If you’re into ham radio, CB radios, or just need a reliable 13.8V power source for your electronics bench, this power supply circuit is exactly what you need. The 13.8V output isn’t random. It’s the standard voltage for running 12V mobile radio equipment, accounting for the voltage drop you’d see in a vehicle’s electrical system when the engine is running.
Today I’m breaking down a straightforward 13.8V 5A regulated power supply that uses the TL431 shunt regulator and TIP35C power transistor. This design delivers up to 69 watts of clean DC power, perfect for powering transceivers, testing automotive electronics, or running any 12V equipment that needs stable voltage.
Why 13.8V Instead of 12V?
Before diving into the circuit, let’s talk about why 13.8V is the magic number. Car batteries sit at about 12.6V when fully charged but not being used. When the alternator is running, the charging voltage typically ranges from 13.8V to 14.4V. Radio equipment designed for mobile use expects this higher voltage.
Running a 12V radio at exactly 12V means you’re operating it at the low end of its range. At 13.8V, you get optimal performance, better transmit power, and headroom for voltage drops in the connecting wires. This is especially important for transmitters that pull heavy current during transmission.
Circuit Overview and Operation
This circuit takes an input voltage of 18-24V DC (already rectified and filtered) and regulates it down to a precise 13.8V output. The regulation happens through a combination of the TL431 voltage reference and a series pass transistor configuration.
The Heart: TL431 Shunt Regulator
The TL431 is a three-terminal adjustable shunt regulator that acts as a precision voltage reference. Think of it as a variable Zener diode that you can program to any voltage between 2.5V and 36V. In this circuit, it monitors the output voltage and controls the base current of the driver transistors to maintain regulation.
Looking at the pinout shown in the diagram, the TL431 has three pins: REF (pin 1), Anode (pin 2), and Cathode (pin 3). The cathode connects to the voltage divider that samples the output voltage. When the output tries to rise above 13.8V, the TL431 conducts more current, which reduces the drive to the pass transistor and brings the voltage back down.
The Power Section: TIP35C
The TIP35C is an NPN power transistor rated for 25A collector current and 125W power dissipation (with adequate heatsinking). In this circuit, we’re using it at 5A, well within its comfort zone. The TIP35C (VT1 in the schematic) is the series pass element that actually controls the current flow to the load.
The physical package shown in the image is a TO-247, which is designed for easy mounting to a heatsink. At full load, this transistor will dissipate roughly (Vin – Vout) × I = (18V – 13.8V) × 5A = 21 watts minimum. With a 24V input, that jumps to about 51 watts. You absolutely need a proper heatsink here.
Driver Stage Transistors
The circuit uses three additional transistors: 2SC733 (VT2), 2SC945 (VT3), and a third transistor in the control loop. These form a compound amplifier that provides enough current gain to drive the TIP35C’s base.
The 2SC733 is a small signal transistor that interfaces with the TL431. The 2SC945 provides additional current gain. This cascaded arrangement means the TL431 doesn’t need to source much current directly, making the regulation more precise and stable.
Component Analysis and Selection
Input Filter Capacitor
The 22000µF (22mF) capacitor at the input is massive, and for good reason. At 5A load, you need substantial energy storage to smooth out the rectified AC ripple. This capacitor should be rated for at least 35V (preferably 50V) to handle the 18-24V input with a safety margin.
A capacitor this large has significant ESR (equivalent series resistance), which matters at high currents. Use a low-ESR type designed for switching power supplies or audio amplifiers. The physical size will be substantial, probably 35-50mm in diameter.
Feedback and Stability Network
The resistor network around the TL431 sets the output voltage and provides loop stability. The 1k resistors in series and parallel with the 0.1 ohm resistor form part of the voltage sensing divider. The exact ratio determines the output voltage.
The 22k resistor connected to the base of VT3 provides bias current, while the 10k resistor helps set the operating point of the driver stage. These values have been chosen to provide stable regulation without oscillation.
The 1n (1nF) capacitor and 10k resistor near VT1 form a compensation network. This prevents high-frequency oscillation that can occur in feedback loops. Without this capacitor, the power supply might oscillate or ring when the load changes suddenly.
Current Sensing: The 0.1 Ohm Resistor
R5, the 0.1 ohm resistor, acts as a current sense element. When 5A flows through it, you get 0.5V drop (V = IR = 5A × 0.1Ω). This voltage can be used for current limiting, though the circuit, as shown, doesn’t have explicit overcurrent protection beyond the transistor ratings.
This resistor needs to be a low-inductance, wire-wound type rated for at least 5 watts (preferably 10 watts). A standard carbon film resistor will burn up instantly at this current level.
Output Filter Capacitor
The 10µF capacitor at the output (rated for at least 25V) reduces high-frequency noise and improves transient response. When you suddenly draw current, this capacitor supplies the initial surge while the regulation loop catches up. A low-ESR electrolytic or a ceramic capacitor works well here.
LED Indicators
LED1 indicates that input power is present. The 1k series resistor limits current to about 18-20mA (depending on the LED forward voltage), which is bright enough to see clearly. LED2 at the output shows when a regulated voltage is available, also with a 1k current-limiting resistor.
Building the Power Supply
Start with a good-quality PCB or a thick perfboard. The high current paths need special attention. Use heavy gauge wire (16-18 AWG minimum) for connections carrying the full 5A load current. The ground return path is especially important. A thin ground trace can introduce voltage drops that affect regulation.
Mount the TIP35C on a substantial heatsink before soldering it into the circuit. Calculate the required thermal resistance based on your maximum input voltage. At 24V input, 13.8V output, and 5A load, the worst-case dissipation is about 51 watts.
Using the formula: Temperature rise = Power × Thermal resistance
For a safe junction temperature of 100°C with 40°C ambient: 60°C temperature rise / 51W = 1.18°C/W maximum thermal resistance
You’ll need a heatsink with thermal resistance under 1°C/W. Add a small fan if your heatsink doesn’t meet this spec. Don’t forget thermal paste between the transistor and heatsink.
Input Power Requirements
This circuit needs a pre-regulated DC input of 18-24V. You’ll need a transformer, a bridge rectifier, and that large input capacitor to create this voltage from AC mains.
For a 5A output, your transformer should be rated for at least 6-7A on the secondary side at 18-20V AC. The extra current capacity accounts for rectifier losses, capacitor charging spikes, and ensures the transformer doesn’t overheat during extended operation.
Use a 10A bridge rectifier rated for at least 200V (like a KBPC1010 or similar). Mount it on a small heatsink because it will dissipate 5-10 watts at full load.
Testing and Adjustment
Before connecting any load, verify the input voltage is within the 18-24V range. Power up the circuit and measure the no-load output voltage. It should be close to 13.8V but might need adjustment.
If adjustment is needed, you’ll modify the voltage divider ratio around the TL431. This typically involves changing one of the resistors in the feedback network. Check your specific TL431 datasheet for the calculation formula.
Test with a dummy load before connecting expensive equipment. A 2.7 ohm 50W resistor will draw about 5A at 13.8V. Connect it for 10-15 minutes and monitor the output voltage and transistor temperature. The voltage should stay rock solid at 13.8V, varying by no more than 0.1V.
Use an oscilloscope to check for ripple at the output. You should see less than 50mV peak-to-peak ripple. If ripple is excessive, check your input filter capacitor and ground connections.
Overcurrent Protection Considerations
As drawn, this circuit relies on the transistor current ratings for protection. For a more robust design, you can add explicit current limiting by sensing the voltage across the 0.1 ohm resistor and using it to trigger a shutdown circuit.
A simple addition would be a transistor that monitors the current sense resistor and shunts base current away from the TIP35C when current exceeds your set limit. This prevents damage from short circuits or excessive loads.
Practical Applications
This power supply is ideal for:
Ham Radio Equipment: Most HF and VHF transceivers designed for mobile use need 13.8V at 3-5A for receive mode, with transmit current potentially higher. This supply handles typical 25-50W transceivers easily.
CB Radio: Similar power requirements to ham gear. A CB radio typically draws 2-3A on receive and can spike to 5A or more on transmit.
Automotive Electronics Testing: When you’re testing car stereos, amplifiers, or other 12V automotive gear on the bench, 13.8V is the right voltage to use.
12V LED Lighting: LED strips and fixtures rated for 12V operation actually perform better at 13.8V, giving you brighter output while staying within safe limits.
Battery Charging: With appropriate current limiting, you can use this to charge 12V lead-acid batteries, though you’d want to add a proper charging controller for sealed batteries.
Common Problems and Solutions
Output voltage low under load: Check the TIP35C heatsink temperature. If it’s extremely hot, the transistor might be going into thermal limiting. Improve cooling or reduce input voltage to lower dissipation.
Oscillation or instability: This usually points to the compensation capacitor or layout issues. Verify the 1nF capacitor is actually 1nF and is placed close to the circuit. Bad ground layout can also cause instability.
Excessive ripple: Increase the input filter capacitor or check for bad solder joints on the capacitor terminals. A defective capacitor can lose capacitance and cause ripple problems.
Low output voltage: The TL431 feedback network might need adjustment, or the TL431 itself could be damaged. Check the resistor values in the voltage divider.
Transistor failure: Usually caused by inadequate heatsinking or an input voltage too high. The TIP35C is rated for 100V collector-emitter voltage, so 24V input is fine, but heat is the killer.
Efficiency and Heat Management
Linear regulators like this are inherently inefficient when the input-output voltage difference is large. At 24V input, 13.8V output, and 5A load:
Efficiency = (Vout × I) / (Vin × I) = 13.8V / 24V = 57.5%
The other 42.5% becomes heat. At full power, that’s about 51 watts of waste heat. This is why proper heatsinking is critical.
If efficiency matters for your application, consider using a buck converter pre-regulator to drop the voltage from 24V to about 16V before the linear regulator. This reduces power dissipation significantly while keeping the benefits of linear regulation (low noise, simple design).
Safety Considerations
Add a fuse on the input side rated for 6-7A. This protects against catastrophic failures. Use a slow-blow type to handle the initial capacitor charging current.
If you’re building the rectifier section too, include proper isolation from mains voltage. The transformer should have adequate creepage and clearance distances. Use a grounded metal enclosure for the entire power supply.
Include reverse polarity protection at the output if you’re using this for radio equipment. A series diode (rated for 10A) prevents damage if someone connects cables backward.
Final Thoughts
This 13.8V 5A power supply design is straightforward and reliable. The TL431 provides excellent regulation, and the TIP35C handles the current easily with proper cooling. Total component cost is probably $15-25 depending on your parts sources.
For ham radio operators, this is a worthwhile project. Commercial 13.8V supplies with similar specs cost $50-100. Building your own saves money and gives you a supply you can repair and modify.
The circuit is also educational. You’ll learn about voltage regulation, power transistor operation, feedback loops, and thermal management. These are fundamental concepts that apply to countless other electronics projects.
Once built and tested, this power supply will serve you reliably for years. It’s one of those tools that becomes indispensable on your workbench.
