Relays are often misunderstood because we confuse coil ratings with contact ratings. This article explains the difference between AC and DC relay coils and shows how relay contact ratings change with voltage, current type, and load type.
Key Takeaways
- Coil ratings and contact ratings are not the same thing.
- Relay coils are designed for either AC or DC operation.
- Contact ratings change with voltage, current type, and load type.
- Inductive and DC loads are harder on contacts than resistive AC loads.
- Always size the relay for the real load, not just the nominal current.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Relay Logic
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 13 Mar 2026
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 shading 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 Load Voltage and 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, AC is easier to accommodate than DC. 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.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.