Page 280 - Op Amps Design, Applications, and Troubleshooting

262 POWER SUPPLY CIRCUITS

Recall that minimum voltage gain occurs when the wiper arm of PI is at its upper
extreme. Under these conditions, the equation for RI can be determined as follows:

Similarly, an equation for R 2 can be derived from the basic gain equation as
shown:

Substituting Equation (6.11) for RI in this equation and performing some algebraic
transposing gives us our final equation for the value of R 2.

We can now compute the required values for R l and JR 2- First, we apply Equation
(6.12) to find K 2.

We are now in a position to apply Equation (6.11) to find the value of Rj.

Because the computed values for RI and R 2 are both standard, we do not have to
make any decisions regarding standard values. For most applications, we simply
choose the nearest standard value.
This completes the design of our series regulator circuit. The final schematic
is shown in Figure 6.10, and the performance of the circuit is indicated by the
oscilloscope displays in Figure 6.11. The waveforms show the effects of minimum
and maximum load current and minimum and maximum line voltage. Figure
6.11(a) shows the output under no-load conditions, and Figure 6.11(b) illustrates