Having a solid understanding of resistors in electrical circuits is crucial for radio amateurs and electronic hobbyists. In this article, I will explain how to design and calculate circuits with resistors.
Serial circuits
When electricity has a single path
A series circuit with resistors is the simplest to calculate. The total resistance in a serial circuit can be calculated by adding all the resistances in the circuit.
- Rs = Sum of resistance in series (Ohm)
- R1 = Resistor one (Ohm)
- R2 = Resistor two (Ohm)
- R3 = Resistor two (Ohm)
- Rn = Any additional resistors (Ohm)
The voltage drop over each resistor is calculated using Ohm's law
Parallel circuits
When electricity has multiple paths
The total resistance in a parallel circuit can be calculated by using the following formula for just two resistors:
- Rp = Sum of resistance in parallel (Ohm)
- R1 = Resistor one (Ohm)
- R2 = Resistor two (Ohm)
The total resistance in a parallel circuit is always less than the smallest resistance.
If you have two resistors with the same value, the parallel resistance will be half of one of them. Eg, two 200 Ohm resistors in parallel will have a total resistance of 100 Ohm.
For circuits with more than two resistors in parallel, the following formula applies.
This can be used to calculate the parallel resistance, using this formula:
Serial/parallel circuits
When you have a combination of resistors in parallel and series, you must calculate them systematically by calculating the innermost serial circuits, then move on to parallel circuits until you can finally calculate the last serial or parallel circuit and find the total resistance in the circuit.
For a circuit where two resistors are placed in series, and those resistors are in parallel with a single resistor, you first need to calculate the resistance of the serial resistors, then use that resistance to calculate the paralell resistance.
For a circuit with two resistors in parallel, that are connected with a single circuit in series, you first need to calculate the parallel resitance, and then use the parallel resistance to calculate the serial resistance.
The same approach can be used to calculate the resistance in complex circuits of any size.
AC/DC
Alternating Current and Direct Current
Resistors behave the same in both AC (Alternating Current) and DC (Direct Current) circuits. This means that the same calculations can be used to calculate power, resistance, current, and voltage in both AC and DC circuits.