The
Electromechanical Relay
The term Relay generally refers to a device
that provides an electrical connection between two or more points in response
to the application of a control signal. The most common and widely used type of
electrical relay is the electromechanical relay or EMR.
An Electrical Relay
The most fundamental control of any equipment is the ability to
turn it “ON” and “OFF”. The easiest way to do this is using switches to
interrupt the electrical supply. Although switches can be used to control
something, they have their disadvantages. The biggest one is that they have to
be manually (physically) turned “ON” or “OFF”. Also, they are relatively large,
slow and only switch small electrical currents.
Electrical Relays however, are
basically electrically operated switches that come in many shapes, sizes and
power ratings suitable for all types of applications. Relays can also have
single or multiple contacts within a single package with the larger power relays
used for mains voltage or high current switching applications being called
“Contactors”.
In this tutorial about electrical relays we are just concerned
with the fundamental operating principles of “light duty” electromechanical
relays we can use in motor control or robotic circuits. Such relays are used in
general electrical and electronic control or switching circuits either mounted
directly onto PCB boards or connected free standing and in which the load
currents are normally fractions of an ampere up to 20+ amperes. The relay
circuit are common in Electronics applications.
As their name implies, electromechanical relays are electro-magnetic devices
that convert a magnetic flux generated by the application of a low voltage
electrical control signal either AC or DC across the relay terminals, into a
pulling mechanical force which operates the electrical contacts within the
relay. The most common form of electromechanical relay consist of an energizing
coil called the “primary circuit” wound around a permeable iron core.
This iron core has both a fixed portion called the yoke, and a
moveable spring loaded part called the armature, that completes the magnetic
field circuit by closing the air gap between the fixed electrical coil and the
moveable armature. The armature is hinged or pivoted allowing it to freely move
within the generated magnetic field closing the electrical contacts that are
attached to it. Connected between the yoke and armature is normally a spring
(or springs) for the return stroke to “reset” the contacts back to their
initial rest position when the relay coil is in the “de-energized” condition,
i.e. turned “OFF”.
Electromechanical
Relay Construction
In our simple relay above, we have two sets of electrically
conductive contacts. Relays may be “Normally Open”, or “Normally Closed”. One
pair of contacts are classed as Normally Open, (NO) or make
contacts and another set which are classed as Normally Closed, (NC) or
break contacts. In the normally open position, the contacts are closed only
when the field current is “ON” and the switch contacts are pulled towards the
inductive coil.
In the normally closed position, the contacts are permanently
closed when the field current is “OFF” as the switch contacts return to their
normal position. These terms Normally Open, Normally Closed or Make
and Break Contacts refer to the state of the electrical contacts when
the relay coil is “de-energized”, i.e, no supply voltage connected to the relay
coil. Contact elements may be of single or double make or break designs. An
example of this arrangement is given below.
The relays contacts are electrically conductive pieces of metal
which touch together completing a circuit and allow the circuit current to
flow, just like a switch. When the contacts are open the resistance between the
contacts is very high in the Mega-Ohms, producing an open circuit condition and
no circuit current flows.
When the contacts are closed the contact resistance should be
zero, a short circuit, but this is not always the case. All relay contacts have
a certain amount of “contact resistance” when they are closed and this is
called the “On-Resistance”, similar to FET’s.
With a new relay and contacts this ON-resistance will be very
small, generally less than 0.2Ω’s because the tips are new and clean, but over
time the tip resistance will increase.
For example. If the contacts are passing a load current of say
10A, then the voltage drop across the contacts using Ohms Law is 0.2
x 10 = 2 volts, which if the supply voltage is say 12 volts then the load
voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and
if they are not properly protected from high inductive or capacitive loads,
they will start to show signs of arcing damage as the circuit current still
wants to flow as the contacts begin to open when the relay coil is de-energized.
This arcing or sparking across the contacts will cause the
contact resistance of the tips to increase further as the contact tips become
damaged. If allowed to continue the contact tips may become so burnt and
damaged to the point were they are physically closed but do not pass any or
very little current.
If this arcing damage becomes to severe the contacts will
eventually “weld” together producing a short circuit condition and possible
damage to the circuit they are controlling. If now the contact resistance has
increased due to arcing to say 1Ω’s the volt drop across the contacts for the
same load current increases to 1 x 10 = 10 volts dc. This high voltage drop
across the contacts may be unacceptable for the load circuit especially if
operating at 12 or even 24 volts, then the faulty relay will have to be
replaced.
To reduce the effects of contact arcing and high
“On-resistances”, modern contact tips are made of, or coated with, a variety of
silver based alloys to extend their life span as given in the following table.
Electrical Relay
Contact Tip Materials
·
Ag (fine silver)
o 1.
Electrical and thermal conductivity are the highest of all the metals.
o 2.
Exhibits low contact resistance, is inexpensive and widely used.
o 3.
Contacts tarnish easily through sulphurisation influence.
·
AgCu (silver copper)
o 1.
Known as “Hard silver” contacts and have better wear resistance and less
tendency to arc and weld, but slightly higher contact resistance.
·
AgCdO (silver cadmium oxide)
o 1. Very
little tendency to arc and weld, good wear resistance and arc extinguishing
properties.
·
AgW (silver tungsten)
o 1.
Hardness and melting point are high, arc resistance is excellent.
o 2. Not
a precious metal.
o 3. High
contact pressure is required to reduce resistance.
o 4.
Contact resistance is relatively high, and resistance to corrosion is poor.
·
AgNi (silver nickel)
o 1.
Equals the electrical conductivity of silver, excellent arc resistance.
·
AgPd (silver palladium)
o 1. Low
contact wear, greater hardness.
o 2.
Expensive.
·
Platinum, Gold and Silver Alloys
o 1.
Excellent corrosion resistance, used mainly for low-current circuits.
Relay manufacturers data sheets give maximum contact ratings for
resistive DC loads only and this rating is greatly reduced for either AC loads
or highly inductive or capacitive loads. In order to achieve long life and high
reliability when switching alternating currents with inductive or capacitive
loads some form of arc suppression or filtering is required across the relay
contacts.
Extending the life of relay tips by reducing the amount of
arcing generated as they open is achieved by connecting a Resistor-Capacitor
network called an RC Snubber Network electrically in parallel with
an electrical relay contact tips. The voltage peak, which occurs at the instant
the contacts open, will be safely short circuited by the RC network, thus
suppressing any arc generated at the contact tips. For example.
Electrical Relay
Snubber Circuit
Electrical Relay
Contact Types.
As well as the standard descriptions of Normally
Open, (NO) and Normally Closed, (NC) used to describe
how the relays contacts are connected, relay contact arrangements can also be
classed by their actions. Electrical relays can be made up of one or more
individual switch contacts with each “contact” being referred to as a “pole”. Each
one of these contacts or poles can be connected or “thrown” together by
energizing the relays coil and this gives rise to the description of the
contact types as being:
·
SPST – Single Pole Single Throw
·
SPDT – Single Pole Double Throw
·
DPST – Double Pole Single Throw
·
DPDT – Double Pole Double Throw
With the action of the contacts being described as “Make”
(M) or “Break” (B). Then a simple relay with one set of
contacts as shown above can have a contact description of:
“Single Pole Double Throw
– (Break before Make)”, or SPDT – (B-M)
Examples of just some of the more common diagrams used for
electrical relay contact types to identify relays in circuit or schematic
diagrams is given below but there are many more possible configurations.
Electrical Relay
Contact Configurations
·
Where:
·
C is the Common terminal
·
NO is the Normally Open contact
·
NC is the Normally Closed contact
Electromechanical relays are also denoted by the combinations of
their contacts or switching elements and the number of contacts combined within
a single relay. For example, a contact which is normally open in the de-energized
position of the relay is called a “Form A contact” or make contact. Whereas a
contact which is normally closed in the de-energized position of the relay is
called a “Form B contact” or break contact.
When both a make and a break set of contact elements are present
at the same time so that the two contacts are electrically connected to produce
a common point (identified by three connections), the set of contacts are
referred to as “Form C contacts” or change-over contacts. If no electrical
connection exists between the make and break contacts it is referred to as a
double change-over contact.
One final point to remember about using electrical relays. It is
not advisable at all to connect relay contacts in parallel to handle higher
load currents. For example, never attempt to supply a 10A load with two relay
contacts in parallel that have 5A contact ratings each, as the mechanically
operated relay contacts never close or open at exactly the same instant of
time. The result is that one of the contacts will always be overloaded even for
a brief instant resulting in premature failure of the relay over time.
Also, while electrical relays can be used to allow low power
electronic or computer type circuits to switch relatively high currents or
voltages both “ON” or “OFF”. Never mix different load voltages through adjacent
contacts within the same relay such as for example, high voltage AC (240v) and
low voltage DC (12v), always use separate relays for safety.
One of the more important parts of any electrical relay is its
coil. This converts electrical current into an electromagnetic flux which is
used to mechanically operate the relays contacts. The main problem with relay
coils is that they are “highly inductive loads” as they are made from coils of
wire. Any coil of wire has an impedance value made up of resistance (R) and
inductance (L) in series (LR Series Circuit).
As the current flows through the coil a self-induced magnetic
field is generated around it. When the current in the coil is turned “OFF”, a
large back emf (electromotive force) voltage is produced as the magnetic flux
collapses within the coil (transformer theory). This induced reverse voltage
value may be very high in comparison to the switching voltage, and may damage
any semiconductor device such as a transistor, FET or micro-controller used to
operate the relay coil.
One way of preventing damage to the transistor or any switching
semiconductor device, is to connect a reverse biased diode across the relay
coil.
When the current flowing through the coil is switched “OFF”, an
induced back emf is generated as the magnetic flux collapses in the coil.
This reverse voltage forward biases the diode which conducts and
dissipates the stored energy preventing any damage to the semiconductor
transistor.
When used in this type of application the diode is generally
known as a Flywheel Diode, Free-wheeling Diode and even Fly-back
Diode, but they all mean the same thing. Other types of inductive loads
which require a flywheel diode for protection are solenoids, motors and
inductive coils.
As well as using flywheel Diodes for protection of semiconductor
components, other devices used for protection include RC Snubber
Networks, Metal Oxide Varistors or MOV and Zener
Diodes.
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