Page 288 - Op Amps Design, Applications, and Troubleshooting

270 POWER SUPPLY CIRCUITS

Let us plan to use a potentiometer between RI and R 2 of Figure 6.12 to adjust the
gain of the error amp. This is similar to the method used with the series voltage
regulator discussed in a prior section.

Select the Potentiometer. Selection of PI is not critical, and the guidelines
discussed for the series regulator may be followed. That is, the minimum value for
PI should be at least 20 times the imnimum equivalent load resistance. This is
computed with Equation (6.10).

The maximum value is generally no more than a few tens of thousands of ohms.
We will use a 10-kilohm potentiometer for P l in this particular application.
Compute Jt 2. Equation (6.12) provides the tool necessary to compute the value
for R 2. We calculate it as follows:

For this application, we select the nearest standard value of 39 kilohms for R 2,

Compute &,. Resistor R l can be calculated with Equation (6.11). For our pres-
ent application, RI is computed as shown:

Again, we use the nearest standard value of 100 kilohms because the application is
not critical.

Determine the Value of K 3. Resistor JR 3 establishes the maximum possible
load current from the shunt-regulator circuit. We can determine its value for a
given application by applying Ohm's Law at a time when the load current and
output voltage are at maximum and the input voltage is at minimum. That is,