Current Sources and Voltage References

392

Some simple zener applications

3 to 300 volts, and current-limits the zener (i.e., 5 to 10 mA). The value chosen for the

zener (V

Z

) depends on the range of the input pulse voltage (i.e., 48 volts ±10%, 500

milliseconds), along with the value of the transistor’s V

BE

(≈ 0.7 volt). Resistor R

2

,

which is in parallel with the zener and Q

1

’s V

BE

, provides a low current bias point that

is much less than I

Z

. The NPN transistor Q

1

functions as a general-purpose switch,

Figure 13.20. Over voltage circuit protection couples a zener to detect the transient, and trigger

an SCR in a crowbar configuration.

Figure 13.21. This example shows a zener voltage detector ...