Resistors in series and parallel

Most circuits have more than one component, called a resistor that limits the current flow in the circuit. It is the measure of this limit on current flow, that is called resistance and its unit is the ohm.


Considering the photo below, the VOLT pushes the AMP (current) through the circuit, while OHM, “the resistance” opposes the current flow.

The simplest combinations of resistors are the series and parallel connections illustrated below.

The total resistance of a combination of resistors depends on both their individual values and how they are connected.

(1.) A series connection of resistors.

(2.) A parallel connection of resistors.

  1. Resistors in Series
    When are resistors in series ? Resistors are said to be connected in series when they are daisy chained together in a single line resulting in a common current flowing through them. More so resistors are said to be in series whenever the flow of current is sequential through the device. For example, if current flows
    through resistors R1, then R2 and R3 as shown below.

Resistors in series can be added with simple arithmetical addition.

R1 + R2 + R3 = Rs

It seems reasonable that the total resistance is the sum of the individual resistances, considering that the current has to
pass through each resistor in sequence.



  1. Resistors in parallel

The diagram above shows resistors in parallel, wired to a voltage source. Resistors R1 and R2 are in parallel as each resistor is connected directly to the voltage source by connecting wires with negligible resistance. Each resistor thus has the full voltage of the source applied to it.

Each resistor draws the same current it would if it alone were connected to the voltage source
(provided the voltage source is not overloaded).

The effective resistance, Rs of paralleled Resistors is given as,


1/Rs= 1/R1+1/R2+1/R3

These analyses of series and parallel connections of Resistors will go a long way in helping us to understand working principles of some sensors like thermistor, LDR and others.