What does it do?
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Resistors resist the flow of electricity. They are used in a wide variety of electronic subsystems to regulate current and control voltage.
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How does it operate?
The circuits for many electronic subsystems include resistors which are needed to enable the subsystem to work. Sometimes, by changing the resistance value of the resistors (measured on ohms – symbol W), we can change the details of the operation of the subsystem, for example, changing the gain of an amplifier.
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The resistor colour code Colour Code |
Colour Code
The resistance of resistors is found by using the resistor colour code.
The three bands close together identify the resistance value.
The colour of the first band gives the first digit. The colour of the second band gives the second digit. And the colour of the third band gives the number of zeros after these first two digits.
The resistor at the bottom of the graphic on the left has bands of brown, black and orange. So its resistance value is ‘1’ (brown), then ‘0’ (black) and then three zeros (orange) i.e. 10,000W.
If you are colour blind, check the resistance value with a multimeter on the ohms setting.
- Silver 10%
- Gold 5%
- Red 2%
- Brown 1%
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Colour: Tolerance
silver ±10%,
gold ±5%
red ±2%,
brown ±1%.
If no fourth band is shown the tolerance is ±20%.
Resistance tolerances |
A colour code is used for the fourth band and this represents the tolerance of the resistor (ie how accurate the value is).
So in the example of the 10,000Ù shown above with a brown tolerance band, the tolerance is + or -1%. The resistance value is therefore between 9900Ù and 10100Ù . |
Thousands and Millions
Resistors used in electronics are often in the range of thousands or sometimes millions of ohms. To make it easier to write down these large values thousand of ohms are called ‘kilohms’ and millions of ohms are called ‘megohms’.
When writing down these values we use the initials ‘k’ or ‘M’. So, 10,000W = 10 kilohms and is written 10k. (Note that ‘k’ is the correct symbol, but ‘K’ is often incorrectly used). The ‘k’ is often used in place of a decimal place. So, 2,700W = 2.7 kilohms and this is often written as 2k7.
Similarly, ‘M’ is used as a symbol for megohms. 3,300,000W is therefore 3.3 megohms and is written 3M3.
In the case of resistors below 1,000W, the symbol ‘R’ is used after the numerical value of the resistance. Eg 270 ohms would be written as 270R
W, the symbol ‘R’ is often used in place of W, so 330W would be written 330R, and 4.7W as 4R7.
Preferred Values
Resistors are available in various preferred values. A common series of resistors is called the E12 series and the preferred values in this series are: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (i.e. 12 values). The next set of values is higher by a factor of 10: 100, 120, 150 … and then 1,000, 1,200, 1,500 …
The E12 series is usually suitable for electronic circuits. Occasionally, for high precision applications, the E24 or E48 series is needed.
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Panel-mounted variable resistor
PCB-mounted preset variable resistor |
Variable Resistors
Sometimes it is necessary to have a resistor in a circuit whose value can be changed after the circuit has been built.
This might be to allow the circuit to be ‘fine tuned’ by the manufacturer, or adjusted by the user e.g. to change the volume on a radio.
The type of resistor required in this situation is called a ‘potentiometer’. The reason for the name is that this type of resistance can also be used as a potential divider (see below).
These resistors can be mounted on the front panel of the case (usually when the operation of the system is adjusted by the user with a knob). Or they can be PCB-mounted (usually to allow adjustment by the manufacturer).
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Voltage, Current and Resistance
When current passes through a resistor it produces a voltage (V) across the resistor, proportional to the current (I). The value of the voltage, divided by the current is the resistance value of the resistor (R). This is expressed by Ohm’s law, which is very useful for calculating the voltage, current or resistance value if we know the other two quantities.
The classic way that Ohm’s law is written is: 
We can re-arrange this equation to find the resistance if we know the current and voltage:
Or, we can find the current if we know the voltage and resistance:
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A potential divider circuit |
Potential Divider
Resistors are often arranged as a ‘potential divider’. A potential divider is used to reduce the supply voltage Vs to a lower value Vout.
The value of Vout depends on R1 and R2. If they are equal then Vout is half the value of Vs. If R2 is less than R1 then Vout is less than half Vs.
The value of Vout for any value of R1 and R2 can be found from the formula:
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A high power resistor |
Power
Usually, the power dissipation in electronic resistors is small so low power resistors e.g. 0.25W, are suitable. Sometimes (for example, if a resistor is being used as a heater) the power dissipated can be larger. In this case higher power resistors need to be used.
The power (P) dissipated in a resistor is given by the formula:
The resistor’s rated power must be greater than this value.
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Resistors in series |
Resistors in Series
Occasionally it is necessary to produce a resistor by combining several resistors together. The easiest way to do this is to combine them in ‘series’ (in a line). The resistance of resistors in series is found by adding them together:
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Resistors in parallel |
Resistors in Parallel
Very occasionally resistors are combined in ‘parallel’ (both ends are connected together). The resistance of resistors in parallel is found from the formula:
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Possible applications
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Resistors can be mounted either way round. |
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Different length resistors on a PCB |
When designing the PCB, the two pads for the ends of the resistor can be spaced at any convenient distance apart greater than 0.3 inches. This can make PCB designs simpler and neater.
It is often convenient to use resistors as ‘bridges’, with PCB tracks running underneath. |
Testing
Check that the voltage across the resistor is the expected value.
Fault finding
Once a resistor has been soldered into a PCB it can be difficult to check it with a multimeter because other components in the circuit can change the apparent resistance. Use the colour code to confirm its value.
Alternatives
There are no alternatives to resistors.
Web links
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