RELAYS
Presented By Engr. Nolan S. Sudaria RME, MIT
Electromechanical Relays
The electromechanical relay (EMR) is a device that
uses an electromagnet to provide
the force to close (or open) switch contacts, in other
words, an electrically powered switch. When the
electromagnet, called
the coil, is energized, it pulls down on the springloaded armature.
Relay contacts are
described as being one of two kinds: normally open
contacts (NO), which are open in the
unenergized state, and normally closed contacts
(NC), which are closed in the unenergized
state.
By convention,
the symbol always depicts the relay in the unenergized
state, so you can easily
determine which are the NC and NO contacts from the
schematic. This symbol is used in a type of drawing
called a ladder diagram. In a ladder diagram, the relay
coil and its contacts are separated on the drawing.
The electrical specifications for the contacts are
different than for the coil. For the
contacts, the maximum current and voltage for DC
and AC operation is specified. For
the coil, the intended voltage and coil resistance are
usually specified.
The coil voltage and
resistance can be used to calculate the steady-state
coil current. Actually, it takes more
voltage and current to pull in the relay contacts than
it does to hold them there because
the armature must be pulled in across an air gap.
Hence, these quantities are called,
respectively, pull-in voltage and pull-in current.
The contacts of a particular
6-V relay actually close at 2.1 V and stay closed until
the voltage is decreased
to 1 V. The values of voltage and current needed to
keep the relay energized are called
the minimum holding voltage and sealed current.
Notice that the actual pull-in voltage
is much less than the rated coil voltage.
This is to guarantee that the relay will pullin
quickly and reliably when operated at the rated
voltage. Coil voltages are specified
to be AC or DC. The difference is that AC coils are
constructed with shaded poles to
prevent “buzzing” with 60-cycle power. A shadedpole relay has a metal ring around
the pole face of the electromagnet. Magnetic flux
induced into this
ring keeps the relay closed when the AC cycles
through 0 V.
Relays are available in a variety of sizes, contact
configurations, and power-handling
capabilities. Some miniature relays can plug into an
IC socket and be powered
directly from a digital logic gate. On the other end of
the spectrum, a power relay, often
called a contactor, is used to switch the current
directly to larger machines and may
handle 50 A.
It is well to remember that for two reasons, relays
have a finite life. First, becauase
the relay is a mechanical device, the moving parts
eventually wear out, and second, the
electrical contacts can become pitted because of
arcing. The contact wear is very dependent on the
electric current that is being switched.
Reed relay
The reed relay is unique because the small reedlike
contacts are encapsulated in a
small sealed glass tube that is evacuated or filled
with an inert gas like dry nitrogen.
The contacts are activated by an external magnetic
field. Contacts are either dry or mercury-wetted.
Mercury-wetted contacts have a thin coating
of mercury that fills in surface irregularities, making
a larger conduction area, and
reduces pitting. Generally, reed relays have a long
life and low coil voltage (frequently
TTL compatible), and are immune to dirty
environments; however, they are generally
low power (contacts rated at 2 A or less) and
vibration sensitive.
Solid-State Relays
A solid-state relay (SSR) is a purely solid-state
device that has replaced the EMR in
many applications, particularly for turning on and
off AC loads such as motors.
Physically, the SSR is packaged in a box (about the
same size as an EMR), with four
electrical terminals.
The two input terminals are analogous to the coil of
an EMR, and the two output terminals are analogous
to the contacts of the
EMR (usually SPST, normally open).
The input or control voltage of the SSR is typically 5
Vdc, 24 Vdc, or 120 Vac.
The 5-Vdc models are designed to be driven directly
from TTL digital logic circuits.
Turning our attention to the output side of the SSR,
we see that the load is placed in
series with the 120-Vac or 240-Vac power.
The output current can be as high as 50 A
in some models. Many SSRs incorporate a feature
called zero-voltage switching: The
line current is switched on at the precise time that
the AC voltage is crossing 0 V. This
eliminates sharp output voltage-rise times and so
minimizes electromagnetic interference
noise (EMI).
The input
voltage drives an LED, and the light from the LED
turns on a photo transistor,
which in turn turns on the triac (a solid-state
switching device ). The LED electrically isolates the
input and output sections of the
SSR.
SSR circuit
The LED electrically isolates the input and output
sections of the SSR. This is important for two
reasons: First, it allows the control electronics to
have a separate ground from the power lines;
second, it prevents high-voltage spikes
in the power circuit from working their way back
upstream to the more delicate control
electronics.
Solid-state relays have a number of important
advantages over electromechanical
relays. Having no moving parts means that
(theoretically) they will never wear
out and makes them practically immune to shock
and vibration. Also, because of the
built-in electronics, they can be driven with a lowvoltage source (such as TTL)
regardless of the output-current capability.
The main disadvantages of the SSR are
the following: (1) They can be “false triggered” by
electrical noise; (2) even when
on, the output resistance is not exactly 0 ohm, so
there is some small voltage drop
and consequent power loss within the relay, and
when off they may have lethal levels
of leakage current; (3) although they are longlasting, unlike an EMR they do not
fail predictably; (4) contact arrangements are
limited, so they may not work for all
relay applications.
Hybrid solid-state relay
The hybrid solid-state relay is similar to the SSR but
uses a low-voltage, fast-acting
reed relay instead of an LED to turn on the output
triac. Using the reed relay provides
good electrical isolation and may work better than
the SSR in some situations.