Page 216 - Op Amps Design, Applications, and Troubleshooting
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198 OSCILLATORS
Similarly, Equation (4.26) tells us the nominal voltage rating for D 6.
Referring to Appendices 5 and 8, we see that there will be difficulty getting a 6.4-
volt zener. However, if we design for a zener current that is less than the test cur-
rent, the actual zener voltage will be less than the rated value. With this in mind,
let us select the next higher standard value. More specifically, let us plan to use
1N5235 zeners for diodes D 5 and D 6.
Compute the Value for R&. R 6 can be found with our basic zener equation,
Equation (4.24). In our present case, we will be finding the value of JRg with +V SAT,
V D5, and I D5 known. Because the output of the op amp must supply currents to
two zener circuits and the Q timing circuit, we will limit the zener currents to no
more than 20 percent of the short-circuit output current. That is,
In our present case,
If we use the typical +V SAT value of +13 volts, we can calculate a value for R 6 by
applying Equation (4.24).
Select the Reference Zener. Although the selection of diodes D 3 and D 4 is
not critical, the following equation provides a good rule of thumb:
