Page 181 - Op Amps Design, Applications, and Troubleshooting

164 VOLTAGE COMPARATORS

If the diode drops are included, the hysteresis will be computed as 4.13 volts.

Zener Currents. The current through Dj can be computed from Equation
(3.20) as follows:

The reverse current through D 3 is computed with the following expression:

Continuing with the calculations, we get

Note that the minus sign simply indicates direction and is of no significance to us
at this time. D 2 current is computed in a similar manner as shown:

Substituting values for the present circuit gives us

The power dissipation of the zeners can be found by applying the basic power
equation P = IV, where I and V are the current and voltages associated with a par-
ticular zener. In our particular case,