Page 184 - Op Amps Design, Applications, and Troubleshooting
P. 184
Voltage Comparator with Output Limiting 1 <§7
Similarly,
Since both of these exceed the short-circuit output current of the 741, we will
have to limit the current well below the maximum amount. Let us plan to limit it
to 5 milliamps.
Next we compute the minimum values for RI in order to limit the diode cur-
rents to the desired value by applying Ohm's Law. The minimum value as dic-
tated by D 2 is found, from Equation (3.20), to be
Similarly, the value required to limit the current through D 3 is computed as
The larger of these two values (1.94 kilohms) sets the lower limit on Rj, We will
select the next higher standard value of 2 kilohms for Rj.
Select the Reference Zener Diode. The required reference voltage can be
determined by using the following equation:
In our present design, the required reference is calculated as follows:
