Building a 0-40V, 50A Adjustable Power Supply

Building a 0-40V, 50A Adjustable Power Supply Using LM317HVT and TIP35 Transistors

Designing and constructing a high-current adjustable power supply can be a rewarding project for electronics enthusiasts and professionals alike. In this blog post, we'll guide you through the process of creating a 0-40V, 50A adjustable power supply using the LM317HVT voltage regulator and TIP35 transistors. This power supply will be suitable for a wide range of applications, providing precise control over voltage and current.

Components Needed

1. LM317HVT voltage regulator
2. 100K trimmer (potentiometer)
3. 0.22 ohms, 5W resistor
4. Two TIP35 power transistors
5. 1K resistor
6. 0.1uF, 275VAC capacitor
7. Heatsinks for LM317HVT and TIP35 transistors
8. Power supply unit (e.g., a transformer, rectifier, and filter capacitor combination)

Circuit Design

Step 1: Understanding the LM317HVT

The LM317HVT is a versatile voltage regulator capable of outputting up to 1.5A by itself. However, with the help of external pass transistors like the TIP35, it can handle much higher currents. The output voltage of the LM317HVT is adjustable via an external resistor network.

Step 2: Basic Voltage Regulator Configuration

1. Set Up the LM317HVT:

   • Connect the input pin of the LM317HVT to the positive output of the rectified and filtered power supply.
   • Connect the adjust pin of the LM317HVT to one end of the 100K trimmer.
   • Connect the other end of the trimmer to the output pin of the LM317HVT.
   • Place a 1K resistor between the output pin and the adjust pin of the LM317HVT.

2. Voltage Adjustment:

   • The 100K trimmer adjusts the output voltage. By changing the resistance, you can set the output voltage anywhere from 0V up to the maximum voltage provided by the power supply.

Step 3: Current Boosting with TIP35 Transistors

To achieve the high current capacity, we need to use external power transistors:

1. Connecting the TIP35 Transistors:

   • Connect the collectors of the two TIP35 transistors to the positive output of the power supply.
   • Connect the bases of the TIP35 transistors to the output pin of the LM317HVT through a 0.22 ohms, 5W resistor.
   • Connect the emitters of the TIP35 transistors to the load.

2. Stabilizing the Circuit:

   • Place a 0.1uF capacitor across the output terminals to filter out high-frequency noise.
   • Use appropriate heatsinks for the LM317HVT and TIP35 transistors to dissipate heat effectively, as high currents generate significant thermal energy.

Step 4: Assembling the Circuit

1. Power Supply Unit:

   • Ensure your power supply unit can deliver the necessary input voltage and current. For a 0-40V, 50A output, you need a transformer with a secondary winding that can handle the required voltage and current, followed by a rectifier bridge and large filter capacitors to smooth the DC output.

2. Connecting Components:

   • Connect the power supply output to the input pin of the LM317HVT and the collectors of the TIP35 transistors.
   • Assemble the voltage adjustment circuit and connect it to the LM317HVT as described.
   • Connect the TIP35 emitters to the output terminal where the load will be connected.

Detailed Explanation

Voltage Regulation

The LM317HVT controls the output voltage by adjusting the voltage at its adjust pin. The 100K trimmer and 1K resistor form a voltage divider network that sets this voltage. The LM317HVT maintains a constant 1.25V difference between its output and adjust pins, thereby regulating the output voltage.

Current Boosting

The TIP35 transistors handle the high current demand. When the LM317HVT outputs a voltage, it drives the bases of the TIP35 transistors through the 0.22 ohms resistor. The current flowing through this resistor ensures the TIP35 transistors provide the necessary current to the load. The resistor also helps balance the current between the two TIP35 transistors, ensuring even distribution.

Power Supply Unit

The transformer steps down the mains voltage to a suitable level, which is then rectified and filtered to provide a smooth DC voltage. The quality of the transformer, rectifier, and filter capacitors directly affects the performance and stability of the power supply.

Running the Circuit

1. Testing the Voltage Range:

   • Turn on the power supply and adjust the 100K trimmer to vary the output voltage. Use a multimeter to measure the output voltage and ensure it ranges from 0V to 40V as expected.

2. Testing the Current Capability:

   • Connect a suitable load to the output and measure the current. Ensure the TIP35 transistors and LM317HVT are properly heatsinked to handle the high current without overheating.

3. Stability and Noise Filtering:

   • Check the output with an oscilloscope to ensure there is minimal ripple and noise. The 0.1uF capacitor helps filter out high-frequency noise, ensuring a stable DC output.

Conclusion

Building a 0-40V, 50A adjustable power supply using the LM317HVT and TIP35 transistors is a practical and educational project. This power supply can be used for various applications, providing a reliable and adjustable source of power for your electronic projects. By following the steps outlined in this blog post, you can successfully design, assemble, and test your own high-current adjustable power supply.