Relays

Relay:
A relay uses a low amperage circuit to switch on a higher amperage circuit. They are very commo on vehicles, and although there are many types, they are very similar in how they work.

The control circuit will have a coil of wire that creates magnetism when the circuit is powered and earthed. The switching circuit will have a set of point contacts that are switched by having the magnetism pull the points over to connect with another set of points. There may or may not be a circuit that is complete when the relay is "off", or un-powered.

The control circuit usually gets power from the battery. And it will have a switching device in it that will turn the circuit on and off. The switch may be in the positive side or the earth side. It may be switched by a switch, a sensor with a switch inside in, or an ECU (electronic control unit) that does the switching based on a logic circuit.

The switching circuit usually also gets power from the battery. Its circuit will contain the device that needs to be turned on or off. And this circuit will usually end in the device being earthed to provide a complete circuit.

The wiring diagram symbol usually looks somthing like this:
Example 1: Battery power will come in on the left, to power both the control coil and switched circuit. Grounding the coil on the bottom would pull the relay switch points closed. Power would go out the right to power whatever had been switched on. This is a normally open switch.

Example 2 shows a transistor to turn on the controlling circuit. And the switched points are "normally open" but will be closed when the relay is turned

Relays will often have a wiring diagram printed on their cover to show how the relay can be used. There may be unmbers on the bottom to show which pins are connected to which terminals.

Protection Diode: Thecontrol circuit may have a diode or resistor in parallel to lower voltage spikes that can hurt electronic control units. Care must be taken to wire up the control circuit so it does not short the diode. The circuit must be set up so the current reaches the diode in the "reverse bias" direction

Pin designations: Bosch style relays use common pin numbers to designate what they are usually connected to.
86 Positive side of control circuit
85 Negative side of control circuit
30 Battery supply for switched circuit
87a Normally closed switch circuit
87 Other switch circuit
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Relay wiring on Electrical Boards: The diagrams and pictures below show which relay pins relate to the different colored terminals on the relays.

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1.0 Measure Resistance (Ω) in the following circuit: ( measure when the relay is off)
86 to 85     67.4Ω
30 to 87a   0Ω
30 to 87     infinity

1.1 Identify Control Circuit and Switching Circuit: Write the terminal numbers for the circuits below:
Control circuit that switches the points     86,85
Switched points that carry the higher amps     30,87,87a

1.2 Calculate amps that would flow in the control circuit 86 to 85, if 12 volts supply was used. Show your calculations:
I=V/R     V=12V;I=67.4A
I=12/67.4=0.18A

1.3 Is there a switching circuit that is Normally Closed when the relay is off?
YES                    Pin number: 30 and 87A

1.4 Is there a switching circuit that is Normally Open when the relay is off?
YES                    Pin number: 30 and 87

2.0 Draw a wiring diagram in the box below showing a relay circuit controlling three light bulbs in parallel. The switching circuit of the relay must turn on all of the light bulbs, and the control circuit of the relay must be negative switched:

2.1 Wire up the Relay on a circuit board so the switching ciecuit of the relay will turn on one of the light bulbs, and the control corcuit of the relay will be controlled by a switch.

2.2 Measure available voltage at the following terminal points when the circuit is off , and then when the circuit is on.
Circuit Off                    Circuit On                    Control circuit
86     11.99V               86     11.94V               current 0.14A
85     11.98V               85     0.02V                 Consumer Circuit
30     11.98V               30     11.93V               current 1.70A
87A  11.98V               87A  0V
87     0V                      87     11.90V

2.3 On which terminals did the voltage change the most? Between the off and on position? Note those that apply
85, 87, 87a

2.4Explain why the voltage change on each terminal of the relay
between the on and off position? Compare and explain:
86. When the switch off, the voltage of terminal 86 was 11.99V, when the switch on, the voltage of terminal 86 was 11.94v,because the load makes the voltage drop 0.05V
85. When the switch off , the voltage of the terminal 85 was 11.98V because that was open circuit, when the switch on the voltage of the terminal 85 was 0.02V, because when the switch on that was close ciecuit, the control circuit gose work and makes the voltage drop to 0.02V.
30. When the switch on, the voltage of terminal 30 droped from 119.98V to 11.93V, becasue the load makes the voltage drop
87A. When the switch off the voltage from positive to terminal 30 and connect to 87A, so the voltage was 11.98V, when the switch on the terminal 87A was not connect to the circuit, the voltage was 0V.
87. When the switch off, the terminal 87 was open circuit. When the switch on, the voltage from positive to terminal 30 and connect to 87, that was close circuit, so the voltage was 11.90V.
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3.0 Draw a wiring diagram in the box below: to wire up a relay to switch between two light, as if you were switching between low beam and two high beam headlamps.

3.1 Wire up a relay with the diagram you drew above to switch between one light bulb and another, as if you were switching between low beams and high beam headlamps.