V (zero volts).
This is explained on the Voltage and Current page.

Circuit Symbols

Circuit symbols are used in circuit diagrams which show how a circuit is connected together. The actual layout of the components is usually quite different from the circuit diagram. To build a circuit you need a different diagram showing the layout of the parts on stripboard or printed circuit board.

Wires and connections
Component	Circuit Symbol	Function of Component
Wire		To pass current very easily from one part of a circuit to another.
Wires joined		A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.
Wires not joined		In complex diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer the 'bridge' symbol shown on the right because the simple crossing on the left may be misread as a join where you have forgotten to add a 'blob'!

Power Supplies
Component	Circuit Symbol	Function of Component
Cell		Supplies electrical energy. The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.
Battery		Supplies electrical energy. A battery is more than one cell. The larger terminal (on the left) is positive (+).
DC supply		Supplies electrical energy. DC = Direct Current, always flowing in one direction.
AC supply		Supplies electrical energy. AC = Alternating Current, continually changing direction.
Fuse		A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.
Transformer		Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.
Earth (Ground)		A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.

Output Devices: Lamps, Heater, Motor, etc.
Component	Circuit Symbol	Function of Component
Lamp (lighting)		A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.
Lamp (indicator)		A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.
Heater		A transducer which converts electrical energy to heat.
Motor		A transducer which converts electrical energy to kinetic energy (motion).
Bell		A transducer which converts electrical energy to sound.
Buzzer		A transducer which converts electrical energy to sound.
Inductor (Coil, Solenoid)		A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.

Switches
Component	Circuit Symbol	Function of Component
Push Switch (push-to-make)		A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.
Push-to-Break Switch		This type of push switch is normally closed (on), it is open (off) only when the button is pressed.
On-Off Switch (SPST)		SPST = Single Pole, Single Throw. An on-off switch allows current to flow only when it is in the closed (on) position.
2-way Switch (SPDT)		SPDT = Single Pole, Double Throw. A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.
Dual On-Off Switch (DPST)		DPST = Double Pole, Single Throw. A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.
Reversing Switch (DPDT)		DPDT = Double Pole, Double Throw. This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.
Relay		An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit. NO = Normally Open, COM = Common, NC = Normally  Closed.

Resistors
Component	Circuit Symbol	Function of Component
Resistor		A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit. Some publications still use the old resistor symbol:
Variable Resistor (Rheostat)		This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.
Variable Resistor (Potentiometer)		This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.
Variable Resistor (Preset)		This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.

Capacitors
Component	Circuit Symbol	Function of Component
Capacitor		A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Capacitor, polarised		A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Variable Capacitor		A variable capacitor is used in a radio tuner.
Trimmer Capacitor		This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.

Diodes
Component	Circuit Symbol	Function of Component
Diode		A device which only allows current to flow in one direction.
LED Light Emitting Diode		A transducer which converts electrical energy to light.
Zener Diode		A special diode which is used to maintain a fixed voltage across its terminals.
Photodiode		A light-sensitive diode.

Transistors
Component	Circuit Symbol	Function of Component
Transistor NPN		A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Transistor PNP		A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Phototransistor		A light-sensitive transistor.

Audio and Radio Devices
Component	Circuit Symbol	Function of Component
Microphone		A transducer which converts sound to electrical energy.
Earphone		A transducer which converts electrical energy to sound.
Loudspeaker		A transducer which converts electrical energy to sound.
Piezo Transducer		A transducer which converts electrical energy to sound.
Amplifier (general symbol)		An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.
Aerial (Antenna)		A device which is designed to receive or transmit radio signals. It is also known as an antenna.

Meters and Oscilloscope
Component	Circuit Symbol	Function of Component
Voltmeter		A voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!
Ammeter		An ammeter is used to measure current.
Galvanometer		A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.
Ohmmeter		An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.
Oscilloscope		An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Sensors (input devices)
Component	Circuit Symbol	Function of Component
LDR		A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
Thermistor		A transducer which converts temperature (heat) to resistance (an electrical property).

Logic Gates Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the Logic Gates page. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).
Gate Type	Traditional Symbol	IEC Symbol	Function of Gate
NOT			A NOT gate can only have one input. The 'o' on the output means 'not'. The output of a NOT gate is the inverse (opposite) of its input, so the output is true when the input is false. A NOT gate is also called an inverter.
AND			An AND gate can have two or more inputs. The output of an AND gate is true when all its inputs are true.
NAND			A NAND gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not AND gate. The output of a NAND gate is true unless all its inputs are true.
OR			An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
NOR			A NOR gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not OR gate. The output of a NOR gate is true when none of its inputs are true.
EX-OR			An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).
EX-NOR			An EX-NOR gate can only have two inputs. The 'o' on the output means 'not' showing that it is a Not EX-OR gate. The output of an EX-NOR gate is true when its inputs are the same (both true or both false).

Sets of circuit symbols to download

You can download complete sets of all the circuit symbols shown above. The sets are 'zipped' for convenience and they are provided in three formats:

• WMF circuit symbols (32K) - Windows Metafiles.
  These vector drawings are the best format for printed documents on most computer systems, including Windows where they can be used in Word documents for example. They can be enlarged without loss of quality. If you are not sure which format is best for you I suggest you try this one first.

   

• GIF circuit symbols (43K) - Graphics Interchange Format.
  These bitmap images are the best format for web pages but they print poorly and their bitmap nature will become obvious if they are enlarged. You can download individual symbols by saving the images used above on this page.

   

• Drawfile circuit symbols (29K) - for RISC OS (Acorn) computers.
  These high quality vector drawings are suitable for almost all documents on a RISC OS computer. All the symbols were originally drawn in this format. They print perfectly and can be enlarged without loss of quality. Sorry, this format is NOT suitable for Windows computers.

Electricity and the Electron

What is electricity?

Electricity is the flow of charge around a circuit carrying energy from the battery (or power supply) to components such as lamps and motors.

Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.

The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.

With the switch open the circuit is broken - so electricity cannot flow and the lamp is off.

With the switch closed the circuit is complete - allowing electricity to flow and the lamp is on. The electricity is carrying energy from the battery to the lamp.

We can see, hear or feel the effects of electricity flowing such as a lamp lighting, a bell ringing, or a motor turning - but we cannot see the electricity itself, so which way is it flowing?

Imaginary positive particles moving in the direction of the conventional current

Which way does electricity flow?

We say that electricity flows from the positive (+) terminal of a battery to the negative (-) terminal of the battery. We can imagine particles with positive electric charge flowing in this direction around the circuit, like the red dots in the diagram.

This flow of electric charge is called conventional current.

This direction of flow is used throughout electronics and it is the one you should remember and use to understand the operation of circuits.

However this is not the whole answer because the particles that move in fact have negative charge! And they flow in the opposite direction! Please read on...

The electron

When electricity was discovered scientists tried many experiments to find out which way the electricity was flowing around circuits, but in those early days they found it was impossible to find the direction of flow.

They knew there were two types of electric charge, positive (+) and negative (-), and they decided to say that electricity was a flow of positive charge from + to -. They knew this was a guess, but a decision had to be made! Everything known at that time could also be explained if electricity was negative charge flowing the other way, from - to +.

The electron was discovered in 1897 and it was found to have a negative charge. The guess made in the early days of electricity was wrong! Electricity in almost all conductors is really the flow of electrons (negative charge) from - to +.

By the time the electron was discovered the idea of electricity flowing from + to - (conventional current) was firmly established. Luckily it is not a problem to think of electricity in this way because positive charge flowing forwards is equivalent to negative charge flowing backwards.

To prevent confusion you should always use conventional current when trying to understand how circuits work, imagine positively charged particles flowing from + to -.

Series and Parallel Connections

Connecting Components

There are two ways of connecting components:

In series so that each component has the same current. The battery voltage is divided between the two lamps Each lamp will have half the battery voltage if the lamps are identical.
In parallel so that each component has the same voltage. Both lamps have the full battery voltage across them. The battery current is divided between the two lamps.

Most circuits contain a mixture of series and parallel connections

The terms series circuit and parallel circuit are sometimes used, but only the simplest of circuits are entirely one type or the other. It is better to refer to specific components and say they are connected in series or connected in parallel.

For example: the circuit on the right shows a resistor and LED connected in series (on the right) and two lamps connected in parallel (in the centre). The switch is connected in series with the two lamps.

See Lamps in Parallel below for another example.

Lamps in Series

If several lamps are connected in series they will all be switched on and off together by a switch connected anywhere in the circuit. The supply voltage is divided equally between the lamps (assuming they are all identical). If one lamp blows all the lamps will go out because the circuit is broken.

Christmas Tree Lights

The lamps on a Christmas tree are connected in series.

Normally you would expect all the lamps to go out if one blew, but Christmas tree lamps are special! They are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one 'fuse' lamp which blows normally.

If there are 20 lamps and the mains electricity voltage is 240V, each lamp must be suitable for a 12V supply because the 240V is divided equally between the 20 lamps: 240V ÷ 20 = 12V.

WARNING! The Christmas tree lamps may seem safe because they use only 12V but they are connected to the mains supply which can be lethal. Always unplug from the mains before changing lamps. The voltage across the holder of a missing lamp is the full 240V of the mains supply! (Yes, it really is!)

Lamps in Parallel

If several lamps are connected in parallel each one has the full supply voltage across it. The lamps may be switched on and off independantly by connecting a switch in series with each lamp as shown in the circuit diagram. This arrangement is used to control the lamps in buildings.

This type of circuit is often called a parallel circuit but you can see that it is not really so simple - the switches are in series with the lamps, and it is these switch and lamp pairs that are connected in parallel.

Switches in Series

If several on-off switches are connected in series they must all be closed (on) to complete the circuit.

The diagram shows a simple circuit with two switches connected in series to control a lamp.

Switch S1 AND Switch S2 must be closed to light the lamp.

Switches in Parallel

If several on-off switches are connected in parallel only one needs to be closed (on) to complete the circuit.

The diagram shows a simple circuit with two switches connected in parallel to control a lamp.

Switch S1 OR Switch S2 (or both of them) must be closed to light the lamp.

--------------------------------------------------------------------------------------------

Voltage and Current

Voltage and Current are vital to understanding electronics, but they are quite hard to grasp because we can't see them directly.

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 The 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

Connecting a voltmeter in parallel

• 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.

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 labelled with 0V as a reminder.

You may find it helpful to think of voltage like height in geography. The reference point of zero height is the mean (average) sea level and all heights are measured from that point. The zero volts in an electronic circuit is like the mean sea level in geography.

Zero volts for circuits with a dual supply

Some 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.

Connecting an ammeter in series

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 acoss the gap, as shown in the diagram.
• The symbol I is used for current in equations.
  Why is the letter I used for current? ... please see FAQ.

1A (1 amp) is quite a large current for electronics, so mA (milliamps) are often used. m (milli) 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

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.
________________________________________________________________

Meters

Analogue 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 continously 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.

Correct reflection hidden	Wrong reflection visible

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

Voltmeters

Connecting a voltmeter in parallel

• 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 chip. 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 labelled 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.

Connecting an ammeter in series

Ammeters

• Ammeters measure current.
• Current is measured in amps (amperes), A.
  1A is quite large, so mA (milliamps) and µA (microamps) 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 Voltmeter A galvanometer with a high resistance multiplier in series to make a voltmeter.	Making an Ammeter 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

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 = 1000000.

Analogue Multimeter	Digital Multimeter
Multimeter Photographs © Rapid Electronics

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.

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

For further information please see the Multimeters page.

Multimeters

Liquid-Crystal Display (LCD)

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 Multimeter Photograph © Rapid Electronics

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 unfused 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.

Top of page | Choosing | Digital | Analogue | Voltage & Current | Resistance | Diode | Transistor

Analogue Multimeter Photograph © Rapid Electronics

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.

Top of page | Choosing | Digital | Analogue | Voltage & Current | Resistance | Diode | Transistor

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 are 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 chip. 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 labelled on the circuit diagram as a reminder.

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.

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)

If you are not familiar with reading analogue scales generally you may wish to see the analogue display section on the general meters page.

Top of page | Choosing | Digital | Analogue | Voltage & Current | Resistance | Diode | Transistor

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!

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 labelled "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!

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

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

If you are not familiar with reading analogue scales generally you may wish to see the analogue display section on the general meters page.

Top of page | Choosing | Digital | Analogue | Voltage & Current | Resistance | Diode | Transistor

Testing a diode with a multimeter

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

Diodes a = anode k = cathode

Testing a diode with a DIGITAL multimeter

• Digital multimeters have a special setting for testing a diode, usually labelled 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).

For further information please see the diodes page.
You may find it easier to test a diode with the simple tester project.

Top of page | Choosing | Digital | Analogue | Voltage & Current | Resistance | Diode | Transistor

Testing a transistor with a multimeter

Testing an NPN transistor

Set a digital multimeter to diode test and an analogue multimeter to a low resistance range such as × 10, as described above for testing a diode.

Test each pair of leads both ways (six tests in total):

• The base-emitter (BE) junction should behave like a diode and conduct one way only.
• The base-collector (BC) junction should behave like a diode and conduct one way only.
• The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.

For further information please see the transistors page.
You may find it easier to test a transistor with the simple tester project.

Some multimeters have a 'transistor test' function, please refer to the instructions supplied with the meter for details.

Resistance

Resistance

Resistance is the property of a component which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component.

Resistance is measured in ohms, the symbol for ohm is an omega .
1 is quite small for electronics so resistances are often given in k and M.
1 k = 1000     1 M = 1000000 .

Resistors used in electronics can have resistances as low as 0.1 or as high as 10 M.

Resistors connected in Series

When resistors are connected in series their combined resistance is equal to the individual resistances added together. For example if resistors R1 and R2 are connected in series their combined resistance, R, is given by:

Combined resistance in series:   R = R1 + R2

This can be extended for more resistors: R = R1 + R2 + R3 + R4 + ...

Note that the combined resistance in series will always be greater than any of the individual resistances.

Resistors connected in Parallel

When resistors are connected in parallel their combined resistance is less than any of the individual resistances. There is a special equation for the combined resistance of two resistors R1 and R2:

Combined resistance of two resistors in parallel:	R =	R1 × R2
		R1 + R2

For more than two resistors connected in parallel a more difficult equation must be used. This adds up the reciprocal ("one over") of each resistance to give the reciprocal of the combined resistance, R:

1	=	1	+	1	+	1	+ ...
R		R1		R2		R3

The simpler equation for two resistors in parallel is much easier to use!

Note that the combined resistance in parallel will always be less than any of the individual resistances.

Conductors, Semiconductors and Insulators

The resistance of an object depends on its shape and the material from which it is made. For a given material, objects with a smaller cross-section or longer length will have a greater resistance.

Materials can be divided into three groups:

• Conductors which have low resistance.
  Examples: metals (aluminium, copper, silver etc.) and carbon.
  Metals are used to make connecting wires, switch contacts and lamp filaments. Resistors are made from carbon or long coils of thin wire.
• Semiconductors which have moderate resistance.
  Examples: germanium, silicon.
  Semiconductors are used to make diodes, LEDs, transistors and integrated circuits (chips).
• Insulators which have high resistance.
  Examples: most plastics such as polythene and PVC (polyvinyl chloride), paper, glass.
  PVC is used as an outer covering for wires to prevent them making contact.

Ohm's Law

To make a current flow through a resistance there must be a voltage across that resistance. Ohm's Law shows the relationship between the voltage (V), current (I) and resistance (R). It can be written in three ways:

V = I × R

or

I =  V  R

or

R =  V  I

where:

V = voltage in volts (V) I  = current in amps (A) R = resistance in ohms ()

or:

V = voltage in volts (V) I  = current in milliamps (mA) R = resistance in kilohms (k)

For most electronic circuits the amp is too large and the ohm is too small, so we often measure current in milliamps (mA) and resistance in kilohms (k). 1 mA = 0.001 A and 1 k = 1000 .

The Ohm's Law equations work if you use V, A and , or if you use V, mA and k. You must not mix these sets of units in the equations so you may need to convert between mA and A or k and .

The VIR triangle

• To calculate voltage, V: put your finger over V,
  this leaves you with I R, so the equation is V = I × R
• To calculate current, I: put your finger over I,
  this leaves you with V over R, so the equation is I = ^V/_R
• To calculate resistance, R: put your finger over R,
  this leaves you with V over I, so the equation is R = ^V/_I
  

Ohm's Law Calculations

1. Write down the Values, converting units if necessary.
   
2. Select the Equation you need (use the VIR triangle).
   
3. Put the Numbers into the equation and calculate the answer.

It should be Very Easy Now!

• 3 V is applied across a 6 resistor, what is the current?
  • Values: V = 3 V, I = ?, R = 6
  • Equation: I = ^V/_R
  • Numbers: Current, I = ³/₆ = 0.5 A
  
   
• A lamp connected to a 6 V battery passes a current of 60 mA, what is the lamp's resistance?
  • Values: V = 6 V, I = 60 mA, R = ?
  • Equation: R = ^V/_I
  • Numbers: Resistance, R = ⁶/₆₀ = 0.1 k = 100
    (using mA for current means the calculation gives the resistance in k)
  
   
• A 1.2 k resistor passes a current of 0.2 A, what is the voltage across it?
  • Values: V = ?, I = 0.2 A, R = 1.2 k = 1200
    (1.2 k is converted to 1200 because A and k must not be used together)
  • Equation: V = I × R
  • Numbers: V = 0.2 × 1200 = 240 V

Power and Energy

What is power?

Power is the rate of using or supplying energy:

Power =	Energy		Power is measured in watts (W) Energy is measured in joules (J) Time is measured in seconds (s)
	Time

Electronics is mostly concerned with small quantities of power, so the power is often measured in milliwatts (mW), 1mW = 0.001W. For example an LED uses about 40mW and a bleeper uses about 100mW, even a lamp such as a torch bulb only uses about 1W.

The typical power used in mains electrical circuits is much larger, so this power may be measured in kilowatts (kW), 1kW = 1000W. For example a typical mains lamp uses 60W and a kettle uses about 3kW.

Calculating power using current and voltage

Calculating power using resistance and current or voltage

P = I² × R or P = V² / R

where:

P = power in watts (W) I  = current in amps (A) R = resistance in ohms () V = voltage in volts (V)

Wasted power and overheating

Normally electric power is useful, making a lamp light or a motor turn for example. However, electrical energy is converted to heat whenever a current flows through a resistance and this can be a problem if it makes a device or wire overheat. In electronics the effect is usually negligible, but if the resistance is low (a wire or low value resistor for example) the current can be sufficiently large to cause a problem.

You can see from the equation P = I² × R that for a given resistance the power depends on the current squared, so doubling the current will give 4 times the power.

Resistors are rated by the maximum power they can have developed in them without damage, but power ratings are rarely quoted in parts lists because the standard ratings of 0.25W or 0.5W are suitable for most circuits. Further information is available on the Resistors page.

Wires and cables are rated by the maximum current they can pass without overheating. They have a very low resistance so the maximum current is relatively large. For further information about current rating please see the Connectors and Cables page.

Energy

The amount of energy used (or supplied) depends on the power and the time for which it is used:

Energy = Power × Time

A low power device operating for a long time can use more energy than a high power device operating for a short time. For example:

• A 60W lamp switched on for 8 hours uses 60W × 8 × 3600s = 1728kJ.
• A 3kW kettle switched on for 5 minutes uses 3000W × 5 × 60s = 900kJ.

The standard unit for energy is the joule (J), but 1J is a very small amount of energy for mains electricity so kilojoule (kJ) or megajoule (MJ) are sometimes used in scientific work. In the home we measure electrical energy in kilowatt-hours (kWh). 1kWh is the energy used by a 1kW power appliance when it is switched on for 1 hour:

1kWh = 1kW × 1 hour = 1000W × 3600s = 3.6MJ

For example:

• A 60W lamp switched on for 8 hours uses 0.06kW × 8 = 0.48kWh.
• A 3kW kettle switched on for 5 minutes uses 3kW × ⁵/₆₀ = 0.25kWh.

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