Page 175 - Op Amps Design, Applications, and Troubleshooting

158 VOLTAGE COMPARATORS

And, for the present circuit,

This same result can be obtained by applying Kirchhoff 's Voltage Law—that is,

3.5.3 Practical Design Techniques
Let us now design a window detector to meet the following specifications:

1. Upper window limit +10 volts
2. Lower window limit +7.5 volts
3. Power supply ±15 volts
4. Input frequency 0 to 100 Hz

Select the Op Amp. Since the circuit is being driven by a very low-frequency
source, the high-frequency characteristics of the op amp are unimportant to us.
The DC stability of the op amp is more important in circuits like this and will be
determined by the requirements of the application being considered. If the switch-
ing speed of the device is important for an application that has a higher input fre-
quency, then you would do well to select an op amp that is specifically designed
for fast comparator applications.
For purposes of our present example, let us choose the 741.

Select the Zener Diodes. Appendix 5 lists a family of zener diodes. The
1N5236 and 1N5240 devices will satisfy the requirements for our lower and upper
reference voltages of 7.5 volts and 10 volts, respectively.

Calculate the Zener-Current Limiting Resistors. Unless we have some
reason to do otherwise (e.g., ultra-low current designs), we can use the zener test
current as the design value. Appendix 5 lists 20 milliamps as the test current for
both diodes. Basic circuit theory, as given in Equation (3.20), allows us to compute
the values of current limiting resistors.

We will choose a standard value of 390 ohms. In a similar manner,