High Current Adjustable LM317 Power Supply Circuit

The IC LM317 is a voltage controlling chip yet it has a 1.5 amp current delivery capacity.

An additional component can be added to the IC LM317 circuit through a TIP35 transistor.

When instructed to do so by the IC LM317 this transistor behaves like a follower.

The IC can now control a significantly higher current of up to 10 amps by adding this transistor.

For projects requiring a lot of power this makes the LM317 a better choice.

Circuit Working:

Parts List:

With a 33V DC input the LM317 functions as a voltage regulator producing an output voltage that can be adjusted between 1.25V and 30V.

Resistors R1 and R2 are configured to set the desired output voltage in order to perform this.

The base of the TIP35 transistor is connected to the LM317 output in an emitter follower configuration.

The emitter voltage closely follows the base voltage in an emitter follower setup.

When the LM317 modifies its output voltage the TIP35s base voltage also changes.

The reason for this is that the TIP35s base is connected to the LM317 output.

Based on the values of the resistors the LM317 controls its output voltage which affects the TIP35s base voltage.

The TIP35s emitter voltage closely follows the base voltage because of its emitter follower design.

As a result the emitter voltage of the TIP35 changes in connection with changes in the LM317 output voltage.

The TIP35s collector is connected to the input supply with a high current.

A proportionate amount of current is allowed to flow from the TIP35s collector to the emitter when the emitter voltage changes.

This allows a major increase in the output current because the collector current exceeds nearly the load current.

By working as a current amplifier the TIP35 may increase the LM317s limited output current of 1.5 amps to as much as 10 amps based on the TIP35s specifications.

Accurate voltage regulation is ensured by the LM317 and the TIP35 increases current without changing the LM317s voltage regulation parameters.

The circuits safety and steady functioning are improved with resistors, capacitors and with proper heat sinks.

Formulas:

There are two main formulas linked high current adjustable LM317 power supply circuit which are mentioned below:

Output Voltage (Vo):

This formula is used to measure the LM317 circuits adjustable output voltage.

The values of two resistors R1 and R2 connected in series are essential to the voltage divider design.

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 which is around 100uA

Current Limit for external transistor:

To increase the LM317s capacity to handle current the circuit includes an external transistor TIP35C.

By limiting the transistors base current the resistor Ry also reduces 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 roughly around 0.7V
• I_limit is the required 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 selected has a rating that matches to the circuits overall power dissipation.

How to Build:

To build a High Current Adjustable LM317 Power Supply Circuit follow the following steps:

Connect Input Voltage:

The LM317 input pin should be connected to the 33V DC input voltage.

Output Voltage Adjustment:

To select the correct output voltage connect resistors R1 and R2.

To get resistor values use an online calculator or the LM317 datasheet.

Output Connection:

Connect the LM317 output pin to the load.

Capacitors for Stability:

Capacitors should be connected across the LM317s input and output for safety.

An input of 0.1µF and an output of 10µF are common choice for capacitors.

Add TIP35 Transistor:

The TIP35 transistors base should be connected to the LM317 output pin.

The output pin of the LM317 is connected to the TIP35 transistors emitter.

Also the TIP35s collector is connected to the high current input source.

Heat Sinks:

Connect heat sinks to the IC LM317 and transistor TIP35 to distribute heat produced during operation.

Final Steps:

• Verify all component values and connections at least twice.
• Use the 33V DC input to supply power to the circuit.
• Increase the output voltage slowly while keeping an eye on how it acts.
• Make sure the components remain within the set temperature range and keep an eye on heat generation.
• To make sure the circuit is stable and performing well test it with different loads.

Conclusion:

A flexible and efficient solution for applications seeking a variable voltage source that is capable producing a higher current output is provided by the TIP35 transistor in the LM317 adjustable high current power supply circuit.

References:

LM317 3-Terminal Adjustable Regulator datasheet

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