Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: Sink or Source? Normally Open or Normally Closed?
Share
Notification Show More
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Basics > Sink or Source? Normally Open or Normally Closed?

Sink or Source? Normally Open or Normally Closed?

We discuss concepts of Sink or Source used in electrical circuits or in PLC wiring. Also we deal with basics of normally open contact or normally closed contact.

Last updated: November 2, 2023 2:44 pm
Editorial Staff
Basics Control Systems PLC Tutorials
4 Comments
Share
9 Min Read
SHARE

We discuss about concepts of Sink or Source used in electrical circuits or in PLC wiring. Also we deal with basics of normally open contact or normally closed contact which are used in relay wiring.

Contents
Sink or SourceExample CircuitNormally Open or Normally Closed

Sink or Source

When it comes to mastering the language of electronics, nothing causes more confusion than the “sink” or “source” question. This area causes problems for those trying to sort out machine electronics. When ordering parts, we often find ourselves unable to answer the question “do you need sink or source?”

Questions such as these often confront automation engineers and technicians:

If you replace a sinking input with a sourcing input, does it mean that everything that was “on” will now be “off”, and vice-versa?

Is “sinking with normally closed” the same as “sourcing with normally open”?

If your application call specifically for “sinking” or “sourcing”, does that mean you need pnp or npn or does it not matter?

You will be surprised to find out just how easy it is to understand this concept.

Take a look at the circuit in Figure 1. This is our most basic circuit. Even though it is a simple circuit, we can use this circuit to explain sinking and sourcing.

The circuit in Figure 1 shows a light bulb that is connected to a battery with two wires. DC (direct current) flows from the battery, through the light bulb, and back to the battery.

That is all it takes to light the bulb. Connect positive (+) to one side of the bulb, connect negative (-) to the other side of the bulb, and it turns on. This circuit contains the basic requirements of a digital system: a power source (the battery), a device that is turned on and off (the bulb) and wires to connect them together.

Example Circuit

Now we will add one more feature: suppose you want to turn the light on and off? You need to add a switch (or two?). Here is the big question:

Do you add the switch to wire A or do you add switch to wire B? Or do you add two switches?
Sink Current Example

Figure 2 shows two switches installed in the circuit. You probably already know that one switch will do the job. After all, you have been turning lights on and off all your life with just once switch.

So you probably also guessed that it would be foolish to use two switches where one will do the job. But which switch do you choose? Do you install a switch on Wire A as shown in Figure 3; or do you install a switch on Wire B as shown in Figure 4.

Drum roll please… the answer is: it doesn’t make any difference; either switch will work fine! Either switch will break the flow of current through the circuit.

Now here is the part that causes all of the confusion: we need a way to describe using switch 1 vs. switch 2. If you examine Figure 3 you will see that switch 1 is installed on the wire that runs from the negative (-) side of the battery to the light bulb.

You can see in Figure 4 that switch 2 is installed on the wire that runs from the positive (+) side of the battery to the light bulb.

Source Current Example

You can see that the final circuit, which contains just one switch (either figure 3 or 4), has two parts: there is an “inactive” part of the system and also an “active” part of the system. Figure 5 shows these two parts. The bulb, the battery and the wire that runs from the (+) side of the battery to the bulb are inactive.

These 3 items sit there all day long and never change. The switch however, is active. It has two states: it can be either ON or OFF. Sometimes people refer to this as OPEN or CLOSED; but those are just different words for the same thing.

Now consider what is happening from the switches point of view in Figure 5. As the switch is opened and closed it takes the negative (-) from the battery and connects or disconnects it with the bulb.

In Figure 4, the switch takes the positive (+) from the battery and connects it to the bulb. Now imagine that you want to tell someone how to wire a circuit. You need to tell them whether they should wire the circuit as shown in Figure 3 or as in Figure 4.

Imagine further that you have lots of people wiring lots of circuits and you need to give these instructions all day long. It’s too much of a mouthful to say “Wire one side of the bulb permanently to positive and install the switch so it connects and disconnects the negative from the battery to the bulb.” Besides, if you say that, you don’t sound very sophisticated.

So someone made up terms to shorten the instructions and help us to sound more sophisticated. And these terms are… SINK and SOURCE. I have no idea why they chose these terms.

They could have used any two terms: “Sink” and “Swim” would have worked just fine; or why not “Source” and “Resource”?

For all we care they can call these two different conditions Moe and Larry. The name is not important, what is important is that when you set up a DC system to turn something on and off, you will switch either the positive or the negative and leave the other permanently wired.

sink and source circuit example

Now, in order to communicate intelligently, all you need to know is… which is Sink and which is Source? Here is something that may surprise you: the terms sink and source are NOT standardized enough across the industry to be relied on.

As incredible as it sounds, when one person refers to what you may call “source” the other person may picture what you would call “sink”. So I recommend that you never use these terms.

Instead, you can review the wiring diagrams, or say something like “switch the negative” or “switch the positive”. Leave the terms “sink” and “source” for those who are so pressed for time that they want to optimize their speech while risking wiring the machine improperly.

So now you know. Next time you here the words “sink” or “source”, just ignore the terminology and focus on what you actually have: are you are switching the positive (+) or the negative (-)? That’s your deciding factor.

Normally Open or Normally Closed

The concept of “normally open” vs. “normally closed” is often confused with sink and source, but these two are not related. will be discussed in next article. Check Here.

If you liked this article, then please subscribe to our YouTube Channel for PLC and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

  • Relay Working Animation
  • Failsafe Wiring Practices
  • Electrical Schematics
  • PLC Ladder Diagrams
  • What is a Limit Switch?
Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Basics of Wellhead Control Panel (WHCP)
Basics of PLC Programming
PLC Conveyor Motor Ladder Logic
PLC Program for Alternate Output Circuit with Latched Function
Single Element Drum Level Control System
DCS versus PLC Architecture
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
4 Comments
  • RSK says:
    July 25, 2016 at 5:17 am

    Nice & Simple 🙂

    Reply
  • Nikhil More says:
    September 29, 2016 at 7:09 am

    Very good example to understand the concept

    Reply
  • Debabrata Tripathy says:
    October 3, 2018 at 9:50 pm

    Perfectly explained. Any one can understand the concept after reading it.Thank you sir.

    Reply
  • junaid khalid says:
    January 8, 2020 at 7:09 pm

    so finally which is sink?
    switch on the negative?

    Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Two Element Drum Level Control System
Energize to Safe Loop philosophy
Studio 5000: Programming Three Motors with Fixed Priorities
Troubleshooting a 4-20mA Current loop
Compare Modbus, Fieldbus, and Profibus
Configuration of Control Valve in Studio 5000
Proportional Integral Derivative Controllers – PID MCQ
Turbine Bypass System

Keep Learning

Simple Conveyor Control PLC Program

XG5000 Example of Simple Conveyor Control PLC Program

Instrumentation Control Systems

What is Instrumentation Control System ?

How to Prevent Ground Loops in Analog Circuits?

How to Prevent Ground Loops in Analog Circuits?

PLC FBD Programming for Burglar Alarm Security System

PLC FBD Programming for Burglar Alarm Security System

Function of Racks in PLC System

Function of Racks in PLC – Types of SIEMENS S7-400 PLC Racks

Pressure Control Loop

Pressure Control Loop Wiring Connections

Exhaust Fan Control

Exhaust Fan Control: Example of PLC Timer Programming

Safety in Automation Systems

Safety in Automation Systems

Learn More

Deep-Bed Filter

Deep-Bed Filters

What is a Wet Contact in PLC

What is a Wet Contact? – Basics of PLC Wiring

Configure the Instruments in Flow Control Loop

Configure the Instruments in Flow Control Loop

Dew Point in Instrument Air

Dew Point in Instrument Air

Digital Electronics Multiple Choice Questions

Parity Generators Objective Questions

State Variable Analysis and Design

State Models for Linear Continuous Time Systems

Flow meters Tube Fittings

Sanitary Pipe Fittings for the Instruments

Best Online Courses to Learn Electrical Engineering

Best Online Courses to Learn Electrical Engineering

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?