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Contact VAC Cha p te r T w o 120 VAC Load
Time
RC Snubber
Contact VAC Time 120 VAC Load
FIGURE 2.71 Snubber circuit.
2.9.3 Transistor DC Switches
Transistors are solid-state DC switching devices. They are most com-
monly used with low-voltage DC-powered inductive and capacitive
sensors as the output switch. Two types are employed, depending on
the function (Fig. 2.72).
In an NPN transistor, the current source provides a contact clo-
sure to the DC positive rail. The NPN current sink provides a contact
to the DC common. The transistor can be thought of as a single-pole
switch that must be operated within its voltage and maximum cur-
rent ratings (Fig. 2.73).
Any short circuit on the load will immediately destroy a transis-
tor that is not short-circuit protected. Switching inductive loads
creates voltage spikes that exceed many times the maximum rating of
+V +V
C C 36 V
Photoelectric B Output Photoelectric B
circuit circuit
E 36 V E Output
Common
FIGURE 2.72 DC circuit logic.
