The relay presents a learning challenge to the uninitiated. The first challenge involves mastering the difference between coil and contact including the distinction between normally open and normally closed. The second challenge involves an understanding of the dynamic nature of coil, contact, and the inductive nature of most loads.
The first learning challenge is solved using ladder logic. Students learn to appreciate the difference between coil and contact by constructing elementary logic gates. They also learn how to construct primitive relay-based memory circuits by constructing the 3-wire start stop circuit.
The learning challenge associated with relay dynamics requires considerably more study. Here the term dynamics is associated with both coil and contact. For example, the relay’s coil acts as an inductor. It takes time to build the magnetic field and time to dissipate the energy. The dynamics associated with dissipating the coil’s energy has a profound impact on the time it takes to open the relay.
The dynamics associated with the load adds another layer of complexity. For example, we know that AC currents are easier to interrupt than DC currents. The fact that the current passes though zero twice a cycle makes it easier to extinguish the arc. This is reflected in the dual specifications for a relay’s contacts such as 277 VAC but only 110 VDC. There is often an additional stipulation about inductive loads as an inductor can sustain an arc across opening contacts thereby reducing the life of the relay through contact pitting. Note that the previously mentioned speed of contact opening will impact relay life especially if the relay/contactor is used to drive large inductive loads.
Must-know facts about power relays
- Family relations: Most relays are offered as member of a family. The simplest relays are often manufactured with discrete coil voltages to meet your needs. Examples include DC coils with voltage such as 5, 12, 24, and 100 VDC or AC coils with voltages such as 24, 120, 240, 277 VAC. Industrial relays are often provided with DIN rail sockets and a host of accessories. For example, consider this Finder brand relay which includes a time delay option.
- Inductive kick: A relay’s coil develops a magnetic field that acts on the armature. By definition, this is a large inductor with energy stored in a magnetic field. Like all inductors, this magnetic field will cause a spike — often several hundred volts — when the coil is turned off. This flyback voltage must be addressed, especially if a BJT or MOSFET is used to control the relay. For smaller relays, a 1N4004 diode may be placed in placed in parallel with the relay’s coil. Larger relays driving inductive loads would benefit from a Zener or TVS diode.
Tech Tip: Don’t forget the flyback diode. I once made this mistake and had a very embarrassing presentation. The equipment clicked on twice and then failed in the closed position. Without the diode I had destroyed the drive transistor.
- Sinking vs Sourcing: The terms sinking and sourcing are often associated with relays, Programmable Logic Controllers (PLC), and associated sensors. The terms are used to describe how a load is powered.
- With a sourcing circuit the relay’s contact is placed between the positive connection of the power source and the load.
- With a sinking circuit the relay’s contact is placed in the return leg of the load.
Tech Tip: Many PLC families offer three different types of digital outputs including relays, semiconductor sourcing, and semiconductor sinking. A complete understanding of the terms sinking and sourcing is required to wire and troubleshoot the PLC-based circuits.
Looking forward to a continuing the conversation on this forum.
Best wishes,
Aaron Dahlen
LCDR USCG (Ret.), MSEE
DigiKey Applications Engineer
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About this 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.