Relays are electromechanical components with specifications for both coil and contact. These specifications are often misunderstood as there are several closely related parameters that are easy to misinterpret. The root of the confusion may be attributed to a misunderstanding of the relay itself.
A classic error is to conflate the contact specifications with the coil specifications.
AC vs DC coil specifications
Let’s start with the simple coil specifications. The coil of a relay is designed for either AC or DC operation. This is a binary specification relating to the physical construction of the coil as shown in Figure 1. The DC coil (blue) is wound on a solid cylindrical core. The AC relay (orange) has laminations and a shade pole winding which appears as the letter D.
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A relay designed for AC may physically operate with DC. We do not recommend this operation as there may be system level considerations as explored in the clarification subsection.
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A relay coil designed for DC operation will not work with an AC. You can try, but all that will happen is the relay will buzz with no motion on the armature. Recall that a DC powered electromagnetic will produce a steady magnetic field while the AC field is time-varying. We can change the properties of the AC field by adding a shade pole winding. As seen in Figure 1, this winding looks like the letter D.
Figure 1: Comparison of a DC relay (top) with an AC relay (bottom). The DC coil is wound on a solid cylindrical core. The AC relay has laminations and a shade pole winding which appears as the letter D.
Clarification
You should always select a relay with a coil voltage designed for the DC or AC in your system. This will ensure the relay operates as expected. For example, the relay must hold when subject to mechanical vibration in mobile equipment or when subject to vibrating machinery such as an industrial air compressor.
Contact specifications depend on voltage as well as AC vs DC
The contact specifications aren’t nearly as straightforward as the coil specifications. The reason for this complication is that not all loads are equal from the perspective of the relay’s contacts.
We recognize that inductive loads such as motors are harder to open than resistive loads as the inductor’s flyback energy tends to sustain the arc. Consequently, we often find relays with multiple contact voltage specifications. Here are the specifications for a representative relay:
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110 VAC, 10 A resistive / 7.5 A inductive
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220 VAC, 7.5 A resistive / 5 A inductive
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30 VDC, 10 A resistive / 5 A inductive
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Specification may be further clarified to account for the power factor of the inductive load and the L/R time constant.
These specifications capture the physics involved in opening a switch under load. Recall that an electrical arc forms when the contacts are in the process of opening. This arc is a function of the voltage, type of load, and the type of current. Lower voltages are easier to accommodate. Also, DC is easier to accommodate than AC. This is represented graphically in Figure 2.
Figure 2: This sketch demonstrates the arc extinguishing relationships between voltage, inductance, and type of current.
Tech Tip: It very important for relay contacts to open and quickly thereby extinguish the arc. Prolonged arcing can quickly destroy the relay’s contacts. The contacts can overheat. In extreme cases the load will not be disconnected when the relay’s contacts open. This is a risk to the equipment as well as the operator.
For reference, here is a picture of a contactor rated for 10 A in 250 VDC system. You can see that it considerably larger than the previous relay.
Figure 3: Image of a large contactor suitable for breaking a 10 A load in a 250 VDC system.
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Best Wishes,
APDahlen
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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.