<s>
A	O
solid	B-Algorithm
state	I-Algorithm
relay	I-Algorithm
(	O
SSR	O
)	O
is	O
an	O
electronic	O
switching	O
device	O
that	O
switches	O
on	O
or	O
off	O
when	O
an	O
external	O
voltage	O
(	O
AC	O
or	O
DC	O
)	O
is	O
applied	O
across	O
its	O
control	O
terminals	O
.	O
</s>
<s>
Solid	B-Algorithm
state	I-Algorithm
relays	I-Algorithm
were	O
invented	O
in	O
1971	O
by	O
the	O
Crydom	O
Controls	O
division	O
of	O
International	O
Rectifiers	O
.	O
</s>
<s>
Packaged	O
SSRs	O
use	O
power	O
semiconductor	O
devices	O
such	O
as	O
thyristors	O
and	O
transistors	B-Application
,	O
to	O
switch	O
currents	O
up	O
to	O
around	O
a	O
hundred	O
amperes	B-General_Concept
.	O
</s>
<s>
This	O
feature	O
is	O
called	O
zero-crossing	B-Algorithm
,	O
or	O
zero-crossover	O
,	O
switching	O
.	O
</s>
<s>
An	O
SSR	O
based	O
on	O
a	O
single	O
MOSFET	B-Architecture
,	O
or	O
multiple	O
MOSFETs	B-Architecture
in	O
a	O
paralleled	O
array	O
,	O
can	O
work	O
well	O
for	O
DC	O
loads	O
.	O
</s>
<s>
MOSFETs	B-Architecture
have	O
an	O
inherent	O
substrate	O
diode	O
that	O
conducts	O
in	O
the	O
reverse	O
direction	O
,	O
so	O
a	O
single	O
MOSFET	B-Architecture
cannot	O
block	O
current	O
in	O
both	O
directions	O
.	O
</s>
<s>
For	O
AC	O
(	O
bi-directional	O
)	O
operation	O
two	O
MOSFETs	B-Architecture
are	O
arranged	O
back-to-back	O
with	O
their	O
source	O
pins	O
tied	O
together	O
.	O
</s>
<s>
It	O
is	O
common	O
to	O
provide	O
access	O
to	O
the	O
common	O
source	O
so	O
that	O
multiple	O
MOSFETs	B-Architecture
can	O
be	O
wired	O
in	O
parallel	O
if	O
switching	O
a	O
DC	O
load	O
.	O
</s>
<s>
Usually	O
a	O
network	O
is	O
provided	O
to	O
speed	O
the	O
turn-off	O
of	O
the	O
MOSFET	B-Architecture
when	O
the	O
control	O
input	O
is	O
removed	O
.	O
</s>
<s>
SSRs	O
for	O
DC	O
switching	O
applications	O
may	O
use	O
mosfets	B-Architecture
,	O
or	O
IGBTs	O
.	O
</s>
<s>
Applications	O
which	O
require	O
Time	O
Critical	O
On/Off	O
switching	O
,	O
should	O
use	O
Transistor	B-Application
or	O
MOSFET	B-Architecture
design	O
types	O
as	O
they	O
are	O
not	O
subject	O
to	O
the	O
inherent	O
Zero	O
Cross	O
variations	O
that	O
SCR	O
or	O
TRIAC	O
devices	O
will	O
exhibit	O
.	O
</s>
<s>
The	O
control	O
voltage	O
energizes	O
an	O
internal	O
LED	O
which	O
illuminates	O
and	O
switches	O
on	O
a	O
photo-sensitive	O
diode	O
(	O
photo-voltaic	O
)	O
;	O
the	O
diode	O
current	O
turns	O
on	O
a	O
back-to-back	O
thyristor	O
(	O
TRIAC	O
)	O
,	O
SCR	O
,	O
or	O
MOSFET	B-Architecture
to	O
switch	O
the	O
load	O
.	O
</s>
<s>
Most	O
of	O
the	O
relative	O
advantages	O
of	O
solid	B-Algorithm
state	I-Algorithm
relays	I-Algorithm
over	O
electromechanical	O
relays	O
are	O
common	O
to	O
all	O
solid-state	O
devices	O
when	O
compared	O
to	O
electromechanical	O
devices	O
.	O
</s>
<s>
SSRs	O
can	O
also	O
include	O
zero	B-Algorithm
crossing	I-Algorithm
hardware	O
to	O
only	O
turn	O
the	O
voltage	O
on	O
or	O
off	O
when	O
the	O
AC	O
voltage	O
is	O
at	O
zero	O
.	O
</s>
<s>
Proportional	O
SSRs	O
can	O
delay	O
the	O
onset	O
of	O
voltage	O
after	O
the	O
zero	B-Algorithm
crossing	I-Algorithm
in	O
order	O
to	O
lower	O
the	O
current	O
output	O
(	O
phase	O
angle	O
control	O
)	O
.	O
</s>
