The resulting value can be calculated using the formula.
Rt = ———————————
——— + ———
For more than two resistors the formula simply expands as follows.
Rt = —————————————————
1 1 1
——— + ——— + ———
R1 R2 R3
The resulting overall resistance, Rt, for the circuit in Sk 6 is arrived at by considering
it in two stages. First, two resistors, R2 and R3, in parallel as in Sk 5 and summing
this value with the R1, as in Sk 4.
Placing two resistors in series will be for one of two reasons. 1, if the value required
is not available and has to be arrived at using two of lower value, not necessarily
the same. Or 2, and more likely, to tap off a smaller voltage than that applied to
the circuit as illustrated by Sk 7.
In this case the ratio of resistor values R2 : R1 + R2 will be identical to the ratio
of the output to input voltages.
The most basic of circuits is were a voltage (V) is feeding a resistor (R) resulting
in a current (I), Sk3.
In this case the current (I) is dependent on the values of V and R. Current will
increase proportionally with voltage and inversely proportional with resistance.
That is, typically, twice the voltage resulting in twice the current but twice the
resistance resulting in half the current. The formula for this basic circuit is
I = ——— amps
Before we expand on that simple circuit it is necessary to understand the result
of using multiple resistors. This can be with them in series (Sk 4), parallel (Sk
5) or complex circuits (Sk 6).
The total resistance, Rt, when connecting two resistors in series is the sum of the
two values. The formula being.
Rt = R1 + R2
Placing two resistors in parallel is more complex but the resulting value will always
be smaller than that of the smallest. Typically, with high and low value resistors
in parallel the result will be a little less than that of the lower value.
Vo = Vin x —————————
R1 + R2
This though is an over simplification as the output voltage, Vo, will be feeding
a load that will have its own resistance value. The circuit Sk 6 is therefore more
appropriate where R3 is the load.
In this case the value of R2 and R3 in parallel should be used in the ratio. I suspect
that some viewers will consider that I am discussing electronic circuits and whilst
my comments could be applicable to these they are equally at home in non electronic