emitter junction of Q1 is 0.6 volts. This voltage is the difference between the Zener voltage

and the voltage drop across R1. The 0.6-volt forward bias of Q1 permits proper operation

regulated current output of 400 milliamperes, the transistor resistance (RQ1) is 9 ohms. This

can be computed by using Ohm's law and the values shown on the schematic. In this case,

current (I) is equal to the voltage (E) drop divided by the resistance (R). Therefore 12 volts

divided by 30 ohms equals 0.4 ampere, or 400 milliamperes.

4-119. Knowing about the basic current regulating circuitry will help you understand how

the various components work to maintain the constant 400-milliampere output. Remember

a decrease in load resistance causes a corresponding increase in current flow. Figure 4-43

larger voltage drop across R1 because of the increased current flow. The voltage drop has

increased from 2.4 volts to 2.5 volts. Of course, the voltage drop across CR1 remains

constant at 9 volts due to its regulating ability. Because of the increased voltage drop

across R1, the forward bias on Q1 is now 0.5 volts. Since the forward bias of Q1 has

decreased, the resistance of the transistor increases from 9 ohms to 14 ohms. Notice that

the 5-ohm increase in resistance across the transistor corresponds to the 5-ohm decrease in

the load resistance. Therefore, the total resistance around the outside loop of the circuit

remains constant. Since the circuit is a current regulator, you know that output voltages

will vary as the regulator maintains a constant current output. In Figure 4-43, the voltage

output is reduced to 4 volts, which is computed by multiplying current (I) times resistance

(R) (400 mA x 10 ohms = 4 volts).

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