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Relays are switches that open and close circuits electromechanically. They control an electrical circuit by opening and closing
the contacts in another circuit. When a relay contact is normally open (NO), an open contact when the relay is energized. When a
relay contact is normally closed (NC), there is a closed contact when the relay is energized. In either case, the application of electric
current to the contact changes its state.
Relays are used to switch currents are used in smaller and a control circuit, usually not control energy consuming devices, except
for small motors and solenoids to attract low amps. However, relays can control voltages and amperes larger amplifier to have an
effect due to a small voltage applied to the coil of a relay may result in a large voltage is controlled by the contacts.
Protective relays can prevent equipment damage by detecting electrical abnormalities, including overcurrent, undercurrent, overload
and reverse currents. Also, the relays are also widely used to change the starting coils, heating elements, pilot lights and alarms.

The relays are either solid state or electromechanical. In an electromechanical relay contacts are opened or closed by a magnetic
force. With a solid state relay, no contacts and is fully electronic exchange. The decision to use a solid state relay or
electromechanical depends on the electrical needs of an application, the cost constraints and life expectancy, using optocouplers
and triacs internally. Although solid state relays have become very popular, electromechanical relays are still common. Many of the
functions performed by the heavy-duty equipment required switching capabilities. A current solid state relay emails using mobile
devices such as silicon controlled rectifiers.
These differences in the two types of broadcast systems result in advantages and disadvantages with each system. Due to a solid
state relay does not have to either energize a coil or contacts, less voltage is required to "ignite" an SSR ON or OFF. Similarly, an
SSR is switched on and off faster because there is no physical parts to move. Despite the lack of contact and moving parts means
that SSR are not subject to the arch and not wear, contacts in an EMR can be replaced, while all the SSR should be replaced when
a part becomes defective. Due to the construction of solid state relays, there is a residual electrical resistance and / or leakage
current when the switches are open and closed. Small voltage drops that are created are not usually a problem, however, provide a
cleaner REM ON or OFF condition due to the relatively large distance between the contacts, which acts as a form of insulation.
Solid State Relays consist of an input circuit, a control circuit and an output circuit. The input circuit is the relay that is connected to
the control component. The input circuit performs the same function as the coil of an electromechanical relay. The circuit is
activated when a voltage higher than that specified relay pickup voltage is applied to the input terminal. The input circuit is disabled
when the applied voltage is less than the minimum output voltage from the specified relay. The range of voltage of 3 Vdc to 32 Vdc,
commonly used in most solid-state relays, which makes it useful for most electronic circuits. The control circuit is part of the relay
that determines when the output component is energized or de-energized. The control circuit functions as the coupling between the
input and output circuits. In an electromechanical relay, the coil performs this function. The output circuit is the relay that switches
the load and performs the same function as mechanical contacts of an electromechanical relay. Solid state relays, however, usually
only have an output contact.

The types of electromechanical relays.
General Purpose Relays are electromechanical switches, usually operated by a magnetic coil. General purpose relays operate on
AC or DC, in common voltages such as 5V, 12V, 18V, 24V, 48V, 120V and 230 V, and can control the currents ranging from 2A-30A
and more.
Control relays are also machine operated by a magnetic coil. High strength relays are used to control inputs and other industrial
components. Although they are more expensive than general purpose relays, which are generally more durable. The biggest
advantage of the relay machine control through general purpose relays expandable MCR functionality by adding accessories. A
wide selection of accessories for machine control relays, including additional poles, convertible contacts, transient suppression of
electrical noise, lock control, time attachments.
Reed Relay is a small design and even magnetic type, compact, quick work with a contact sensor, which is NO. The relay is
hermetically sealed in a glass envelope, which makes contacts unaffected by contaminants, smoke or moisture, allows reliable
exchange, and provides greater contact lifespan. The ends of the contact, which often are plated with gold or other low resistance
material to increase conductivity, are drawn together and closed by a magnet. Reed Relays are able to switch industrial
components such as solenoids, contactors and starter motors. A reed relay consists of two rods. When applying a magnetic force,
such as an electromagnet or coil, creating a magnetic field in which the end of the rod assumes opposite polarity. When the
magnetic field is strong enough, the attraction of opposites than the rigid rods and are united. When the magnetic force is removed,
the reeds jump back to its original position open. These relays work very quickly because of the short distance between the reeds.



Types of solid state relays.
Zero-Switching Relay - relay turns on when the control load (minimum run) was applied and the charging voltage is near zero. The
relay is deactivated when the load is removed the control voltage and load current is near zero. This relay is the most widely used.
Relay instant - is activated immediately when the load pickup voltage is present. This allows the load is turned ON at any point that
is above and below the waves.
load is activated when the control voltage is present, and the load voltage is at its peak. The relay is deactivated when removed the
control voltage and the load current is close to zero.
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