Relays (Mod.3 Sect.3 Part 2) 1 of 2

086021 19 (binary/octet-stream)

What is a relay? (Clapper Relays)

A relay is an electrically operated switch. The construction of one type of relay called a clapper relay is shown in Figure 19. In this relay, a coil surrounds a soft iron core. When current is applied to the coil, the core becomes magnetized. The soft iron armature is attracted by the magnetized core and moves toward the core. (The armature is mounted so that it can move on the relay frame.) As the armature moves downward, the armature contacts touch. The armature contacts act like switch contacts in that when they come together, the circuit connected to the terminals x and y is completed. If current stops flowing through the coil, the core becomes demagnetized. The spring then pulls the armature away from the core, and the contacts are separated. That opens (breaks) the circuit.A relay is an electrically operated switch. The construction of one type of relay called a clapper relay is shown in Figure 19. In this relay, a coil surrounds a soft iron core. When current is applied to the coil, the core becomes magnetized. The soft iron armature is attracted by the magnetized core and moves toward the core. (The armature is mounted so that it can move on the relay frame.) As the armature moves downward, the armature contacts touch. The armature contacts act like switch contacts in that when they come together, the circuit connected to the terminals x and y is completed. If current stops flowing through the coil, the core becomes demagnetized. The spring then pulls the armature away from the core, and the contacts are separated. That opens (breaks) the circuit.

Caption: : The armature of a relay is mounted so it can move on the relay frame. When current is applied to the coil, the core becomes magnetized, attracting the armature toward it. When the armature moves downward, the contacts touch and close the circuit.

Clapper Relay

086021 20 (binary/octet-stream)

Caption: : A clapper-type relay is shown here.

086021 21 (binary/octet-stream)

Normally Closed/Normally Open Relay Circuits

Caption: : A basic relay circuit is shown here. In Figure A, the switch (SW) is open and the relay is in its de-energized condition. Figure B shows the same circuit with the switch (SW) closed and the relay is in its energized condition.

The relay in this circuit has normally closed contacts when the relay is de-energized. In Figure A, the switch (SW) is open and the relay is in its de-energized condition. Note that the lamp circuit is connected through the normally closed contacts, so Lamp 2 is on when the relay is de-energized. Figure B shows the same circuit with the switch (SW) closed. Current is now flowing through the coil, and the moveable contact (the contact connected to the armature) has switched to its position when recoil is energized. In this relay position, Lamp 1 is turned on and Lamp 2 is turned off.

086021 26 (binary/octet-stream)

AC Relay

Figure A shows a standard bar magnet. In a magnet, the magnetic lines of force (flux lines) leave the north pole of the magnet, make a complete path around the magnet, and return into the south pole of the magnet. This basic magnetic principle can be used to operate the clapper-type relay shown in Figure B.

086021 28 (binary/octet-stream)

Contact Forms

Caption: : Each of the contact forms shown here is identified by a letter. For example, the form K contact is labeled “SPDTNO” which means single-pole, double-throw, normally open

A relay’s contact arrangement is called its contact form. When many contacts are moved at the same time, the contact arrangement is called the pileup. Some standard contact forms are shown in a chart in this figure. These contact forms are established by the American Standards Association (ASA). For each of the contact forms shown in this chart, the movable contact moves down when the relay is energized. Note that each of the different contact forms is identified with a letter.

086021 29 (binary/octet-stream)

Although many relays are represented in schematic drawings by contact symbols, some industrial electronics applications use a different set of symbols. This figure shows these alternate symbols. These symbols are used by the International Electromechanical Commission (IEC), the Joint Interagency Council (JIC), and the National Machine Tool Builders Association (NMTBA). These symbols are also used in most telephone circuit diagrams.

Caption: : These relay symbols are used instead of contact symbols in many industrial electronics applications.

Contact Forms #2

086021 32 (binary/octet-stream)

Telephone Relays

The telephone relay shown in Figure A can switch a large number of contacts. That is, this relay can have a large pileup. This particular relay design can also have contacts added as needed. Figure B shows a simplified drawing of the relay’s operation. When the armature is attracted by the magnetic field of the core, the armature pushes up the moving contact springs, and the contact connections are changed. The dropout voltage is changed by varying the distance of the gap between the armature and the core with an adjustable screw. When the unit operates, the screw hits the stationary core to limit the distance of the armature’s movement.

Caption: : This figure illustrates the operation of a telephone relay.

Reed Relay or Switch

086021 33 (binary/octet-stream)

Caption: : Figure A shows a reed relay. Figure B is a cross-sectional view of the relay. Figure C shows how the relay operates.

Another important type of relay is the reed relay. The small size and low power requirements of the reed relay have made it popular in modern circuits. The relay is composed of small magnetic metal strips (reeds) enclosed in a glass envelope (Figure A). The coils in a reed relay either surround the glass envelope, or are wound on an auxiliary hollow iron core structure to provide the magnetic field required for relay operation. This is an ideal arrangement, because the magnetic field of a coil is concentrated in its center. Since the reeds are made of a magnetic material, they can be operated by a permanent magnet. In that case, they’re called reed switches. A cross-sectional view of a reed relay is shown in Figure B. Figure C shows how the relay operates. When a permanent magnet with the polarity shown is put next to the tube, the reeds become magnetized and attract each other, closing the switch.

086021 34 (binary/octet-stream)

Caption: : The basic parts of a stepping relay are shown here.

Stepping Relays

Stepping relays (also called stepping switches) have many switch positions. The stepping relay is actually a rotating switch that’s used to establish connections between the wiper blade and the contacts, one step at a time. Each input pulse causes the relay to step to the next position. Stepping relays are widely used in control circuits, particularly in signaling and counting circuits. When the coil is energized, the armature is attracted, overcoming the pull of the return spring. This pushes the pawl against a tooth on the ratchet. When the ratchet turns, it moves the contact or wiper blade to a new switch position. Note that the ratchet in the illustrated device can only turn clockwise. It holds each position until the next pulse arrives.