Introduction: LM317 Current Boosting Secrets!


LM317 is one of the most popular adjustable regulator chips. The output voltage of the regulator can be adjusted from 1.25V to 35V. However, the chip can deliver currents up to 1.5A which is not enough for some power applications. In this article, I will discuss two methods of LM317 current boosting, using power PNP and NPN pass transistors.

[A] Circuit Analysis

According to the LM317 datasheet: “The LM317 [1, 2] device is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. It requires only two external resistors to set the output voltage. The device features a typical line regulation of 0.01% and a typical load regulation of 0.1%. It includes current limiting, thermal overload protection, and safe operating area protection. Overload protection remains functional even if the ADJUST terminal is disconnected.” This information proves to us that this cheap 3-terminal device is suitable for many applications but it comes with a drawback for power applications and that is the limitation of the regulator’s output current handling (1.5A in the best conditions). This problem can be solved using a pass power transistor.

[A-1] Current Boosting Using a PNP Power Transistor (MJ2955)

Figure-1 shows the schematic diagram of the circuit. This is an adjustable high current regulator circuit that the output voltage can be adjusted using a 5K potentiometer.

Step 1: Figure 1: LM317 Current Boosting Circuit Using MJ2955

The 10R resistor defines the Turn-On time of the pass-transistor and by the way, it defines how much current should passe through the LM317 and MJ2955 [3, 4]. Based on this parameter, the power rate of the resistor must be calculated. 1N4007 is a protective diode and 270R resistor provides the necessary ADJ pin’s current. As mentioned earlier, the 5K potentiometer defines the output voltage. 1000uF, 10uF, and 100nF capacitors have used to reduce noises. Don’t forget to install the transistor on a big heatsink.

[A-2] Current Boosting Using an NPN Power Transistor (2N3055)

Figure-2 shows the schematic diagram of the circuit. The 10K resistor at the output draws a small amount of current to avoid the floating output and it helps to stabilize the output voltage. Here 2N3055 [5, 6] plays the role of pass-transistor also.

Step 2: Figure 2: LM317 Current Boosting Circuit Using 2N3055

[B] PCB Board

The schematic diagrams are simple, so I decided to implement them on a prototyping board to test and show the operation. I decided to test the figure 1 (MJ2955 boosting). It has been demonstrated in figure 3.
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Step 3: Figure 3: Boosting Circuit Implementation Using an MJ2955

[C] Test and Measurements
You can watch the complete test process in the video, however, I also put an oscilloscope captured image from the circuit output. I used the Siglent SDS1104X-E oscilloscope that offers a nice low noise front end. I intended to measure the possible output ripple of the circuit. Figure 4 shows the output noise/ripple of the MJ2955 current boosting circuit. The circuit has been built on the prototyping board and the oscilloscope’s probe ground connection has been made through the ground lead, so these high-frequency noises are normal. If you plan to use any of these two circuits, design a proper PCB for it, then replace the probe’s ground lead with a ground spring, then you can re-examine the output noises.

Step 4: Figure 4: Oscilloscope Capture From the Current Booster Output (read the Text)