<s>
Phase-fired	O
control	O
(	O
PFC	O
)	O
,	O
also	O
called	O
phase	B-Device
cutting	I-Device
or	O
"	O
phase	B-Device
angle	I-Device
control	I-Device
"	O
,	O
is	O
a	O
method	O
for	O
power	O
limiting	O
,	O
applied	O
to	O
AC	O
voltages	O
.	O
</s>
<s>
PFC	O
differs	O
from	O
Pulse-width	B-Algorithm
modulation	I-Algorithm
(	O
PWM	O
)	O
in	O
that	O
it	O
addresses	O
supplies	O
that	O
output	O
a	O
modulated	O
waveform	O
,	O
such	O
as	O
the	O
sinusoidal	O
AC	O
waveform	O
that	O
the	O
national	O
grid	O
outputs	O
.	O
</s>
<s>
Phase-fired	B-Device
controllers	I-Device
take	O
their	O
name	O
from	O
the	O
fact	O
that	O
they	O
trigger	O
a	O
pulse	O
of	O
output	O
at	O
a	O
certain	O
phase	O
of	O
the	O
input	O
's	O
modulation	O
cycle	O
.	O
</s>
<s>
Most	O
phase-fired	B-Device
controllers	I-Device
use	O
thyristors	O
or	O
other	O
solid-state	O
switching	O
devices	O
as	O
their	O
control	O
elements	O
.	O
</s>
<s>
Thyristor-based	O
controllers	O
may	O
utilise	O
gate	O
turn-off	O
(	O
GTO	O
)	O
thyristors	O
,	O
allowing	O
the	O
controller	O
to	O
not	O
only	O
decide	O
when	O
to	O
switch	O
the	O
output	O
on	O
but	O
when	O
to	O
turn	O
it	O
off	O
,	O
rather	O
than	O
having	O
to	O
wait	O
for	O
the	O
waveform	O
to	O
return	O
to	O
the	O
next	O
zero	B-Algorithm
crossing	I-Algorithm
.	O
</s>
<s>
A	O
phase-fired	B-Device
controller	I-Device
,	O
like	O
a	O
buck	O
topology	O
switched-mode	O
power	O
supply	O
,	O
is	O
only	O
able	O
to	O
deliver	O
an	O
output	O
maximum	O
equal	O
to	O
that	O
which	O
is	O
present	O
at	O
its	O
input	O
,	O
minus	O
any	O
losses	O
occurring	O
in	O
the	O
control	O
elements	O
themselves	O
.	O
</s>
<s>
Derating	O
of	O
mains	O
powered	O
,	O
phase-fired	B-Device
controllers	I-Device
is	O
important	O
as	O
they	O
are	O
often	O
used	O
to	O
control	O
resistive	O
loads	O
,	O
such	O
as	O
heating	O
elements	O
.	O
</s>
<s>
For	O
industrial	O
applications	O
previously	O
,	O
extremely	O
expensive	O
and	O
heavy	O
multi-tapped	O
transformers	B-Algorithm
were	O
used	O
as	O
the	O
supplies	O
for	O
such	O
elements	O
,	O
with	O
the	O
corresponding	O
winding	O
tap	O
being	O
connected	O
to	O
the	O
element	O
to	O
produce	O
the	O
desired	O
temperature	O
.	O
</s>
<s>
In	O
modern	O
,	O
usually	O
high	O
power	O
,	O
equipment	O
,	O
the	O
transformer	B-Algorithm
is	O
replaced	O
with	O
phase-fired	B-Device
controllers	I-Device
connecting	O
the	O
load	O
directly	O
to	O
the	O
mains	O
,	O
resulting	O
in	O
a	O
substantially	O
cheaper	O
and	O
lighter	O
system	O
.	O
</s>
<s>
This	O
is	O
because	O
the	O
removal	O
of	O
the	O
mains	O
transformer	B-Algorithm
means	O
that	O
the	O
load	O
has	O
electrical	O
continuity	O
with	O
the	O
input	O
.	O
</s>
<s>
The	O
first	O
patent	O
for	O
phase-fired	B-Device
controllers	I-Device
derives	O
from	O
1912	O
.	O
</s>
