Page 287 - Op Amps Design, Applications, and Troubleshooting

Shunt Voltage Regulation 269

the circuit in Figure 6.12 is shorted directly to ground, then the maximum current
flow can be computed with Ohm's Law as

The resulting power dissipation in R 3 is computed with the basic power formula as

Since this clearly exceeds the 0.5-watt rating of the resistor as listed in Figure 6.12,
we can expect the resistor to overheat and burn open. However, it can withstand
momentary short circuits without having a higher power rating. A common way
for momentary short circuits to occur is for your probe to slip off of a test point
while troubleshooting a circuit.

6.3.3 Practical Design Techniques
Let us now design a shunt regulator circuit similar to the one shown in Figure
6.12. We will design it to meet the following design specifications:

1. Unregulated input voltage +18 to+22 volts DC
2. Regulated output voltage +12 to+15 volts DC
3. Load current 0 to 150 milHamps
4. Line regulation <2 percent
5. Load regulation <2 percent
6. Op amp 741

Determine the Error Amp Voltage Gain. Since the design calls for a variable
output voltage, we will need to compute a range of error amp gains. As with the
previous design, we will use the +4-volt reference circuit designed earlier in the
chapter. The required voltage gains can be computed with Equations (6.8) and (6.9).