Design & Technology Resources
Resistors are only made in certain values otherwise you would need millions of different resistors! Instead, manufacturers do a range of resistors know as preferred values. In an exam you may be asked to convert an irregular resistance value into a value that can actually be bought.
If you do a calculation and find you need a 122.3R resistor for example, you wouldn’t be able to buy one. Instead you would have to choose the closest available value.
There are two series that you need to know:
E12 - a range of 12 different values for each decade;
Converting resistances into preferred values
Iin the exam you will be given the following information:
E12 Resistor series:
10, 12, 15, 18, 22, 27, 33, 39, 47, 57, 68, 82
E24 Resistor series
10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91
You usually round up the value. The tables below shows some examsples...
There are only 12 different values for each decade (10s,100s, 100s etc)
There are 24 different values for each decade. You can therefore select more accurate values.
In an ideal world the resistance of a resistor would match exactly to the actual resistance value in real life.In reality however, the real value is different because of tolerance.
What is tolerance? Imagine if you were asked to cut a length of wood 100mm long, it would be almost impossible for you to cut it to exactly that length. In reality it would probably be alright to get it between 99mm and 101mm long, in other words you would be allowed a tolerance of +/- 1%.
Resistors work in exactly the same way. You can tell what the tolerance of a resistor is by looking at the fourth coloured band:
Gold Band (+/- 5% tolerance)
A gold band means that the actual value can be 5% higher or 5% lower than the colour bands tell you the resistance is.
Silver Band (+/-10% tolerance)
A silver band means that the actual value can be 10% higher or 10% lower tham the colour bands tell you the resistance is.
Does it really matter?
In most circuits it doesn’t matter if the resistors are not exactly bang on the desired value, although in some timing circuits (such as astables and monostables) it can be important.
If a resistor is marked as having a resistance of 100R but has a tolerance of +/-5% then the actually resistance value could be anywhere between:
95R to 105R
When two or more resistors are connected together in a row they are said to be connected in series. In an exam you may be asked to calculate the total resistance value.
It is just a matter of adding the values together...
Rtotal = R1 + R2 + R3
Rtotal = 1000 + 4000 + 6000
Rtotal = 11K
A potentiometer is a component who’s resistance can be varied by rotating a shaft.
A potentiometer is usually used as a potential divider in an electronic circuit. This means it can be used to produce a voltage depending on the position of the shaft.
Preset potentiometers are usually found inside pieces of equipment where the value needs to be adjusted in the factory, but once it is set it doesn’t need to be changed.
The diagrams below show what voltages the potentiometers produce depending on the position of the shaft: