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| circuit symbol | |
- Icy water 0°C: high resistance, about 12k
.
- Room temperature 25°C: medium resistance, about 5k
- Boiling water 100°C: low resistance, about 400
Friday, January 21, 2011
Thermistor
Light Dependent Resistor (LDR)
An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is |
| Photograph © Rapid Electronics |
 |
| circuit symbol |
A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR:
- Darkness: maximum resistance, about 1M
- Very bright light: minimum resistance, about 100
Capacitors
Polarised (> 1µF) | Unpolarised (< 1µF) | Real Values | Variable & trimmers
Also see: Capacitance and Uses of CapacitorsFunction
Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.Capacitance
This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):- µ means 10-6 (millionth), so 1000000µF = 1F
- n means 10-9 (thousand-millionth), so 1000nF = 1µF
- p means 10-12 (million-millionth), so 1000pF = 1nF
There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.
Polarised capacitors (large values, 1µF +)
Examples:
Circuit symbol: 
Electrolytic Capacitors
Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.
Tantalum Bead Capacitors
Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size.Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.
For example: blue, grey, black spot means 68µF
For example: blue, grey, white spot means 6.8µF
For example: blue, grey, grey spot means 0.68µF
Unpolarised capacitors (small values, up to 1µF)
Examples:
Circuit symbol: 
Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!
Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!For example 0.1 means 0.1µF = 100nF.
Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.
Capacitor Number Code
A number code is often used on small capacitors where printing is difficult:
- the 1st number is the 1st digit,
- the 2nd number is the 2nd digit,
- the 3rd number is the number of zeros to give the capacitance in pF.
- Ignore any letters - they just indicate tolerance and voltage rating.
| Colour | Number |
| Black | |
| Brown | |
| Red | |
| Orange | |
| Yellow | |
| Green | |
| Blue | |
| Violet | |
| Grey | |
| White | |
Capacitor Colour Code
A colour code was used on polyester capacitors for many years. It is now obsolete, but of course there are many still around. The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).
For example:brown, black, orange means 10000pF = 10nF = 0.01µF.
Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band.
For example:
wide red, yellow means 220nF = 0.22µF.
Polystyrene Capacitors
This type is rarely used now. Their value (in pF) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint.Real capacitor values (the E3 and E6 series)
You may have noticed that capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not!Why is this? Imagine that you decided to make capacitors every 10µF giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.To produce a sensible range of capacitor values you need to increase the size of the 'step' as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.
The E3 series (3 values for each multiple of ten)
10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 etc.
Notice how the step size increases as the value increases (values roughly double each time).
The E6 series (6 values for each multiple of ten)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how this is the E3 series with an extra value in the gaps.
The E3 series is the one most frequently used for capacitors because many types cannot be made with very accurate values.
Variable capacitors
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| Variable Capacitor Symbol |
 |
| Variable Capacitor |
Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.
Trimmer capacitors
 |
| Trimmer Capacitor Symbol |
 |
| Trimmer Capacitor |
Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF.
Trimmers are the capacitor equivalent of presets which are miniature variable resistors.
Diodes
Signal diodes | Rectifier diodes | Bridge rectifiers | Zener diodes
Also see: LEDs | AC and DC | Power SuppliesExample:
Circuit symbol: 
Function
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).Reverse Voltage
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LEDs(which have their own page) and Zener diodes (at the bottom of this page).Connecting and soldering
Diodes must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is marked by a line painted on the body. Diodes are labelled with their code in small print, you may need a magnifying glass to read this on small signal diodes!Small signal diodes can be damaged by heat when soldering, but the risk is small unless you are using a germanium diode (codes beginning OA...) in which case you should use a heat sink clipped to the lead between the joint and the diode body. A standard crocodile clip can be used as a heat sink.Rectifier diodes are quite robust and no special precautions are needed for soldering them.
Testing diodes
You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. A lamp may be used to test a rectifier diode, but do NOT use a lamp to test a signal diode because the large current passed by the lamp will destroy the diode!Signal diodes (small current)
Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.
For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.
Protection diodes for relays
Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil.Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.| Diode | Maximum Current | Maximum Reverse Voltage |
| 1N4001 | 1A | 50V |
| 1N4002 | 1A | 100V |
| 1N4007 | 1A | 1000V |
| 1N5401 | 3A | 100V |
| 1N5408 | 3A | 1000V |
Rectifier diodes (large current)
Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode.All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The table shows maximum current and maximum reverse voltage for some popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.Also see: Power Supplies
Bridge rectifiers
There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled
.The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct.Also see: Power Supplies
 |  |  |  |  |
| Various types of Bridge Rectifiers Note that some have a hole through their centre for attaching to a heat sink | ||||
Integrated Circuits (Chips)
Pin numbers | IC holders | Static | Datasheets | Sinking/sourcing | Combining outputs |
555 and 556 Timers | Logic ICs | 4000 Series | 74 Series | PIC microcontrollersAlso see: 4000 Series ICs | 74 Series ICs | 555 and 556 Timer Circuits
Integrated Circuits are usually called ICs or chips. They are complex circuits which have been etched onto tiny chips of semiconductor (silicon). The chip is packaged in a plastic holder with pins spaced on a 0.1" (2.54mm) grid which will fit the holes on stripboard and breadboards. Very fine wires inside the package link the chip to the pins.
Pin numbers
The pins are numbered anti-clockwise around the IC (chip) starting near the notch or dot. The diagram shows the numbering for 8-pin and 14-pin ICs, but the principle is the same for all sizes.IC holders (DIL sockets)
ICs (chips) are easily damaged by heat when soldering and their short pins cannot be protected with a heat sink. Instead we use an IC holder, strictly called a DIL socket (DIL = Dual In-Line), which can be safely soldered onto the circuit board. The IC is pushed into the holder when all soldering is complete.IC holders are only needed when soldering so they are not used on breadboards.Commercially produced circuit boards often have ICs soldered directly to the board without an IC holder, usually this is done by a machine which is able to work very quickly. Please don't attempt to do this yourself because you are likely to destroy the IC and it will be difficult to remove without damage by de-soldering.
Removing an IC from its holder
If you need to remove an IC it can be gently prised out of the holder with a small flat-blade screwdriver. Carefully lever up each end by inserting the screwdriver blade between the IC and its holder and gently twisting the screwdriver. Take care to start lifting at both ends before you attempt to remove the IC, otherwise you will bend and possibly break the pins.Static precautions
 |
| Antistatic bags for ICs |
resistor in series.Datasheets
| PDF files To view and print PDF files you need an Acrobat Reader which may be downloaded free for Windows, Mac,RISC OS, or UNIX/Linux computers. If you are not sure which type of computer you have it is probably Windows. |
Sinking and sourcing current
IC outputs are often said to 'sink' or 'source' current. The terms refer to the direction of the current at the IC's output.If the IC is sinking current it is flowing into the output. This means that a device connected between the positive supply (+Vs) and the IC output will be switched on when the output is low (0V).If the IC is sourcing current it is flowing out of the output. This means that a device connected between the IC output and the negative supply (0V) will be switched on when the output is high (+Vs).
It is possible to connect two devices to an IC output so that one is on when the output is low and the other is on when the output is high. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately.
The maximum sinking and sourcing currents for an IC output are usually the same but there are some exceptions, for example 74LS TTL logic ICs can sink up to 16mA but only source 2mA.
Using diodes to combine outputs
The outputs of ICs must never be directly connected together. However, diodes can be used to combine two or more digital (high/low) outputs from an IC such as a counter. This can be a useful way of producing simple logic functions without using logic gates!The diagram shows two ways of combining outputs using diodes. The diodes must be capable of passing the output current. 1N4148 signal diodes are suitable for low current devices such as LEDs.For example the outputs Q0 - Q9 of a 4017 1-of-10 counter go high in turn. Using diodes to combine the 2nd (Q1) and 4th (Q3) outputs as shown in the bottom diagram will make the LED flash twice followed by a longer gap. The diodes are performing the function of an OR gate.
Example projects: Traffic Light | Dice | Model Lighthouse
The 555 and 556 Timers
The 8-pin 555 timer IC is used in many projects, a popular version is the NE555. Most circuits will just specify '555 timer IC' and the NE555 is suitable for these. The 555 output (pin 3) can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply LEDs, relay coils and low current lamps. To switch larger currents you can connect a transistor.The 556 is a dual version of the 555 housed in a 14-pin package. The two timers (A and B) share the same power supply pins.Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.
For further information please see the page on 555 and 556 timer circuits.
Logic ICs (chips)
Logic ICs process digital signals and there are many devices, including logic gates, flip-flops, shift registers, counters and display drivers. They can be split into two groups according to their pin arrangements: the 4000 series and the 74 series which consists of various families such as the 74HC, 74HCT and 74LS.For most new projects the 74HC family is the best choice. The older 4000 series is the only family which works with a supply voltage of more than 6V. The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation.The table below summarises the important properties of the most popular logic families:
| Property | 4000 Series | 74 Series 74HC | 74 Series 74HCT | 74 Series 74LS |
| Technology | CMOS | High-speed CMOS | High-speed CMOS TTL compatible | TTL Low-power Schottky |
| Power Supply | 3 to 15V | 2 to 6V | 5V ±0.5V | 5V ±0.25V |
| Inputs | Very high impedance. Unused inputs must be connected to +Vs or 0V. Inputs cannot be reliably driven by 74LS outputs unless a 'pull-up' resistor is used (see below). | Very high impedance. Unused inputs must be connected to +Vs or 0V. Compatible with 74LS (TTL) outputs. | 'Float' high to logic 1 if unconnected. 1mA must be drawn out to hold them at logic 0. | |
| Outputs | Can sink and source about 5mA (10mA with 9V supply), enough to light an LED. To switch larger currents use a transistor. | Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor. | Can sink and source about 20mA, enough to light an LED. To switch larger currents use atransistor. | Can sink up to 16mA (enough to light an LED), but source only about 2mA. To switch larger currents use a transistor. |
| Fan-out | One output can drive up to 50 CMOS, 74HC or 74HCT inputs, but only one 74LS input. | One output can drive up to 50 CMOS, 74HC or 74HCT inputs, but only 10 74LS inputs. | One output can drive up to 10 74LS inputs or 50 74HCT inputs. | |
| Maximum Frequency | about 1MHz | about 25MHz | about 25MHz | about 35MHz |
| Power consumption of the IC itself | A few µW. | A few µW. | A few µW. | A few mW. |
 |
| Driving 4000 or 74HC inputs from a 74LS output using a pull-up resistor. |
Mixing Logic Families
It is best to build a circuit using just one logic family, but if necessary the different families may be mixed providing the power supply is suitable for all of them. For example mixing 4000 and 74HC requires the power supply to be in the range 3 to 6V. A circuit which includes 74LS or 74HCT ICs must have a 5V supply.A 74LS output cannot reliably drive a 4000 or 74HC input unless a 'pull-up' resistor of 2.2k is connected between the +5V supply and the input to correct the slightly different logic voltage ranges used.Note that a 4000 series output can drive only one 74LS input.
| Quick links to individual ICs4000 4060 4001 4068 4002 4069 4011 4070 4012 4071 4017 4072 4020 4073 4023 4075 4024 4077 4025 4081 4026 4082 4028 4093 4029 4510 4030 4511 4040 4516 4049 4518 4050 4520 |
4000 Series CMOS
This family of logic ICs is numbered from 4000 onwards, and from 4500 onwards. They have a B at the end of the number (e.g. 4001B) which refers to an improved design introduced some years ago. Most of them are in 14-pin or 16-pin packages. They use CMOS circuitry which means they use very little power and can tolerate a wide range of power supply voltages (3 to 15V) making them ideal for battery powered projects. CMOS is pronounced 'see-moss' and stands for Complementary Metal Oxide Semiconductor.However the CMOS circuitry also means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. For the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces.For further information, including pin connections, please use the quick links on the right or go to 4000 Series ICs.
| Quick links to individual ICs7400 7432 7402 7442 7403 7447 7404 7486 7405 7490 7408 7493 7409 74132 7410 74160 7411 74161 7412 74162 7414 74163 7420 74192 7421 74193 7427 74390 7430 7439374HC4017 74HC4020 74HC4040 74HC4060 74HC4511 |
74 Series: 74LS, 74HC and 74HCT
There are several families of logic ICs numbered from 74xx00 onwards with letters (xx) in the middle of the number to indicate the type of circuitry, eg 74LS00 and 74HC00. The original family (now obsolete) had no letters, eg 7400.The 74LS (Low-power Schottky) family (like the original) uses TTL (Transistor-Transistor Logic) circuitry which is fast but requires more power than later families.The 74HC family has High-speed CMOS circuitry, combining the speed of TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.
The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations.
Beware that the 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL!
For further information, including pin connections, please use the quick links on the right or go to 74 series ICs.
The CMOS circuitry used in the 74HC and 74HCT series ICs means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them.
PIC microcontrollers
A PIC is a Programmable Integrated Circuit microcontroller, a 'computer-on-a-chip'. They have a processor and memory to run a program responding to inputs and controlling outputs, so they can easily achieve complex functions which would require several conventional ICs. |
| www.picaxe.co.uk |
If you think PICs are not for you because you have never written a computer program, please look at the PICAXE system! It is very easy to get started using a few simple BASIC commands and there are a number of projects available as kits which are ideal for beginners. The system is stocked by Rapid Electronics.
Resistors
Colour Code | Tolerance | Real Values (E6 & E12 series) | Power Rating
Also see: Resistance | Ohm's LawExample:
Circuit symbol: 
Function
Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.Connecting and soldering
Resistors may be connected either way round. They are not damaged by heat when soldering.Colour Code | |
| Colour | Number |
| Black | |
| Brown | |
| Red | |
| Orange | |
| Yellow | |
| Green | |
| Blue | |
| Violet | |
| Grey | |
| White | |
Resistor values - the resistor colour code
Resistance is measured in ohms, the symbol for ohm is an omega.1
is quite small so resistor values are often given in k and M.1 k
= 1000 1 M = 1000000 .Resistor values are normally shown using coloured bands.Each colour represents a number as shown in the table.
Most resistors have 4 bands:
- The first band gives the first digit.
- The second band gives the second digit.
- The third band indicates the number of zeros.
- The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given below.
This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.So its value is 270000
= 270 k.On circuit diagrams the
is usually omitted and the value is written 270K.Find out how to make your own Resistor Colour Code Calculator
Small value resistors (less than 10 ohm)
The standard colour code cannot show values of less than 10. To show these small values two special colours are used for the third band: gold which means × 0.1 and silverwhich means × 0.01. The first and second bands represent the digits as normal.For example:red, violet, gold bands represent 27 × 0.1 = 2.7
green, blue, silver bands represent 56 × 0.01 = 0.56
Tolerance of resistors (fourth band of colour code)
The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390, between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390).A special colour code is used for the fourth band tolerance:silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.
Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.
Resistor shorthand
Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.For example:- 560R means 560
2K7 means 2.7 k = 2700
39K means 39 k
1M0 means 1.0 M = 1000 kReal resistor values (the E6 and E12 series)
You may have noticed that resistors are not available with every possible value, for example 22k and 47k are readily available, but 25k and 50k are not!Why is this? Imagine that you decided to make resistors every 10 giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.
The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.
The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.
Notice how this is the E6 series with an extra value in the gaps.
The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390
could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429.Resistors in Series and Parallel
For information on resistors connected in series and parallel please see the Resistance page,Power Ratings of Resistors
 |
 |
| High power resistors (5W top, 25W bottom) |
) or high voltages (more than 15V).The power, P, developed in a resistor is given by:
| P = I² × R or P = V² / R | where: | P = power developed in the resistor in watts (W) I = current through the resistor in amps (A) R = resistance of the resistor in ohms ( )V = voltage across the resistor in volts (V) |
Examples:
- A 470
In this case a standard 0.25W resistor would be suitable. - A 27
A high power resistor with a rating of 5W would be suitable.
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