The 13.8v power supply Circuit Diagrams shown below are based on a type 723 voltage regulator because it is reliable, widely available and cheaper than many of the latest high-power (>1.5 A) three-terminal voltage regulators. The 723 IC comes in two packages; a 14-pin DIL case or a 10-lead metal can. The pin connections and pin numbers are shown in the 2nd circuit diagram. The pin numbers shown in this power supply are for the 14-pin DIL case.
The supply is intended for use with mobile ham radio transceivers. As many of today’s FM mobile rigs are capable of supplying r.f. powers of more than 50 watts, a heavy-duty power supply is required, if such a set is not to run off a vehicle battery. This supply is capable of providing output currents of up to 12 amperes at 13.8 V.
Circuit Diagram of 13.8v Power Supply Circuit
This project can be designed using a few basic components. The circuit diagram of this project is shown below.
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Working Explanation of 13.8v Power Supply Circuit
The regulator, IC1, is conventionally wired, drives a power transistor array and monitors the output current by measuring the voltage drop across series resistor Rsc. The normal supply output voltage of 13.8 V is set with preset P1. The input voltage for the supply is obtained from a 22-25 V, 15 A transformer, a 25-A bridge rectifier and a 10,000 μF smoothing capacitor. This section is shown in the first circuit diagram.
The current booster consists of a darlington transistor T1 and three parallel-connected power transistors T2 – T4. Resistors RA, RB and RC are emitter current distribution elements.
The current sensing resistor for the short circuit protection, Rsc, has a value of 0.05 2 which results in a protection onset level of about 0.6/0.05 12 A. This resistor is either made from resistance wire or from two parallel connected 0.1 2, 5 W resistors.
A 12 A or 16 A fuse in the positive supply rail provides additional protection against output short circuits. If this fuse blows, transistor T5 briefly actuates an active buzzer and a flashing LED. It does so by draining the charge built up in C9.
The 15 V overvoltage protection of the output of the supply is a crowbar circuit. If the supply is set to an output voltage other than 13.8 V, the zener diode, thyristor and associated resistor have to be omitted or disconnected by breaking the wire link indicated in the circuit diagram. If used, the thyristor should have a current rating of about 25 A; a type 2N6506 is suggested.
The darlington and power transistors have to be mounted on a large heat-sink with appropriate insulating washers. A small fan may be provided in necessary.
As the choice of components used for the supply is not critical, the circuits easily modified for smaller output currents and/or different output voltages. The use of a 10 A transformer with two power transistors is fine for an 8 A supply. Similarly, a 5 A transformer and one power transistor are sufficient for a 4.A version of the power supply.
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