It is often necessary to operate or test equipment used in automotive applications. This supply, as shown in Figure 6-67, provides up to 8 A at 13.8 V.
The uA723 is used as the control element, furnishing drive current to series-pass transistors which are connected in a Darlington configuration. Two 2N3055 npn transistors are used as the pass transistors, so proper heat sinking is necessary to dissipate the power.
This supply is powered by a transformer operating from 120 VAC on the primary and providing approximately 20 VAC on the secondary. Four 10-A diodes with a 100 PIV rating are used in a full-wave bridge rectifier. A 10,000µF/36 VDC capacitor completes the filtering, providing 28 VDC.
The de voltage is fed to the collectors of Darlington connected 2N3055’s. Base drive for the pass transistors is from pin 10 of the uA723 through a 200Ω current limiting resistor, Rl. The reference terminal (pin 6) is tied directly to the non-inverting input of the error amplifier (pin 5). providing 7.15 V for comparison.
The inverting input to the error amplifier (pin 4) is fed from the center arm of a 10 kΩ potentiometer connected across the output of the supply. This control is set for the desired output voltage of 13.8 V. Compensation of the error amplifier is accomplished with a 500-pF capacitor connected from pin 13 to pin 4.
The 1 kΩ resistor on the output is a light load to provide stability when the supply has a no load condition. The 100ΩF/16 VDC electrolytic capacitor completes the filter action and reduces the ripple voltage.
The current output of the supply is sampled through resistor RSC between the output transistor and the output terminal. The resistor value for a 10A maximum current is calculated from the formula:
If the power supply should exceed 8A or develop a short circuit, the uA723 regulator will bias the transistors to cutoff and the output voltage will drop to near zero until the short circuit condition is corrected. This circuit features a no-load-to-full-load (8A) voltage regulation of no more than 0.2 VDC variation (better than 2% regulation).