High Current Adjustable LM317 Power Supply Circuit

The LM317 is a chip that controls voltage, but it can only deliver a small amount of current 1.5 amps.

We can add another part a TIP35 transistor to the LM317 circuit.

This transistor acts like a follower doing whatever the LM317 tells it to do.

By adding this transistor the LM317 can control a much larger current up to 10 amps.

This makes the LM317 more useful for projects that need a lot of power.

Circuit Working:

Parts List:

Below is the detailed information how circuit works:

The LM317 operates as a voltage regulator taking the 33V DC input and providing an adjustable output voltage between 1.25V and 30V.

This is achieved by configuring resistors R1 and R2 to set the desired output voltage.

The TIP35 transistor is configured as an emitter follower with its base connected to the LM317 output.

In an emitter follower configuration the emitter voltage closely follows the base voltage.

As the LM317 adjusts its output voltage, the base voltage of the TIP35 also changes.

This is because the LM317 output is connected to the base of the TIP35.

As the LM317 regulates its output voltage based on the resistors values it influences the base voltage of the TIP35.

Due to the emitter follower configuration the emitter voltage of the TIP35 closely tracks the base voltage.

Therefore, when the LM317 output voltage changes the emitter voltage of the TIP35 also varies accordingly.

The collector of the TIP35 is connected to the high current input supply.

As the emitter voltage changes the TIP35 allows a proportionate amount of current to flow from its collector to the emitter.

Since the collector current is essentially the load current this allows for a substantial increase in the output current.

The TIP35 acts as a current amplifier transforming the LM317 limited output current 1.5 amps into a higher output current potentially up to 10 amps or as determined by the TIP35 specifications.

The LM317 ensures precise voltage regulation, and the TIP35 amplifies the current without affecting the LM317 voltage regulation characteristics.

Resistors, capacitors, and proper heat sinks contribute to stability and reliable operation of the circuit.

Formulas:

There are two main formulas associated with, high current adjustable LM317 power supply circuit mentioned below:

Output Voltage (Vo): This formula is used to determine the LM317 circuits adjustable output voltage.

The voltage divider design relies on the values of two resistors R1 and R2 linked in series.

V_o = ( 1.25V + ( R2 x I_adj ))

where,

• V_o is the output voltage
• 1.25V is the internal reference voltage of the LM317
• R2 is the value of the variable resistor (in ohms)
• I_adj is the current flowing through the adjustment pin (typically around 100uA)

Current Limit for external transistor: To increase the LM317s capacity to handle current, the circuit you submitted includes an external transistor TIP35C.

By restricting the transistors base current, the resistor Ry also limits the output current.

The formula for Ry:

R_y = V_be / I_limit

where:

• R_y is the value of the current limit resistor (in ohms)
• V_be is the base emitter voltage of the TIP35C transistor typically around 0.7V
• I_limit is the desired maximum output current (in amps)

Note:

To maintain stability, the value of R1 is usually maintained between 120 and 240 ohms.

Only 1.5A of current can be handled by the LM317 alone.

Higher current output is possible with the external transistor circuit, but there is also more heat dissipation.

Make sure the heat sink you select has a rating that corresponds to the circuits overall power dissipation.

Circuit Construction:

To build the LM317 adjustable high current power supply circuit with the TIP35 transistor follow these steps:

Connect Input Voltage:

• Connect the 33V DC input voltage to the LM317 input pin.

Output Voltage Adjustment:

• Connect resistors R1 and R2 to set the desired output voltage.
• Use the LM317 datasheet or an online calculator to determine resistor values.

Output Connection:

Connect the load to the LM317 output pin.

Capacitors for Stability:

• Connect capacitors across the input and output of the LM317 for stability.
• A typical choice is 0.1µF for input and 10µF for output.

Add TIP35 Transistor:

• Connect the LM317 output to the base of the TIP35 transistor.
• Connect the emitter of the TIP35 to the LM317 output pin.
• Connect the collector of the TIP35 to the high current input supply.

Heat Sinks:

• Attach heat sinks to both the LM317 and TIP35 to dissipate heat generated during operation.

Final Steps:

• Double check all connections and component values.
• Power the circuit using the 33V DC input.
• Gradually increase the output voltage and observe the behavior.
• Monitor heat generation and ensure that the components stay within their specified temperature limits.
• Test the circuit with various loads to ensure its stability and performance.

Conclusion

The LM317 adjustable high current power supply circuit with the TIP35 transistor provides an efficient and flexible solution for applications requiring a controllable voltage source with the ability to deliver a higher current output.

References

LM317 3-Terminal Adjustable Regulator datasheet

Linear (LM317 based?) high current / voltage regulator ( 30 amps at 0 – ~35 volts)