High Current Adjustable LM317 Power Supply Circuit

In this post IC LM317 is chip for controlling voltage but it give only 1.5 amp current.

We can add one more part called TIP35 transistor with LM317.

When LM317 tell it and this transistor act like a follower.

Now with this transistor LM317 can handle big current up to 10 amp.

This article for High Current Adjustable LM317 Power Supply Circuit helps in big power projects to make LM317 more good choice.

Circuit Working:

Parts List:

With 33V DC input LM317 work as voltage regulator.

It give output voltage from 1.25V to 30V which we can adjust it.

Resistor R1 and R2 are used to set how much voltage come out.

TIP35 transistor base is connected to LM317 output like emitter follower.

In this setup emitter voltage follow base voltage almost the same.

When LM317 change output voltage TIP35 base voltage also changes.

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

LM317 control its output voltage with R1 and R2 so this also changes TIP35 base voltage.

Because of emitter follower design the TIP35 emitter voltage follow base voltage.

So when LM317 output changes TIP35 emitter also change same way.

Collector of TIP35 connect to high current input supply.

When emitter voltage changes same current passes from collector to emitter.

This make output current go high which is much more than LM317 can give alone.

TIP35 act like current amplifier and increases LM317 1.5A to maybe 10A which depends on TIP35 power.

LM317 keep voltage stable and TIP35 just help to give more current without changing voltage control.

For safe and good working we used resistors, capacitors and heat sinks in circuit.

Formulas:

This LM317 power supply circuit have two main formula for high current and adjustable voltage:

1. Output Voltage (Vo):

This formula help to find LM317 output voltage.

Two resistors R1 and R2 in series make voltage divider.

Formula:

Vo = 1.25V + (R2 × Iadj)

where:

• Vo is output voltage
• 1.25V is inside fixed voltage of LM317
• R2 is variable resistor value in ohms
• Iadj is small current from adjust pin to around 100uA

2. Current Limit for external transistor:

To give more current TIP35C transistor used with LM317.

Ry resistor control base current of transistor so it also control output current.

Formula:

Ry = Vbe / Ilimit

where:

• Ry is value of resistor in ohms
• Vbe is voltage between base and emitter of TIP35C which is around 0.7V
• Ilimit is how much max current we want in amps

Note:

For good working R1 value should be between 120 and 240 ohms.

LM317 alone gives only 1.5A current.

With transistor it can give more current but more heat also come.

Always use good heat sink for safe use.

How to Build:

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

Connect Input Voltage:

• Connect 33V DC to input pin of LM317.

Output Voltage Adjustment:

• Use resistor R1 and R2 to set output voltage.
• We can find right resistor values using LM317 datasheet or online calculator.

Output Connection:

• Connect LM317 output pin to the load.

Capacitors for Stability:

• Put capacitors on input and output of LM317 for safety and smooth working.
• Use 0.1µF on input side and 10µF on output side and these values are common.

Add TIP35 Transistor:

• Connect base of TIP35 to LM317 output pin.
• Connect emitter of TIP35 to output side.
• Connect collector of TIP35 to high current 33V input.

Heat Sinks:

• Put heat sinks on LM317 and TIP35 to keep them cool when working.

Final Steps:

• Check all parts and connections one more time.
• Use 33V DC to give power to circuit.
• Increase output voltage slowly and watch how circuit work.
• Be sure parts do not get too hot just keep an eye on temperature.
• Try different loads to test if circuit is working strong and stable.

Conclusion:

This High Current Adjustable LM317 Power Supply Circuit is good and flexible for project where we need changeable voltage and more current output.

References:

LM317 3-Terminal Adjustable Regulator datasheet

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