Download Relays and Solenoids

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