Lesson : Parallel Circuits

In the previous lessons, we’ve looked at series circuits with resistors. Series circuits acts differently than a parallel circuits and we will now look at how a parallel circuits behave. The parallel circuit is represent below. In this case, we have only two resistors but you could have a huge amount of resistors in parallel if you want. In a parallel circuit, the voltage across the resistors is the same for all resistors but the current is divide among the two resistors. The current is not necessarily divide equally as we will see later. In our case, the voltage for R1 and R2 is going to be 12V. The current is split between the two resistors (I1 and I2). The total current (Itot) of the circuit will be equal to the sum of I1 and I2. Parallel circuit are easier to solve since we only have to apply Ohm’s law to all the resistors to find either the voltage, currents or resistances.

Figure 1 : Parallel Circuit Example

We will now solve the example above. For this example: R1 = 100 Ω and R2 = 150 Ω. The voltage source is 12V. We will calculate the current I1, I2 and Itot. After that, we will calculate the equivalent circuit.

Using Ohm’s law, we can calculate both I1 and I:

I=\cfrac{V}{R}

I_{1}=\cfrac{12V}{100\Omega}=0.12A=120mA

I_{2}=\cfrac{12V}{150\Omega}=0.08A=80mA

We can now calculate Itot :

I=I_{1}+I_{2}

I_{tot}=0.12A+0.08A=200mA

Like series circuit, we can simplify the circuit to an equivalent circuit with only one resistor. For parallel circuit, it is rarely used to solve problem since it doesn’t give us a lot of information. It could be used to find the voltage if we only had the total current and the values of the resistors. This would allow us to find the voltage applied to both resistors then you will be able to calculate the current flowing into both resistors (I1 and I2).

Figure 2 : Equivalent circuit for a parallel circuit

The equation for the resistor Req of the equivalent circuit is :

Req=\cfrac{1}{\cfrac{1}{R1}+\cfrac{1}{R2}}

If we reuse the example above, we would have :

Req=\cfrac{1}{\cfrac{1}{100\Omega}+\cfrac{1}{150\Omega}}=60\Omega

To verify if we have the appropriate value for Req, we can calculate Itot of the equivalent circuit and we should get the same value that we have calculated before.

\cfrac{V}{R}=I_{tot}=\cfrac{12V}{60\Omega} = 0.2A = 200mA

We obtained the same value as before so we know our equivalent circuit is correct. If we had more than two resistors, the generic formula to calculate Req would be:

Req=\cfrac{1}{\cfrac{1}{R1}+\cfrac{1}{R2}+\cfrac{1}{Rn}+\dots}

The next lesson will be dedicated to multiples examples of series and parallel circuit. We will review Ohm’s law and Power law at the same time.