Bredhurst Receiving and Transmitting Society

Syllabus Sections:-

Resistance.

3b.1 Understand and apply the formulae for calculating the combined values of resistors in series and/or in parallel.

Resistors of different values of may be used in series, parallel or combined series and parallel circuits.

Resistors

Resistors and what they are was introduced to you in the foundation Licence course and developed on that in the Intermediate course

There are two quite difference and for some challenging formulae associated with resistor combinations whether they are in series or in parallel

Resistors in Series

The diagram below shows resistors connected in series.

For a calculation any number of resistors, (from as few as to two) may be connected as shown above.

The effective resistance of the combination of R1 R2 and R3 is given by

R(total) = R1 + R2 + R3

BUT you can of course add together as many resistors as you need to and calculate the total effective resistance of all the resistors.

When a connection is taken from the junction between two resistors in series this is know as as fixed potentiometer or potential divider.

With the two resistors in this configuration a voltage somewhere between the maximum and minimum would be measured at the point marked "out" depending upon the values of the resistors. This fixed combination is usually replaced by a single resistor in which the output can be changed by moving a slider along the track.

The output could be taken from anywhere along the resistor.

Without necessarily knowing it you have all used such potentiometer on the audio gain control on your transceiver / receiver !!!

Resistors in Parallel

The diagram below shows resistors connected in parallel.

Note that all the resistors are linked to each other by the wire that is across the end of of them all so it does not matter where the other connecting wires into the circuit actually connect the result would be the same.

For a calculation any number of resistors, (from as few as two) may also be connected as shown above.

The effective resistance of the combination of R1 R2 and R3 is given by

which can also be written as :- 1/R(total)= 1/R1 + 1/R2 + 1/R3

BUT as with the earlier example you can of course add together as many resistors as you need to and calculate the total effective resistance of all the resistors.

Let's put in some values. R1 = 100 ohms R2 = 150ohms R3 =270ohms

Thus 1/R(total) = 1/100 + 1/150 + 1/270

so 1/R(total) = 0.01 + 0.0066 + 0.0037

so 1/R(total) = 0.0203

so R(total) = 1/0.0203

so R(total) = 49.26

So the total resistance of R1, R2 and R3 in parallel is about 49 ohms

Note:- that the resulting resistance is always less than the lowest resistor value in the group.

For the exam do the calculation as shown and then compare you answer to that in the question and you will see what figure is like yours and pick that as the correct answer.

If you are weak on maths then ask your lead instructor for some extra work on this to help you learn it.