How to Wire Series and Parallel Control Circuits

In this lesson we go through how to wire a series and parallel circuit and start to understand what voltage will be at each load depending on which you use.

You may not realized it but you've already wired series and parallel circuits in out Electrical Contacts - Normally open and normally closed contacts exercises.  Now we'll do some basic exercises to understand series and parallel circuits and when each should be used.  But first let's talk about each type of circuit.

  • Series circuits - Each device is wired in a chain so that all current must flow through each device.  


  • Parallel circuits - Each device is wired so that there are multiple paths that current can travel along.


  • Series/Parallel circuits - By far the most common is a combination or series and parallel circuits.  Below is a basic start stop motor circuit.


  • Voltage divider - There are many important principles that you will learn in an electricity class but the one that you absolutely must understand for series wiring in industrial environments is voltage dividers.  
    • Every device that current flow through, even wire, has resistance.  And as we learned in Understanding voltage (AC and DC), current, resistance, and power, if there is voltage and resistance, then there is power being consumed.  In cases of devices that we don't think of as consuming power such as switches, wire, etc the power is is in the form of heat loss.  While we will not consider these losses, "real loads" in series do consume power and the voltage is divided across them according to their resistance.
    • For a simple example which we will use for an exercise later, see image below.  We have two lights in series powered by 24VDC.  While not normally shown, each of those lights is a resistor which when voltage is applied consumes power which is released in the form of light.  However each light does not get 24VDC.  Given that the resistance is the same for each light, the voltage will be divided evenly between the two lights providing 12VDC to each light.  While you would not normally wire two lights in series like this, you would wire these lights in parallel, you do wire analog devices like this.  We will learn more about it in a later lesson.


Trainer Exercises

  1. Create a parallel circuit with Light 1, 2, 3, and 4 controlled by Selector 1.  Check the voltage across each light.  Is it 24VDC?  Explain why.
  2. Create a series circuit through Light 1, 2, 3, and 4 controlled by Selector 1.  Check the voltage across each light.  Is it 24VDC?  Explain why.

Next Steps

Go to the Control and Power Wiring Lesson series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.