# Selection Guide for Power Relays Over 2 A

## What is a power relay?

Power relays over 2 Amps are electromechanical devices that turn a large load on or off through the use of a smaller control signal. The category includes a variety of relays ranging from small PCB mount as well as DIN rail mounted products suitable for industrial control applications.

## An engineer’s perspective

Click to expand

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

## Introduction to DigiKey’s Product Selection Guide

This page is one of many in DigiKey’s Product Selection Guide (PSG). This particular page is focused on power relays, over 2 Amps. It provides a description of the individual parameters used to characterize the part. This information will allow you to better understand and navigate DigiKey’s parametric search engine. Click here for a case study showing how to use the search tools.

## How are power relays categorized?

The DigiKey search tools include the following specification for power relays.

• Mounting Type: Indicates how the device is physically attached.

• Coil Voltage: The nominal voltage that must be applied to a relay’s control coil to actuate the device. Observe that the coil voltage is specified as either AC or DC. This distinction is paramount as a coil designed for DC operation will not activate when an AC signal is applied to the coil.

• Contact Form: Describes the contact closures provided by a relay or relay-output device, in terms of the number of independent circuits that are actuated in parallel, the number of contact positions available for each, and their default states. For more information, please see this article introducing concepts of poles and throws.

• Contact Rating (Current): The amount of current that a relay’s contacts are rated to interrupt when switching a resistive load. Ratings may differ for reactive loads and when switching AC or DC current.

• Switching Voltage: The voltage that a relay’s output contacts are rated to interrupt.

• Coil Current: The nominal current flow through a relay’s control coil when the indicated coil voltage is applied.

• Coil Type: Whether the coil is latching or non-latching, single or dual coil.

• Features: These are different capabilities or properties of the device such as a diode, lighted indicator, or test button.

• Termination Style: Selection of termination style used to connect the device to a system, such as PC pins, socketable, or wire leads.

• Seal Rating: Defines what type of sealing is on the relay.

• Coil Insulation: The class of insulation the relay is built to meet. For more information, please see transformer insulation class.

• Must Operate Voltage: The minimum voltage needed to energize the coil.

• Must Release Voltage: The maximum applied voltage at which a relay must return to an inactive state.

• Operate Time: Characterizes the time delay between the application of rated coil voltage and the closure (or opening) of all contacts that occurs as a result. Typically, does not include contact bounce time.

• Release Time: Characterizes the time delay between the removal of power from the device coil and all contacts returning to their rest positions. Does not include bounce time, and may be affected by the choice of control circuitry.

• Operating Temperature: Recommended operating temperature, typically given in a range or as a maximum. Exceeding these temperatures may affect performance or damage the device and other system components.

• Contact Material: The material used for the electrical contacts on the relay.

• Relay Type: Indicates a relay’s internal operating mechanism, typical usage, or other distinguishing feature.

## Schematic symbol

Schematic symbols courtesy of Scheme-it.