Automotive Relays
Description and Function
The relay is an electro magnetically operated switch, where with an low level input current typically in the range between 100 mA and 150 mA, can be switched an high level current up to 80 A, in some cases and more. When the input current flows through the copper coil, the magnetic field is generated and the hinged soft iron plate is fast attracted, which in turn is mechanically connected to the one movable contact of the switch. The other contact of the switch is non movable, which is in very short distance near to. Depending of the relay type, the switch contacts can be normally open or normally closed. The number of poles refers to the number of switches, so a single pole relay has one switch.
On figure 1 is shown a typical single pole normally open relay, where the contacts are normally open when the relay is not activated (OFF), i.e. coil is not energized. When the relay is activated (ON), i.e. coil is energized, in that case the contacts are closed (connected 8 and 9), so the relay switch is switched ON.
Figure 1. Single pole normally open relay:
1. Housing
2. Pole piece
3. Return spring
4. Copper coil
5. Hinge
6. Flexible copper braid
7. Soft iron core
8. Movable electrical contact
9. Non movable electrical contact
On figure 2 is shown a typical single pole normally open tied pin relay. When the relay is not activated (OFF) the contacts are normally open. When the relay is activated (ON), in that case the contacts are closed, i.e. the relay is switched ON.
Figure 2. Single pole normally open tied pin relay:
1. Housing
2. Pole piece
3. Return spring
4. Copper coil
5. Hinge
6. Flexible copper braid
7. Soft iron core
8. Movable electrical contact
9. Non movable electrical contact with two pins
On figure 3 is shown a typical single pole normally closed relay. When the relay is not activated (OFF) the contacts are normally closed. When the relay is activated (ON), in that case the contacts are open.
Figure 3. Single pole normally closed relay:
1. Housing
2. Pole piece
3. Return spring
4. Copper coil
5. Hinge
6. Flexible copper braid
7. Soft iron core
8. Movable electrical contact
9. Non movable electrical contact
On figure 4 is shown a typical single pole changeover relay. In this case the contact A is normally open and the contact B is normally closed. When the relay is not activated (OFF) the contact A is open (switched OFF), and the contact B is closed (switched ON). When the relay is activated (ON), the contact A is closed (switched ON), and the contact B is open (switched OFF).
Figure 4. Single pole changeover relay:
1. Housing
2. Pole piece
3. Return spring
4. Copper coil
5. Hinge
6. Flexible copper braid
7. Soft iron core
8. Non movable electrical contacts (A and B)
9. Movable electrical contact
Specifications, Characteristics, Wiring Symbols and Marking of Pins
The relays are usually supplied with 12 V directly from the car battery. The electrical resistance (impedance) of the coil is vary and is different depending upon the manufacturer of the relay as well as relay’s type, but in general a typical value is between 50 ohms and 200 ohms. Input current typically is in the range between 100 mA and 150 mA.
Figure 5 shows the usual marking of pins (terminals) and layout for a single pole normally open relay. The mainly, marking of pins are with numbers given in wiring symbols below. Sometimes pin numbering (marking) can be different, for example with numbers 1, 2, 3, 4 or similar. In that case, to find out the pins, must follow the relay symbol scheme, which is usually drawn on the top or on the side of the housing.
Figure 5. Single pole normally open relay:
Pin 85 minus electric pole of the coil (mass)
Pin 86 plus electric pole of the coil (command signal)
Pin 30 permanent plus 12V
Pin 87 switched plus
When on pin 86 is brought a command signal the relay is activated (ON). In that case the switching contacts are closed (pin 30 and pin 87 are connected), so the switch is switched ON.
Some vehicle/engine management systems require to be used a resistor (R) to limit the current flow through the coil or the use of a diode (D) to dissipate the stored energy in the coil. In both cases the layout of pins are same and are shown on figure 5.
Figure 6 shows the standard marking of pins and layout for a single pole normally open tied pin relay. The construction and pin numbering can vary depending upon the manufacturer.
Figure 6. Single pole normally open tied pin relay:
Pin 85 minus electric pole of the coil (mass)
Pin 86 plus electric pole of the coil (command signal)
Pin 30 permanent plus 12V
Pin 87 switched plus (tied pin)
When on pin 86 is brought a command signal the relay is activated (ON). In that case the switching contacts are closed (pin 30 and tied pin 87 are connected), so the switch is switched ON.
Figure 7 shows the standard marking of pins and layout for a single pole normally closed relay.
Figure 7. Single pole normally closed relay:
Pin 85 minus electric pole (mass)
Pin 86 plus electric pole of the coil (command signal)
Pin 30 permanent plus 12V
Pin 87 switched plus
This type of relay works opposite than previous types. In normal position when coil is without command signal (not activated), the switching contacts are closed (pin 30 and pin 87 are connected), i.e. the switch is switched ON. When on pin 86 is brought a command signal the relay is activated. In that case the switching contacts are open (pin 30 and pin 87 are disconnected), so the switch is switched OFF.
Figure 8 shows the standard marking of pins and layout for a single pole changeover relay. The construction and pin numbering can vary depending upon the manufacturer.
Figure 8. Single pole changeover relay:
Pin 85 minus electric pole of the coil (mass)
Pin 86 plus electric pole of the coil (command signal)
Pin 30 permanent plus 12V
Pin 87 switched plus (normally open)
Pin 87a switched plus (normally closed)
In this case, at normal position when coil is without command signal (not activated) the pin contact 87 is normally open (switched OFF), and the contact 87a is normally closed (switched ON). When the relay is activated with command signal, the contact 87 is closed (switched ON), and the contact 87a is open (switched OFF).
Rarely, in some cases can be found a relay type with an integral fuse added for protection. This type is shown below.
Figure 9. Single pole relay type with an integral fuse
Pin 85 minus electric pole of the coil (mass)
Pin 86 plus electric pole of the coil (command signal)
Pin 30 permanent plus 12V
Pin 87 switched plus