<s>
The	O
traditional	O
form	O
of	O
a	O
relay	O
uses	O
an	O
electromagnet	O
to	O
close	O
or	O
open	O
the	O
contacts	O
,	O
but	O
relays	O
using	O
other	O
operating	O
principles	O
have	O
also	O
been	O
invented	O
,	O
such	O
as	O
in	O
solid-state	B-Algorithm
relays	I-Algorithm
which	O
use	O
semiconductor	O
properties	O
for	O
control	O
without	O
relying	O
on	O
moving	O
parts	O
.	O
</s>
<s>
Such	O
diodes	O
were	O
not	O
widely	O
used	O
before	O
the	O
application	O
of	O
transistors	B-Application
as	O
relay	O
drivers	O
,	O
but	O
soon	O
became	O
ubiquitous	O
as	O
early	O
germanium	O
transistors	B-Application
were	O
easily	O
destroyed	O
by	O
this	O
surge	O
.	O
</s>
<s>
For	O
higher-power	O
relays	O
switching	O
many	O
amperes	O
,	O
such	O
as	O
motor	O
circuit	O
contactors	B-Algorithm
,	O
contacts	O
are	O
made	O
with	O
a	O
mixtures	O
of	O
silver	O
and	O
cadmium	O
oxide	O
,	O
providing	O
low	O
contact	O
resistance	O
and	O
high	O
resistance	O
to	O
the	O
heat	O
of	O
arcing	O
.	O
</s>
<s>
DPST	O
–	O
Double-Pole	O
Single-Throw	O
relays	O
are	O
equivalent	O
to	O
a	O
pair	O
of	O
SPST	O
switches	O
or	O
relays	O
actuated	B-Algorithm
by	O
a	O
single	O
coil	O
.	O
</s>
<s>
These	O
are	O
equivalent	O
to	O
two	O
SPDT	O
switches	O
or	O
relays	O
actuated	B-Algorithm
by	O
a	O
single	O
coil	O
.	O
</s>
<s>
The	O
S	O
(	O
single	O
)	O
or	O
D	O
(	O
double	O
)	O
designator	O
for	O
the	O
pole	O
count	O
may	O
be	O
replaced	O
with	O
a	O
number	O
,	O
indicating	O
multiple	O
contacts	O
connected	O
to	O
a	O
single	O
actuator	B-Algorithm
.	O
</s>
<s>
Where	O
radio	O
transmitters	O
and	O
receivers	O
share	O
one	O
antenna	O
,	O
often	O
a	O
coaxial	B-Algorithm
relay	I-Algorithm
is	O
used	O
as	O
a	O
TR	O
(	O
transmit-receive	O
)	O
relay	O
,	O
which	O
switches	O
the	O
antenna	O
from	O
the	O
receiver	O
to	O
the	O
transmitter	O
.	O
</s>
<s>
A	O
contactor	B-Algorithm
is	O
a	O
heavy-duty	O
relay	O
with	O
higher	O
current	O
ratings	O
,	O
used	O
for	O
switching	O
electric	O
motors	O
and	O
lighting	O
loads	O
.	O
</s>
<s>
Continuous	O
current	O
ratings	O
for	O
common	O
contactors	B-Algorithm
range	O
from	O
10	O
amps	O
to	O
several	O
hundred	O
amps	O
.	O
</s>
<s>
Contactors	B-Algorithm
with	O
overload	O
protection	O
devices	O
are	O
often	O
used	O
to	O
start	O
motors	O
.	O
</s>
<s>
Force-guided	O
contacts	O
are	O
also	O
known	O
as	O
"	O
positive-guided	O
contacts	O
"	O
,	O
"	O
captive	O
contacts	O
"	O
,	O
"	O
locked	O
contacts	O
"	O
,	O
"	O
mechanically	O
linked	O
contacts	O
"	O
,	O
or	O
"	O
safety	B-Algorithm
relays	I-Algorithm
"	O
.	O
</s>
<s>
These	O
safety	B-Algorithm
relays	I-Algorithm
have	O
to	O
follow	O
design	O
rules	O
and	O
manufacturing	O
rules	O
that	O
are	O
defined	O
in	O
one	O
main	O
machinery	O
standard	O
EN	O
50205	O
:	O
Relays	O
with	O
forcibly	O
guided	O
(	O
mechanically	O
linked	O
)	O
contacts	O
.	O
</s>
<s>
Safety	B-Algorithm
relays	I-Algorithm
are	O
used	O
as	O
part	O
of	O
an	O
engineered	O
safety	O
system	O
.	O
</s>
<s>
A	O
latching	O
relay	O
,	O
also	O
called	O
impulse	O
,	O
bistable	O
,	O
keep	O
,	O
or	O
stay	O
relay	O
,	O
or	O
simply	O
latch	B-General_Concept
,	O
maintains	O
either	O
contact	O
position	O
indefinitely	O
without	O
power	O
applied	O
to	O
the	O
coil	O
.	O
</s>
<s>
Very	O
early	O
computers	O
often	O
stored	O
bits	O
in	O
a	O
magnetically	O
latching	O
relay	O
,	O
such	O
as	O
ferreed	O
or	O
the	O
later	O
remreed	O
in	O
the	O
1ESS	B-Device
switch	I-Device
.	O
</s>
<s>
Some	O
early	O
computers	O
used	O
ordinary	O
relays	O
as	O
a	O
kind	O
of	O
latch	B-General_Concept
—	O
they	O
store	O
bits	O
in	O
ordinary	O
wire-spring	O
relays	O
or	O
reed	B-Algorithm
relays	I-Algorithm
by	O
feeding	O
an	O
output	O
wire	O
back	O
as	O
an	O
input	O
,	O
resulting	O
in	O
a	O
feedback	O
loop	O
or	O
sequential	O
circuit	O
.	O
</s>
<s>
Although	O
such	O
relays	O
once	O
were	O
the	O
backbone	O
of	O
automation	O
in	O
such	O
industries	O
as	O
automobile	O
assembly	O
,	O
the	O
programmable	B-Architecture
logic	I-Architecture
controller	I-Architecture
(	O
PLC	O
)	O
mostly	O
displaced	O
the	O
machine	O
tool	O
relay	O
from	O
sequential	O
control	O
applications	O
.	O
</s>
<s>
Relay	O
logic	O
is	O
the	O
predecessor	O
of	O
ladder	O
logic	O
,	O
which	O
is	O
commonly	O
used	O
in	O
programmable	B-Architecture
logic	I-Architecture
controllers	I-Architecture
.	O
</s>
<s>
A	O
mercury	B-Algorithm
relay	I-Algorithm
is	O
a	O
relay	O
that	O
uses	O
mercury	O
as	O
the	O
switching	O
element	O
.	O
</s>
<s>
A	O
mercury-wetted	O
reed	B-Algorithm
relay	I-Algorithm
is	O
a	O
form	O
of	O
reed	B-Algorithm
relay	I-Algorithm
that	O
employs	O
a	O
mercury	O
switch	O
,	O
in	O
which	O
the	O
contacts	O
are	O
wetted	O
with	O
mercury	O
.	O
</s>
<s>
These	O
auxiliary	O
contacts	O
are	O
in	O
series	O
with	O
the	O
motor	O
's	O
contactor	B-Algorithm
coil	O
,	O
so	O
they	O
turn	O
off	O
the	O
motor	O
when	O
it	O
overheats	O
.	O
</s>
<s>
thumb|( from	O
top	O
)	O
Single-pole	O
reed	O
switch	O
,	O
four-pole	O
reed	O
switch	O
and	O
single-pole	O
reed	B-Algorithm
relay	I-Algorithm
.	O
</s>
<s>
A	O
reed	B-Algorithm
relay	I-Algorithm
is	O
a	O
reed	O
switch	O
enclosed	O
in	O
a	O
solenoid	O
.	O
</s>
<s>
The	O
switch	O
has	O
a	O
set	O
of	O
contacts	O
inside	O
an	O
evacuated	B-General_Concept
or	O
inert	O
gas-filled	O
glass	O
tube	O
that	O
protects	O
the	O
contacts	O
against	O
atmospheric	O
corrosion	O
;	O
the	O
contacts	O
are	O
made	O
of	O
magnetic	O
material	O
that	O
makes	O
them	O
move	O
under	O
the	O
influence	O
of	O
the	O
field	O
of	O
the	O
enclosing	O
solenoid	O
or	O
an	O
external	O
magnet	O
.	O
</s>
<s>
Reed	B-Algorithm
relays	I-Algorithm
can	O
switch	O
faster	O
than	O
larger	O
relays	O
and	O
require	O
very	O
little	O
power	O
from	O
the	O
control	O
circuit	O
.	O
</s>
<s>
Safety	B-Algorithm
relays	I-Algorithm
are	O
devices	O
which	O
generally	O
implement	O
protection	O
functions	O
.	O
</s>
<s>
A	O
solid-state	O
contactor	B-Algorithm
is	O
a	O
heavy-duty	O
solid	B-Algorithm
state	I-Algorithm
relay	I-Algorithm
,	O
including	O
the	O
necessary	O
heat	O
sink	O
,	O
used	O
where	O
frequent	O
on-off	O
cycles	O
are	O
required	O
,	O
such	O
as	O
with	O
electric	O
heaters	O
,	O
small	O
electric	O
motors	O
,	O
and	O
lighting	O
loads	O
.	O
</s>
<s>
They	O
are	O
activated	O
by	O
AC	O
control	O
signals	O
or	O
DC	O
control	O
signals	O
from	O
programmable	B-Architecture
logic	I-Architecture
controllers	I-Architecture
(	O
PLCs	O
)	O
,	O
PCs	O
,	O
transistor-transistor	B-General_Concept
logic	I-General_Concept
(	O
TTL	B-General_Concept
)	O
sources	O
,	O
or	O
other	O
microprocessor	B-Architecture
and	O
microcontroller	O
controls	O
.	O
</s>
<s>
A	O
solid-state	B-Algorithm
relay	I-Algorithm
(	O
SSR	O
)	O
is	O
a	O
solid	O
state	O
electronic	O
component	O
that	O
provides	O
a	O
function	O
similar	O
to	O
an	O
electromechanical	O
relay	O
but	O
does	O
not	O
have	O
any	O
moving	O
components	O
,	O
increasing	O
long-term	O
reliability	O
.	O
</s>
<s>
A	O
solid-state	B-Algorithm
relay	I-Algorithm
uses	O
a	O
thyristor	O
,	O
TRIAC	O
or	O
other	O
solid-state	O
switching	O
device	O
,	O
activated	O
by	O
the	O
control	O
signal	O
,	O
to	O
switch	O
the	O
controlled	O
load	O
,	O
instead	O
of	O
a	O
solenoid	O
.	O
</s>
<s>
An	O
optocoupler	O
(	O
a	O
light-emitting	O
diode	O
(	O
LED	O
)	O
coupled	O
with	O
a	O
photo	O
transistor	B-Application
)	O
can	O
be	O
used	O
to	O
isolate	O
control	O
and	O
controlled	O
circuits	O
.	O
</s>
<s>
Modern	O
microprocessor-based	O
timing	O
relays	O
provide	O
precision	O
timing	O
over	O
a	O
great	O
range	O
.	O
</s>
<s>
A	O
vacuum	B-General_Concept
relay	O
is	O
a	O
sensitive	O
relay	O
having	O
its	O
contacts	O
mounted	O
in	O
an	O
evacuated	B-General_Concept
glass	O
housing	O
,	O
to	O
permit	O
handling	O
radio-frequency	O
voltages	O
as	O
high	O
as	O
20,000	O
volts	O
without	O
flashover	O
between	O
contacts	O
even	O
though	O
contact	O
spacing	O
is	O
as	O
low	O
as	O
a	O
few	O
hundredths	O
of	O
an	O
inch	O
when	O
open	O
.	O
</s>
<s>
Low	O
power	O
devices	O
such	O
as	O
microprocessors	B-Architecture
can	O
drive	O
relays	O
to	O
control	O
electrical	O
loads	O
beyond	O
their	O
direct	O
drive	O
capability	O
.	O
</s>
<s>
Electromechanical	O
switching	O
systems	O
including	O
Strowger	B-Device
and	O
Crossbar	O
telephone	O
exchanges	O
made	O
extensive	O
use	O
of	O
relays	O
in	O
ancillary	O
control	O
circuits	O
.	O
</s>
<s>
Early	O
electro-mechanical	O
computers	O
such	O
as	O
the	O
ARRA	B-Device
,	O
Harvard	B-Device
Mark	I-Device
II	I-Device
,	O
Zuse	B-Device
Z2	I-Device
,	O
and	O
Zuse	B-Device
Z3	I-Device
used	O
relays	O
for	O
logic	O
and	O
working	O
registers	O
.	O
</s>
<s>
thumb|Several	O
30-contact	O
relays	O
in	O
"	O
Connector	O
"	O
circuits	O
in	O
mid-20th	O
century	O
1XB	B-Device
switch	I-Device
and	O
5XB	B-Device
switch	I-Device
telephone	O
exchanges	O
;	O
cover	O
removed	O
on	O
one	O
.	O
</s>
<s>
Reed	B-Algorithm
relays	I-Algorithm
which	O
are	O
actuated	B-Algorithm
by	O
low	O
currents	O
and	O
switch	O
fast	O
are	O
suitable	O
for	O
controlling	O
small	O
currents	O
.	O
</s>
<s>
Inside	O
the	O
Number	B-Device
One	I-Device
Electronic	I-Device
Switching	I-Device
System	I-Device
(	O
1ESS	B-Device
)	O
crossbar	O
switch	O
and	O
certain	O
other	O
high-reliability	O
designs	O
,	O
the	O
reed	O
switches	O
are	O
always	O
switched	O
"	O
dry	O
"	O
(	O
without	O
load	O
)	O
to	O
avoid	O
that	O
problem	O
,	O
leading	O
to	O
much	O
longer	O
contact	O
life	O
.	O
</s>
