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Figure 2-18.—P-channel JFET equivalent circuit | Up
Neets Module 21-Test Methods and Practices | Next
Figure 2-21.—MOSFET (depletion/enhancement type) equivalent circuit |
2-25
N-Channel Test
Using an ohmmeter set to the R ´ 100 scale, measure the resistance between the drain and the
source; then reverse the ohmmeter leads and take another reading. Both readings should be equal (in the
100- to 10,000-ohm range), regardless of the meter lead polarity. Connect the positive meter lead to the
gate. Using the negative lead, measure the resistance between the gate and the drain; then measure the
resistance between the gate and the source. Both readings should indicate a low resistance and be
approximately the same. Disconnect the positive lead from the gate and connect the negative lead to the
gate. Using the positive lead, measure the resistance between the gate to the drain; then measure the
resistance between the gate and the source. Both readings should show infinity.
P-Channel Test
Using an ohmmeter set to the R ´ 100 scale, measure the resistance between the drain and the
source; then reverse the ohmmeter leads and take another reading. Both readings should be the same (100
to 10,000 ohms), regardless of meter lead polarity. Next, connect the positive meter lead to the gate.
Using the negative lead, measure the resistance between the gate and the drain; then measure it between
the gate and the source. Both readings should show infinity. Disconnect the positive lead from the gate
and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate
and the drain; then measure it between the gate and the source. Both readings should indicate a low
resistance and be approximately equal.
MOSFET TESTING
Another type of semiconductor you should become familiar with is the metal oxide semiconductor
field-effect transistor (MOSFET), as shown in figures 2-19 and 2-20. You must be extremely careful
when working with MOSFETs because of their high degree of sensitivity to static voltages. As previously
mentioned in this chapter, the soldering iron should be grounded. A metal plate should be placed on the
workbench and grounded to the ship’s hull through a 250-kilohm to 1-megohm resistor. You should also
wear a bracelet with an attached ground strap and ground yourself to the ship’s hull through a 250-kilohm
to 1-megohm resistor. You should not allow a MOSFET to come into contact with your clothing, plastics,
or cellophane-type materials. A vacuum plunger (solder sucker) must not be used because of the high
electrostatic charges it can generate. Solder removal by wicking is recommended. It is also good practice
to wrap MOSFETs in metal foil when they are out of a circuit. To ensure MOSFET safety under test, use
a portable volt-ohm-milliammeter (vom) to make MOSFET resistance measurements. A vtvm must
never be used in testing MOSFETs. You must be aware that while you are testing a MOSFET, you are
grounded to the ship’s hull or station’s ground. Use of a vtvm would cause a definite safety hazard because
of the 115-volt, 60-hertz power input. When the resistance measurements are complete and the MOSFET
is properly stored, unground both the plate on the workbench and yourself. You will understand MOSFET
testing better if you visualize it as equivalent to a circuit using diodes and resistors, as shown in figures
2-21 and 2-22.
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