Velleman DCA75 - Manual (Page 18)

Atlas DCA Pro User Guide December 2012 – Rev 1
Page 18
NPN Darlington
Red-B Green-E Blue-C
HFE=9410
NPN Darlington
Red-C Green-E Blue-B
HFE=67
Darlington Transistors
If the device is a Darlington transistor (two BJTs connected
together), the unit will display a similar message to this:
For Darlington devices that do not have
internal resistors, the gain (H
FE
) can be
very high as expected.
This second example however, (left)
shows the display for a Darlington
transistor that has internal resistors
connected
to the base-emitter
connections. This causes the H
FE
measurement to become much lower at the test currents used by the DCA Pro.
This is normal and is not a fault with the transistor or the DCA Pro.
It is important to note that if a Darlington does contain a base-emitter
shunt resistor network, any measurements of current gain (H
FE
) will be
very low at the test currents used by the DCA Pro. This is due to the
resistors providing an additional path for the base current. The
readings for gain however can still be used for comparing transistors
of a similar type for the purposes of matching or gain band selecting.
Note that the DCA Pro will determine that the transistor under test is a
Darlington type if the base-emitter voltage drop is greater than 1.00V
for devices with a base-emitter shunt resistance of greater than 60kΩ
or if the base-emitter voltage drop is greater than 0.80V for devices
with a base-emitter shunt resistance of less than 60kΩ. The measured
base-emitter voltage drop is displayed as detailed later in this section.