TESTING SILICON CONTROLLED RECTIFIERS (SCR's)
SCR's (also called thyristors) are four l
ay
er p-n-p-n
semiconductors with three terminals; cathode, anode
and gate. During conduction, the SCR has the char
acteristics of two series diodes. However, the device
is normally nonconductive until a trigger current is
applied to its gate. Once triggered into conduction,
the SCR cannot be turned off until the anode-cathode
current drops below a holding current value neces
sary to sustain conduction. This holding current is
usually a small percentage of the permissable peak
current.
The following characteristics of an SCR can be
displayed and measured with the semiconductor
curve tracer:
Forward Blocking Voltage
Reverse Blocking Voltage
Leakage Current
Holding Current
Forward Voltage Drop for Various Forward
Current
Gate Trigger Voltage for Various Forward
Voltages
SCR Connections to Curve Tracer (Figure 33)
For all measurements except gate trigger voltage
the SCR should be connected to the curve tracer as
follows:
CATHODE to EMITTER jack or emitter pin of
socket
ANODE to COLLECTOR jack or collector pin of
socket
GATE to BASE jack or base pin of socket
Forward Blocking Voltage (Figure 34)
Forward blocking voltage is the maximum anode
cathode voltage in the forward direction that the
device will withstand before conduction, at zero gate
current. The curve tracer will measure forward
blocking voltage up to 100 volts.
To measure forward blocking voltage, set the
STEP SELECTOR to the IcES position. This shorts the
gate and cathode to satisfy the zero gate current re
quirement. Set the POLARITY switch to NPN. In
crease the SWEEP VOLTAGE until the SCR "fires",
that is, the anode current suddenly increases and the
anode voltage drops to near zero. Read the highest
anode voltage point in the display. This is the max
imum forward blocking voltage. Any anode current
at anode voltage below the "firing" point is forward
leakage current. and can be read directly from the
display.
Reverse Blocking Voltage
Reverse blocking voltage is the maximum reverse
anode-cathode voltage at zero gate current that the
device can withstand before voltage breakdown. It
is similar to the peak inverse voltage of a diode.
Reverse blocking voltage is normally higher than
forward blocking voltage. The Model 501A can
measure reverse blocking voltage up to 100 volts.
The procedure for measuring reverse blocking
voltage is the same as for measuring forward block
ing voltage except that the POLARITY switch is set
to PNP. The voltage at which voltage breakdown
occurs, which is a sudden increase in anode current,
is the reverse blocking voltage value. Any anode
current at voltages below breakdown is reverse
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Figure 33. SCR Connections to Curve Tracer
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(75V)
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20
40
60
80
100
VF
Figure 34. Testing Forward Blocking Voltage and
Holding Current of SCR's
leakage current. and can be read directly from the
displ
ay
.
Holding Current
Holding current is the minimum anode current re
quired to sustain conduction once the SCR has been
fired. With the same procedure as used for the for
ward blocking voltage test, note the lowest current
displayed for the "on" condition. This is the holding
current. The measurement may also be made with
the STEP SELECTOR in one of the "Current per Step"
positions so that less sweep voltage is required to
place the SCR in the "on" condition.
Forward Voltage Drop
The forward voltage drop during the "on" condi
tion at various forward current levels may be meas
ured by increasing the horizonal sensitivity of the
oscilloscope and displ
ay
ing a low voltage portion
of the forward voltage. Increase the STEP SELEC
TOR "Current per Step" setting so that sweep volt
age m
ay
be reduced. The VERTICAL SENSITIVITY
may be reduced to 10 mA per division for a greater
27
