–
Use the
Current Limit
or
Power Limit
setting to protect the transistor from excessive current.
If the collector current tries to exceed this value, the measurement halts and the measurement currents
and voltages are removed.
–
Because the measurement is pulsed with time between each pulse, it may be possible to exceed the
actual power dissipation of the device without destroying it. This is however not guaranteed, and
should be approached very carefully - or with a stack of spare devices.
3.6
PNP Transistor (BJT)
(a) Control Settings
(b) Result
Figure 7: PNP Transistor
For a PNP transistor, setup and operation of the curve tracer is similar to the NPN BJT transistor, section 3.5,
page 9.
The control settings for an PNP Power Transistor (TIP32C), and the results of that measurement, are shown
in figure 7.
As mentioned in section 3.5, one must exercise caution not to destroy the PNP device under test by excessive
current or power dissipation.
3.7
N-Channel MOSFET
The control settings for an N-Channel MOSFET (IRF820), and the results of that measurement, are shown in
figure 8.
Notice that the MOSFET drain current is essentially zero when the gate-source voltage is less than the thresh-
old voltage. It then increases very rapidly as Vgs is increased.
Like the BJT, the drain current of a MOSFET is more-or-less constant with increasing drain-source voltage.
That is, it behaves as a current source (or, more accurately, a current sink in the case of an N-MOSFET).
Internally, the gate-source voltage originates with the same circuit that generates base current for a BJT. A
4k7
Ω
resistor is switched between the current generator and ground, converting it into a voltage source with
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