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Relays and Solenoids Information taken from “How Stuff Works.com” Read the information carefully and complete the written assignment at the end. This assignment will be evaluated for thinking/inquiry. A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of devices. An open relay In this article, we will look at how relays work and a few of their applications. Relay Construction Relays are amazingly simple devices. There are four parts in every relay: Electromagnet Armature that can be attracted by the electromagnet Spring Set of electrical contacts The following figure shows these four parts in action: In this figure, you can see that a relay consists of two separate and completely independent circuits. The first is at the bottom and drives the electromagnet. In this circuit, a switch is controlling power to the electromagnet. When the switch is on, the electromagnet is on, and it attracts the armature (blue). The armature is acting as a switch in the second circuit. When the electromagnet is energized, the armature completes the second circuit and the light is on. When the electromagnet is not energized, the spring pulls the armature away and the circuit is not complete. In that case, the light is dark. When you purchase relays, you generally have control over several variables: The voltage and current that is needed to activate the armature The maximum voltage and current that can run through the armature and the armature contacts The number of armatures (generally one or two) The number of contacts for the armature (generally one or two -- the relay shown here has two, one of which is unused) Whether the contact (if only one contact is provided) is normally open (NO) or normally closed (NC) Relay Applications In general, the point of a relay is to use a small amount of power in the electromagnet -- coming, say, from a small dashboard switch or a low-power electronic circuit -- to move an armature that is able to switch a much larger amount of power. For example, you might want the electromagnet to energize using 5 volts and 50 milliamps (250 milliwatts), while the armature can support 120V AC at 2 amps (240 watts). Relays are quite common in home appliances where there is an electronic control turning on something like a motor or a light. They are also common in cars, where the 12V supply voltage means that just about everything needs a large amount of current. In later model cars, manufacturers have started combining relay panels into the fuse box to make maintenance easier. For example, the six gray boxes in this photo of a Ford Windstar fuse box are all relays: In places where a large amount of power needs to be switched, relays are often cascaded. In this case, a small relay switches the power needed to drive a much larger relay, and that second relay switches the power to drive the load. When You Step on the Gas The gas pedal in your car is connected to the throttle valve -this is the valve that regulates how much air enters the engine. So the gas pedal is really the air pedal. When you step on the gas pedal, the throttle valve opens up more, letting in more air. The engine control unit (ECU, the computer that controls all of the electronic components on your engine) "sees" the throttle valve open and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate as soon as the throttle valve opens; otherwise, when the gas pedal is first pressed, there may be a hesitation as some air reaches the cylinders without enough fuel in it. Sensors monitor the mass of air entering the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine-tune the fuel delivery so that the air-to-fuel ratio is just right. The Injector A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel pump in your car, and it is capable of opening and closing many times per second. When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel -- to make as fine a mist as possible so that it can burn easily. A fuel injector firing The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU. Fuel injectors mounted in the intake manifold of the engine The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors. In this picture, you can see three of the injectors. The fuel rail is the pipe on the left. Relays and Fuel Injector Assignment Name: _______________________________ Relays: A relay is a simple _____________________________ switch that is made up of an ______________________ and a set of __________________. The purpose of a relay is to allow a ______________ amount of electrical current flow (amperage) to control a ____________ amount of current. The four components of any relay are: ____________________________ ____________________________ ____________________________ ____________________________ (3 marks) Explain the basic operation of a relay from page one of the article. (3 marks) ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Name 4 electrical components on a car that might require a relay. Remember, a relay is used to control high amperage devices. (Use the text book if you wish – page 374-375) (4 marks) _____________________________ _____________________________ _____________________________ _____________________________ Fuel Injectors: The gas pedal in a car is connected to a ________________ ______________ that regulates how much ________ enters the engine. What does the vehicles computer do when it ‘sees’ the throttle valve opening more? ________________________________________________________________________ ________________________________________________________________________ Why is it important for the computer to do this very quickly? ________________________________________________________________________ ________________________________________________________________________ What sensor tells the computer the position of the throttle valve? ____________________________________ A fuel injector is nothing more than an __________________ controlled ____________. (4 marks) When the injector is energized, an ________________ moves a ____________ that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to ___________ the fuel. (1 mark) What does atomize mean? (2 marks) ________________________________________________________________________ ________________________________________________________________________ The amount of fuel supplied to the engine is determined by the amount of _______ the fuel injector stays open. This is called the _________ _________, and it is controlled by the ECU (computer). The fuel injectors spray fuel directly at the __________________ _______________. What does the fuel rail do? _________________________________________________ _______________________________________________________________________ (3 marks) How is a fuel injector similar in operation to a relay? What electrical principal do they both use? (5 marks) ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Evaluation: Thinking/ Inquiry /25