Lincoln Electric NA-3S, Service Manual, Page: 65 / 110

THEORY OF OPERATION
E-6 E-6
NA-3 AND NA-4
SCR OPERATION
Since the wire speed of the NA-3 and NA-4
is controlled by silicon-controlled rectifier
(SCR) circuitry, a basic understanding of
how an SCR functions is important. An SCR
is a three-terminal semiconductor device
used to control currents to a load. An SCR
acts very much like a switch. When a gate
signal is applied to the SCR, it is turned on
and there is current flow from anode to
cathode. In the on state, the SCR acts like
a closed switch. When the SCR is turned off,
there is no current flow. Thus, the device
acts like an open switch. As the name
suggests, the SCR is a rectifier, so it passes
current only during positive half cycles of the
AC supply. The positive half cycle is the
portion of the sine wave in which the anode
of the SCR is more positive than the
cathode.
When an AC supply is applied to the SCR,
the device spends a certain portion of the
AC cycle time in the on state and the
remainder of the time in the off state. The
amount of time spent in the on state is
controlled by the gate.
An SCR is fired by a short burst of current
into the gate. This gate pulse must be more
positive than the cathode voltage. Since
there is a standard pin junction between the
gate and cathode, the voltage between
these terminals must be slightly greater than
0.6 VDC. Once the SCR has fired, it is not
necessary to continue the flow of the gate
current. As long as the current continues to
flow from anode to cathode, the SCR will
remain on. When the anode to cathode
current drops below a minimum value, called
the holding current, the SCR will shut off.
This normally occurs as the AC supply
voltage passes through zero into the
negative portion of the sine wave. If the
SCR is turned on early in the positive half
cycle, the conduction time is longer, resulting
in greater SCR output. If the gate firing
time is later in the cycle, the conduction time
is less, resulting in lower SCR output. See
Figure E.5.
FIGURE E.5 – SILICON-CONTROLLED RECTIFIER OPERATION.
INPUT
CATHODE
GATE
GATE
OUTPUT
ANODE
NOTE: AS THE GATE PULSE IS
APPLIED LATER IN THE
CYCLE, THE SCR OUTPUT
IS DECREASED.