Resistance is measured in Ohms and was given the Greek letter Omega with the symbol Ω
but this is difficult to print so the capital letter R is now used.
200 ohms is the same as 200 Ω and 200R
As many resistors are used in circuits, each resistor is given the letter R with a different number from R1 upwards. Not to be confused with their values 1R1 See later details.
Any material which allows an electrical current to flow through it, is know as a conductor. All conductors, however good, try to resist the current flowing through them, even copper wire has a resistance. The resistance is the same whichever direction the current is flowing.
Ohms Law
George Ohms (1789 - 1854) found that Current flowing through a component is related to its Resistance and the Voltage across it. He produced the formula:-
Voltage(in volts) = Current (in amps) x Resistance (in ohms)
V = I x R I = V/R R = V/I
1. Fixed Resistors.
These are components which are made with one value of resistance. The look like small tubes with wires at each end. As they are so small it is not possible to print their resistance value on the side so a colour code system is used.
It is not possible to make resistors to an exact value, so after manufacture their values are measured and then they are sorted into groups of preferred values.
E24 Series resistors.
The E24 Series have resistance values with a 5% Tolerance Range.
As an example a 510 ohm resistor could have a value
as high as 510 +5% = 535.5 ohms or
as low as 510 - 5% = 484.5 ohms
The next higher preferred value is 560 ohms giving values from 588 to 532 ohms.
510 ohms giving values from 535 to 484 ohms.
The next lowest preferred value is 470 ohms giving values from 498 to 441 ohms.
As you can see the highest value of one preferred number overlaps the lowest value of the next preferred. This way the manufacturer can use all the resistors made.
If you look in the Rapid Catalogue you will see the most commonly used resistor values are the E24 Series and the preferred values are multiples of the following numbers:-
1.0/ 1.1 / 1.2 / 1.3 / 1.5 / 1.6 / 1.8 / 2.0 / 2.2 / 2.4 / 2.7 / 3.0
3.3 / 3.6 / 3.9 / 4.3 / 4.7 / 5.1 / 5.6 / 6.2 / 6.8 / 7.5 / 8.2 / 9.1 ohms
To get the values you add zeros ie, 1 ohms 12 ohms 150 ohms 2200 ohms
27000 ohms 620000 ohms up to 9100000 ohms.
These numbers are too big and difficult use so the values are changed to KILO (thousand ohms) and MEGA (million ohms).e.g. 1R 12R 220R 2.7K 30K 390K 4.7M 62M
From experience many mistakes were made reading the position of the decimal point, so the correct way is to use the letter to show the position of the decimal point i.e.
1R0 1R5 18R 240R 3K3 43K 510K 6M2 68M 750M 8G2
E12 Series resistors.
The E12 Series have resistance values with a 10% Tolerance Range.
The prefered values are 10 12 15 18 22 27 33 39 47 56 68 82
CAUTION Do not expect your resistors to be close to the preferred values.
Electronics is more of an art than a science. You must also take into account that all components have a resistance.
Speakers have values given in ohms. This is not their resistance but impedance!
Colour Code
As Resistors are so small it is not possible to print their resistance value on the side so a colour code system is used.
| | Band 1 | Band 2 | Band 3 |
|---|
| Colour | 1st Number | 2nd Number | Zeros |
| Black | 0 | 0 | |
| Brown | 1 | 1 | 0 |
| Red | 2 | 2 | 00 |
| Orange | 3 | 3 | 000 |
| Yellow | 4 | 4 | 0,000 |
| Green | 5 | 5 | 00,000 |
| Blue | 6 | 6 | 000,000 |
| Violet | 7 | 7 | 0,000,000 |
| Grey | 8 | 8 | |
| White | 9 | 9 | |
4th Band Gold = 5% Tolerance
Examples.
| | Band 1 | Band 2 | Band 3 | |
|---|
| 1R0 | black | brown | black | 1 Ω |
| 24R | red | yellow | black | 24 Ω |
| 330R | orange | orange | brown | 330 Ω |
| 470R | yellow | violet | brown | 470 Ω |
| 5K6 | green | blue | red | 5600 Ω |
| 68K | blue | grey | orange | 68000 Ω |
| 750K | violet | green | yellow | 750000 Ω |
| 8M2 | grey | red | green | 8200000 Ω |
| 91M | white | brown | blue | 91000000 Ω |
2. Resistor combinations
A. Resistors in series
When two or more resistors are placed end to end they are said to be connected in series.
The position of resistors in circuits are shown by a sequence of numbers from R1, R2, R3, upwards. Not to be confused with their values 2R2 4K7 etc.
Total resistance for resistors in series is
R = R1 + R2
B. Resistors in Parallel
When two or more resistors are placed side by side they are said to be connected in parallel.
Total resistance for two resistors in parallel is
R = (R1 x R2) / (R1 + R2)
C. Resistors in Series and Parallel
The total resistance for resistors placed in a combination of series and parallel is found in stages. First add the two resistors in series. Then find the total resistance.
R4 = R1 +R2 Then R = R4 x R3 / R4 + R3
This can be combined into
(R1 + R2) x R3 / (R1 + R2) + R3
3. Variable Resistors
Theses are resistors whose resistance can be altered and they have three connections. There are two connections at either end of the resistance material, which is commonly known as the track. The third connection is made to a conducting slider, commonly known as the wiper, which is in contact with the track and can be slid along it from one end to the other. The current or voltage available at the wiper is then related to the position that it has along the track.
Variable Resistors can be used in a circuit to alter resistance and in this situation only two connections are used. One end and the wiper.
It is good practice to connect the free end to the wiper so in the event that the wiper fails to connect, the variable resistor will go to maximum resistance protecting the circuit
Potentiometer or Pot is the name given when the variable resistor is used as a Potential Divider to alter voltage in part of the circuit from 9 to 0 volts or as a speaker balance control. All three connections are used.
There are two types of variable resistors
1. Preset:-
The resistance of part of the circuit needs to be adjusted once during manufacture to allow for component variations. Presets are soldered directly onto the Printed Circuit Board. (PCB)
Pre Set
2. Control:-
The resistance of part of the circuit is altered frequently such as volume control. These are available as single gang, double gang (stereo volume) and single gang switched. They are available as either rotary shaft or slider.
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Rotary Control
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Slide Control
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Values of Variable Resistors
Typical values:- 100R 220R 470R 1K 2K2 4K7 10K 22K 47K 100K 220K 470K 1M
Resistance is available in two forms:- Linear and Log values
Linear values change directly with the amount of movement giving a straight line on the graph.
Log values start off with a small change in resistance for a large movement and gradually alters to a large change in resistance for a small amount of movement. Curved line on the graph.
Graph of Linear and Log Resistance values against Rotation
4. Light Dependant Resistors (LDR)
LDR is a component whose resistance changes in response to the amount of light shining on it. The standard component is the ORP 12 with a dark resistance of 1M ohms and a light resistance of 500 ohms. Typical use is on top of a lamppost to turn the street light on and off depending on the light level.
Graph of Voltage against Current for different light levels.
Photo Diodes are used when a quick response is required, as LDR's are slow to respond.
(Found in Optronics section of Rapid Catalogue)
5. Thermistors
Thermistors are small components whose resistance changes in response to heat.
NTC (negative temperature coefficient) type thermistors.
The resistance decreases as the temperature increases and the output voltage increases.
(Found in Sensors section of the Rapid Catalogue)
6. Power Rating
Heat is produced when current flows through a conductor, so you need to calculate the power rating of the resistor before ordering them.
For normal use specify 0.25 W Carbon Film
Others available:- 0.125W and 0.5W carbon film.
1W and 2W metal film.
2.5W, 3W and 5W wire wound.
Power Rating (Watts) = Voltage (Volts) x Current (Amps)
P = V x I
Issue 30 - 20/2/03
