 Electricity Explained: Types of Electrical Circuits
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We’re going to take a look at the three basic types of automobile circuits: series, parallel and series-parallel and how volts, amps and ohms react in each one. In my February 2018 article, I explained electricity and the relationship between volts, amps and ohms, then followed it up in April with how to use a digital multimeter (DMM) to measure electrical properties. This month, we’re going to take a look at the three basic types of automobile circuits: series, parallel and series-parallel and how volts, amps and ohms react in each one.

Series Circuits
In a series circuit, the current flows from the power source, through a fuse and switch, through one load and then another and so on with only one path for current to flow to ground. If there is a break anywhere in a series circuit, no loads will operate since there is only one path for current to flow.

A series circuit has three rules:

1. The total circuit resistance is the sum of the resistance of all the loads. If a series circuit had a 4-ohm (R1) and a 2-ohm (R2) load, the total circuit resistance would be 6 ohms.

2. Current flow is same throughout the circuit. In a 12-volt system this circuit would pass 2 amps of current. Using Ohm’s law to calculate current flow, we would divide 12 (volts) by 6 (ohms) and get 2 amps. Regardless of where you place your amp meter, in this example, you would measure 2 amps.

3. The sum of the voltage drop across each load will equal source voltage. If you calculate the voltage drop across each load using Ohm’s law, the voltage drop across the 4-ohm load would be 8 volts and the voltage drop across the 2-ohm load would be 4 volts; 8 volts + 4 volts = 12 volts, equaling source voltage.

Parallel Circuits
In a parallel circuit, current flows from the power source, through a fuse and switch, then splits into two or more different branches that each contain a load, giving the current multiple paths to flow to ground. Because each branch gets source voltage and ground, if there is a break in the circuit on one branch, the other will still function as normal.

A parallel circuit also has three rules:

1. The total circuit amperage is the sum of all the branches. In our example, the branch with the 4-ohm load (B1) would pass 3 amps (12 volts divided by 4 ohms) and the branch with the 2-ohm load (B2) would pass 6 amps (12 volts divided by 2 ohms). Add the amperage of both branches together and you get the total circuit amperage of 9 amps.

2. Each branch receives source voltage. If we were to check the voltage going into each branch in our example, we would measure 12 volts. In addition, the voltage drop across each branch always equals source voltage. To calculate this using Ohm’s law, for branch B1, 3 amps multiplied by 4 ohms equals 12 volts. For B2, 6 amps multiplied by 2 ohms equals 12 volts.

3. Total circuit resistance is less than the smallest resistor. In our example we have 12 source volts and 9 amps of total circuit current. Using Ohm’s law to calculate circuit resistance we would divide 12 (volts) by 9 (amps) and get 1.3 ohms, which is of course less than the smallest resistor of 2 ohms. Keep in mind in a parallel circuit, current has more than one path to flow so resistance to current flow drops. Think about a boat with a hole in it; water would come in. If you added a second hole, the boat’s resistance to keeping water out would drop, allowing more water to flow. In a parallel circuit, every time you add another branch circuit resistance would go down and circuit amperage would go up. This is why we just can’t add an additional accessory load to any given circuit in a car.

Series-Parallel Circuits
In a series-parallel circuit, current flows from the power source, through a fuse and switch, through a series load and then splits off into two or more parallel branches that each contain a load. If there is a break in the series portion of the circuit, the entire circuit will not work. If there is a break in one of the parallel branches, only that portion of the circuit will not work. Since these circuits are a combination of both series and parallel, they have characteristics of each.

A series-parallel circuit is not as common as the other two, but you will encounter them from time to time so it is important to be familiar with them. An example of this would be the dash lights. All the bulbs are wired in parallel but the current first travels through a variable resistor that the driver can adjust to drop the voltage to the bulbs, making the dash lighting dimmer or brighter.

Even though it may seem complicated at first, to calculate the relationship between volts, amps and ohms in a series-parallel circuit, it is easiest to divide the circuit up into series and parallel as if it were two separate circuits.
In our example we have one resistor (R1) in the series portion of the circuit and two branches (B1 and B2) in the parallel portion of the circuit, each with one resistor (R2 and R3).

To calculate the total circuit resistance, first calculate the resistance of the series portion of the circuit. In this circuit it is 2 ohms since it is the only resistor (R1), but if there were more than one, follow the first rule of a series circuit.

Next, calculate the resistance of the parallel portion of the circuit.

Finally, calculate total circuit resistance by adding the two together.

Total circuit current can now be calculated using source voltage, total circuit resistance and Ohm’s law. The current flow will be the same across the series and parallel portions of this circuit (however different across the parallel branches).

Voltage drop can be calculated by using Ohm’s law, the total circuit current and the resistance of each portion of the circuit.

Voltage drop across each leg of the parallel portion will be the same since we already know that the available voltage to each leg of a parallel circuit is the same. In this example, the available voltage at B1 and B2 is 4.719V, allowing us to finally calculate the current flow across each branch.