The term Normally Closed (N.C.) identifies the relay contact(s) that are closed when the relay is deenergized. This is the resting position for the relay with spring tension holding the N.C. contacts closed. The term Normally Open (N.O.) identifies the relay contacts(s) that have continuity when the relay coil is energized. The relay’s armature holds the contacts in position.
Many articles have been written about relays specifically focusing on the difference between N.O. and N.C. contacts. These are informative, but often too abstract. Students often have little to no intuitive understanding of the difference. The real learning takes place later when the students make mistakes connecting the relay circuits in the lab.
In this article, we will describe a simple experiment that highlights the difference between the types of relay contacts. These hands-on tasks provide immediate feedback allowing you to see and hear the relay operation. The lessons are directly applicable to continued study of relays including state machines.
The article assumes that you have some familiarity with switches and understand the differences between devices such as DPDT and SPST.
Required parts for the experiment
The experiment requires three components including a relay, two pushbuttons, and a 24 VDC power supply. In this example, we will use the parts as shown in Figure 1:
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2 each snap action switch EAO manufactured 704.900.1 contact blocks. Other switches may be used. However, the chosen part is a good way to familiarize the student with common industrial control components.
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1 each relay assembly. Note that the assembly in Figure 1 includes a Finder relay, socket, and diode module
Other relay types could be substituted. However, be sure to select a relay with the ability to be mechanically forced. Relays such as the Finder 46 series featured in Figure 2, have a physical forcing mechanism. This mechanical “switch” may be physically pushed to place relay contacts into the “energized” positions. The mechanism may also be rotated, provided the pip is removed.
In addition to the previously mentioned components, you may also want to include a N.C. switch such as the 704.900.2 and a 470 uF 50 VDC capacitor. The purpose of these additional components is suggested in the additional learning question located at the end of this article.
Tech Tip: Mechanically forcing a relay or contactor is generally considered bad practice. It is tempting to do so while troubleshooting. However, unexpected machine action can occur. This can damage equipment or harm personnel. If you are a PLC programmer, you may want to monitor the contacts and immediately enter a fault state if the device is forced.
Figure 1: Lab setup to explore the relay’s normally closed contacts.
Figure 2: The Finder (46 series) relay may be locked into position by first removing the plastic pip.
Schematic
The experiment is conducted in two phases as shown in the Figure 3 schematic. On the left is the N.O. configuration while the N.C. configuration is on the right. Observe that there is a single wire connection difference between the two configurations. We see that that the wire is moved from the N.C. (pin 12) to the N.O. (pin 14).
Figure 3: Schematic for the normally open and the normally closed relay experiment.
Circuit construction
The circuit construction is shown in Figure 1. Here are a few tips:
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Use of precut wires with ferrule terminations is helpful.
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Use of twin-wire ferrules may be helpful
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The contacts are clearly labeled for the featured relay and socket. The common contact is located the socket’s lower deck. The N.O. is on the middle and N.C. is on the upper deck.
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Watch out for coil polarity as the relay will only activate when the power supply positive terminal is connected to terminal A1.
Experiment results
Sorry – that’s not the point of the hands-on learning identified in this article. Instead of presenting the results we will seek deeper learning by researching a series of guided questions.
Basic questions and experiment observations
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Classify the pushbutton snap action switch as either SPST or DPST.
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Classify the relay as SPDT or DPDT.
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Describe the operation of the relay when the coil is wired in series with the N.C. contact. Be specific by describing the interaction associated with switches A and B.
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Describe the operation of the relay when the coil is wired series with the N.O. contact. Be specific by describing the interaction associated with switches A and B.
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What is the difference between the “B” switch and mechanically forcing the relay?
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Research and define the term “latch” as it relates to a relay.
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Research and define the term “vibrator (electronic)” as it relates to a relay.
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Research the operation of a tattoo gun and describe how it relates to these experiments.
Advanced questions and areas for further research
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Research the term “3-wire motor starter.” Using this configuration, modify the N.O. circuit, label the switches, and identify each of the three wires. You may want to swap a N.C. for a N.O. pushbutton.
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Research the operation of a “trembler coil” as found in a Ford Model T. Describe how it relates to these experiments.
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Locate the DigiKey part number for a DPDT switch element to replace SPDT the EAO mechanism.
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For an additional challenge, return to the N.C. experiment and add a 100 uF 35 VDC capacitor in parallel with the coil. Describe the results and speculate on the cause of the change. Do watch for polarity as a mistake will result in rapid disassembly (explosion) of the capacitor.
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The relay described in this document may be mechanically forced and then locked into position for testing purposes. Describe what could go wrong?
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As a maintenance supervisor, would you recommend removal of the relay’s locking pip? Would you consider such a relay to be damaged and in need of replacement? Why?
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Suppose a technician inadvertently locked a relay into a forced condition resulting in 1 hour of plant down time. Speculate on the total cost of this mistake.
Conclusion
The hands-on activities described in this article are very simple. Yet, the implications are profound. They lay the foundation for your understanding of industrial control system.
Now, go build the circuit to discover the deeper meaning and implications of a relay’s N.O. and N.C. contacts.
Best Wishes,
APDahlen
About the author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey
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