range of voltage vs current. Read the forward volt
age drop directly from the displ
ay
.
Gate Trigger Voltage (Figure 35)
The "turn on" point of an SCR is dependent upon
the forward voltage and gate voltage. As gate volt
age is increased, less forward voltage is required to
switch on the SCR. Conversely, as forward voltage
is increased, less gate voltage is required to switch
on the SCR. Gate trigger voltage values can be
measured by connecting a de bias supply to the gate
terminal of the SCR. The bias supply reference must
be connected to the emitter jack, or the cathode of
C
B
®
®
E
®
DC BIAS
SUPPLY
the SCR. Otherwise, the curve tracer is set up as for
forward blocking voltage measurement. Two types
of measurements can be made:
1. Set the sweep voltage a to a specified forward
anode-cathode voltage and increase the de bias
supply until the SCR switches on. Measure the
value of gate voltage at which switching
occurred.
2. Set the de bias supply to a specified gate volt
age and increase the sweep voltage until the
SCR switches on. Read the peak value of sweep
voltage which was required.
10
0
10
20
30
40
�o
Figure 35. Testing Gate Trigger Voltage of SCR's
TESTING TRIACS
Triacs are four-layer p-n-p-n semiconductors with
the same characteristics in both directions, and may
be used for ac applications. A triac is the equivalent
of two SCR's connected in parallel, but oriented in
opposite directions (See Figure 36). The device has
MAIN TERMINAL l
MAIN TERMINAL 2
SCHEMATIC
REPRESENTATION
three terminals; main terminal 1, main terminal 2
and gate.
Triacs may be tested exactly as SCR's except that
forward tests should be repeated for both directions,
and there will be no reverse blocking voltage meas
urement.
MAIN TERMINAL 2
MAIN TERMINAL 1
GATE
EQUIVALENT
CIRCUIT
Figure 36. Triacs
28
