13.8V 10A Power Supply Build Guide

Circuit diagram

Description

Amateurs sometimes choose to build their own power supplies rather than purchasing one from a reputable ham radio retailer. The benefit of constructing your own power supply is that it allows for learning about its operation and provides a simpler method for troubleshooting and repairing other power supply units within a ham radio station. A significant cost advantage is only realized if a large power transformer and a substantial heat sink can be obtained at a drastically reduced price. However, self-construction does offer the flexibility to customize the circuit and potentially enhance its reliability beyond commercially available units. The circuit depicted in Figure 1 provides a 10 amp (12 amp surge) output with performance that meets or exceeds that of commercial units. This circuit incorporates a current limiting feature, representing a more dependable system compared to many commercial power supplies. Similar to other commercial units, this circuit employs the LM723 integrated circuit (IC), which delivers excellent voltage regulation. The circuit utilizes three pass transistors, necessitating heat sinking. Resistor R9 facilitates fine-tuning of the output voltage to exactly 13.8 volts, and the resistor network formed by resistors R4 through R7 controls the current limiting function. The LM723 limits current when the collector current of the pass transistors approaches 0.7 volts. To minimize expenses, many commercial units rely on the forward current gain (HFE) of the pass transistors to determine the current limiting. A key drawback of this approach is that the HFE of the transistors increases as they heat up, potentially leading to a thermal runaway condition and a failure of the pass transistors. Because this circuit samples the collector current of the pass transistors, thermal runaway is not a concern, making it a more dependable power supply. The only adjustment required is setting R9 to the desired output voltage, ranging from 10 to 14 volts. A 1K potentiometer (RS271-280) can be used for this purpose if preferred. Resistor R1 primarily enhances temperature stability and can be eliminated by connecting pins 5 and 6 of IC-1 together. Although not strictly necessary due to the chosen current limiting circuit, overvoltage protection can be added to the circuit by connecting the circuit depicted in Figure 2 to the output voltage (Vout). Overvoltage can only occur if transistors Q2 or Q3 fail with a short circuit between the collector and emitter. However, collector-emitter shorts are less likely to occur; it is more probable that the transistors will open up when they fail.

I actually tested this design and deliberately destroyed several 2N3055 transistors by shorting the emitters to ground. In all instances, the transistors opened up and no collector-emitter short occurred. In any event, the optional circuit in Figure 2 provides added peace of mind when a costly radio is used with this power supply. The circuit in Figure 2 detects voltage exceeding 15 volts and triggers the zener diode to conduct. Upon conduction, the gate of the silicon-controlled rectifier (SCR) is activated, causing the SCR to short and blow the 15 amp fuse, thereby shutting off the output voltage. A 2N6399 (Tech America) was used for the SCR in the prototype, but any suitable SCR can be utilized. While overvoltage protection is advisable, it should not be considered a substitute for substantial heat sinks. I personally believe the best protection against overvoltage is the use of large heat sinks and a reliable current limiting circuit. It is important to use large heat sinks combined with heat sink grease for the 2N3055 transistors. I have employed this power supply in my ham radio station for several months on a variety of transceivers, ranging from HF, VHF to UHF, with excellent results and absolutely no hum. This power supply will be a welcome addition to your ham radio station and will substantially enhance your knowledge of power supplies.

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