Parallel Circuits Explained
Yes sir, taking another shot at keeping on with the basic electrical training. A few weeks ago we started this section with series circuits and I mentioned that we’d eventually put a post up about parallel circuits. This is it baby.
Hopefully when you get to the bottom of the page, you will be able to:
- Recognize parallel circuits.
- Be able to define what a parallel circuit is.
- Be able to solve for values in parallel circuits.
There are advantages and disadvantages to both parallel circuits and series circuits. We are not going to get into that here but didn’t want you getting confused between the two already. At some point after this post, probably in another 2 months ha ha, we will go over the last type of circuit that I don’t really know about, a combination of the two, a series-parallel circuit.
Parallel circuits have unique characteristics. Because of them, parallel circuits are the most widely used type of circuit. Power distribution in large cities is sometimes accomplished through parallel circuits, where feeder lines that deliver the power are connected in parallel to each other.
What Are Parallel Circuits?
Great question Steve, great question. I have no idea so let’s look it up. I went to two places, one had a simple definition, the other had like an entire website. So we will try to summarize what exactly parallel circuits are. So you don’t have to. I’m cool like that.
Here is the simple definition of a parallel circuit:
- A parallel circuit is a closed circuit in which the current divides into two or more paths before recombining to complete the circuit.
Each load connected in a separate path receives the full circuit voltage, and the total circuit current is equal to the sum of the individual branch currents.
And here is a somewhat longer definition, kind of copied, pasted, and altered:
- In a parallel circuit, each device is placed in its own separate branch. The presence of branch lines means that there are multiple pathways by which a charge can traverse the external circuit.
When a charge hits the branching location, it makes a choice. They always want to go towards the terminal with the least potential.
Since there are multiple routes which a charge can take, adding another resistor in a separate branch provides another path by which to direct charge through the main area of resistance within the circuit. This decreased resistance resulting from increasing the number of branches will have the effect of increasing the rate at which charge flows.
This is not my example but it’s a good one and was taken from the physics site. Think of parallel circuits like tollbooths on the freeway.
A tollbooth can introduce resistance to car flow. If we were to add morel tollbooths on the same freeway, all of the cars wouldn’t have to pass through the same tollbooth. More tollbooths means more flow. These extra tollbooths decrease the overall resistance to car flow and increase the rate at which they can pass through.
Voltage in Parallel Circuits
Using the above drawing as reference, you can see that each resistor is placed directly across the main source of voltage. This means that each device runs at the same voltage as the source. A device should never be installed into a parallel circuit if its voltage rating is lower than that of the source voltage.
For example, don’t install a 120V ballast in a 277V circuit.
The following equation shows us that in parallel circuits:
- All devices operate at the same voltage.
The equation is as follows:
- ET = E1 = E2 = E3
Current in Parallel Circuits
In parallel circuits, the devices operate independently of each other. Each device will “take on” as much current as it can, in line with its resistance.
The flow paths of current is equal to the number of devices in parallel. This would be the same as how many tollbooths we have. If we have 10 lanes, we have 10 flow paths, same with parallel circuits.
- The total current in a parallel circuit is equal to the individual currents in each device.
The equation for this is:
- IT = I1 + I2 + I3
Resistance in Parallel Circuits
If we again look at the equation for current just above here, we can tell that if we add more parallel branch lines, or installed devices to the parallel circuit, the total current for our circuit will increase.
Since total current has increased, and the source voltage has remained constant, Ohm’s Law tell us that the total resistance in parallel circuits must decrease. Again:
- An increase in parallel branches means a decrease in total resistance.
Parallel Circuits – Unequal Resistors
In parallel circuits with devices that have equal resistance, the total resistance of the circuit is equal to the resistance value of one device divided by the number of devices connected in parallel. Confused? Good. You should be, I am. But it makes sense using the above picture and following equation.
- RT = R/N
RT = Resistance, total
R = Resistance of one device
N = Number of parallel devices or resistors
For the drawing above then:
- RT = R/N
- RT = 12/3
- RT = 4 ohms
Parallel Circuits – Equal Resistors
More often, you will see parallel circuits with devices installed that have unequal resistance values. This is because most motors are different for example, they will have different resistance values. Remember that each different resistor will use a different value of current for the same amount of source voltage.
In order to find the total resistance of a parallel circuit, you may have to apply a known source voltage and then determine the total current. Now we use Ohm’s Law again.
- RT = ET/IT
RT = Resistance, total
ET = Applied voltage
IT = Current, total
Or, you can use this equation:
- 1/RT = 1/R1 + 1/R2 + 1/R3
RT = Resistance, total
R1 = device 1 resistance
R2 = device 2 resistance
R3 = device 3 resistance
Now, let’s go back up to the voltage section and use that drawing of a parallel circuit with uneven resistors. Let’s use those values:
- 1/RT = 1/3 + 1/6 + 1/9
To add the fractions, we need a common denominator, for this it looks like that would be 18, so:
- 1/RT = 6/18 + 3/18 + 2/18
- 1/RT = 11/18
- RT = 18/11 or 1.63 ohms.
Basically you should now be able to lead a seminar on parallel circuits. Not really. You now know as much as I do, which isn’t even the tip of the iceberg.
In this post on parallel circuits, we went over the definition of a parallel circuit as well as how to recognize when you are looking at, or operating, an electrical distribution system that uses parallel circuits. Then finally there were some very basic examples of parallel circuits that you had to understand in order to solve for the different values like resistance or voltage.
Parallel circuits, for me at least, aren’t as straightforward as series circuits but they aren’t all that difficult either. Especially if you remember how to apply Ohm’s Law in each case.
If you have any questions on parallel circuits, this post, women, life, or anything else, feel free to hit me up or contact me through the site. Have a good one and until next time my friends.
If You Want To Learn More
Ain’t going to lie and say this even close to everything you need to know about parallel circuits. There is no way I know enough to make myself comfortable let alone visitors and readers of the site.
If you do have a thirst for more learning and knowledge, you may want to check out this book over at Amazon. It is called Basic Electricity and it was put together by the U.S. Navy to train their electricians. If you read the reviews on Amazon it gets pretty good marks if you can get past the Navy typos.
It is basic, thorough, simplified for people like me, and inexpensive. If you are really looking for more knowledge on parallel circuits or even basic electricity, you may want to give a good look at that book.