Relays

Publish Date: Sep 19, 2014 | 20 Ratings | 3.35 out of 5 | Print

Overview

This document is part of a comprehensive tutorial on industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.

For more information return to the Complete Industrial Digital I/O and Counter/Timer Tutorial

Table of Contents

  1. Introduction to Relays
  2. Recommended NI Products that Connect to Relays
  3. Technical Overview

1. Introduction to Relays

Relays are simple devices which are made up of electromagnet and a set of contacts. They come in a variety of form factors, styles, and technologies. Depending on your application, only one relay type may be suitable In other cases multiple relay types may be appropriate.

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2. Recommended NI Products that Connect to Relays


NI 6509
NI 6514
NI 6515
NI 6528

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3. Technical Overview


The most common types of relays used in ATE applications are:

· Electromechanical Relays
· Reed Relays
· Solid State Relays (SSRs)
· FET Switches


1. Electromechanical relays:

They are perhaps the most widely used relays in ATE applications today. They are made of a coil, an armature mechanism, and electrical contacts. When the coil is energized, the induced magnetic field moves the armature that opens or closes the contacts. See Figure 1.



Figure 1: Electromechanical relay: Current through the coil creates a magnetic field that moves the armature between the contacts


2. Reed Relays:

Reed relays, like electromechanical relays, have physical contacts that are mechanically actuated to open/close a path. For reed relays, however, the contacts are much smaller and lower mass than those used in electromechanical relays. Dry reed relays are made of coils wrapped around reed switches. The reed switch is composed of two overlapping ferromagnetic blades (called reeds) hermetically sealed within a glass capsule that is filled with an inert gas. The reeds have contacts on their overlapping ends. When the coil is energized, the two reeds are drawn together such that their contacts complete a path through the relay. When the coil is de-energized, the spring force in the reeds pulls the contacts apart. See Figure 2.


Figure 2: Dry reed relay: Current through the coil creates a magnetic field which draws the two reed contacts together.

3. Solid State Relays:

SSRs are constructed using a photo-sensitive MOSFET device with an LED to actuate the device. See Figure 3.



Figure 3: Solid State Relay (SSR): Light from the encapsulated LED actuates the photo-sensitive MOSFET and allows current to flow through it.


4. FET Switches

Like SSRs, FET switches are not mechanical devices. FET switches use a series of CMOS transistors to implement the switching. Unlike SSRs, the control circuitry drives the gates of the transistors directly instead of driving an LED. Direct drive of the transistor gate allows for much faster switching speeds because the power-on power-off time of the LED is not an issue. In general, FET switches are the fastest of the switches discussed here. Also, because there are no mechanical parts or LEDs in the packaging, FET switches can be very small. One major drawback of the FET switch, however, is that it lacks a physical isolation barrier and thus may only be used with low-voltage signals.

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