LM350 3A Adjustable Voltage Regulator

LM350 3A Adjustable Voltage Regulator

The LM350 is a proven 3A adjustable voltage regulator. It accepts input voltages up to 35V and provides output voltages from 1.25V to 33V. Its durability is beyond imagination. While a fairly large buck converter smokes and burns out, the LM350 only takes a short break. And a short break is far better than a wisp of smoke. The LM350 can also be used for current limiting, but this requires a separate LM350/LM317 in series with our voltage regulator.

I needed an extremely low-noise variable power supply for a trivial project, and after testing several buck converters on an oscilloscope, I was quite annoyed. They were fine, but incredibly noisy. This circuit required as little noise as possible, and to make matters worse, my modified bench power supply was twice as noisy. The M317's absolute maximum current limit of 1.5A was far too small for my project, so I got my hands on the LM350 and was immediately satisfied.

The LM350 accepts input voltages up to 35V, outputs 3A of current, and provides an output voltage range of 1.25V to 33V. It enters a temporary slump if overheated and requires severe abuse to fail. As an old-school voltage regulator, it does require some cooling, but heatsinks are inexpensive. For this variable voltage regulator, a heatsink capable of dissipating 5 watts is used. It's best to keep the input voltage as close to the output voltage as possible because the formula 'P_dissipate = (Vin – Vout) * Iload' tells us that, for example, (12V – 5V) * 0.5A = 3.5 watts must be dissipated. And this is just the standard effect from a USB 2.0 port regulated from 12V. With P_dissipate = (6.5V – 5V) * 0.5, we get 0.75 watts, which is a more reasonable waste of energy converted into heat and easily dissipated. The formula for calculating the output voltage is:

Vout = 1.25 * ( 1 + ( 150 / ( R1+10 ) ) = ?V

Note that there is a 10-ohm resistor in series with the trimmer potentiometer, producing a minimum output voltage of 1.33V.

The LM350T's pinout is: Pin 1: Adj(ust), Pin 2: Vout, Pin 3: Vin. While I've seen the LM350's power output perfectly clean under load, we'll add some capacitors to trim the input and output. These capacitors absorb noise, and while not absolutely necessary, each one helps. We use a 0.33uF capacitor (C1) on Vin and a 1uF electrolytic (polarized) capacitor (C2) on Vout to improve transient response. The LM350 really likes having a 150-ohm resistor (R1) between Adj and Vout. For R2, we use a 10-ohm resistor in series with a 10K-ohm trimmer potentiometer to achieve a variable output voltage. You can use the lm317_calc.py script from our 12V battery charging controller article. We don't use any protection diodes here, but if you plan to use an output voltage greater than or equal to 25V, you should definitely include them.

For added convenience, a digital voltmeter has been added, but because it causes output current spikes (a few millivolts), it remains off until you press the small button next to the input terminal. Adjusting the small trim potentiometer while holding that button down is easy. Whenever you need to know the output voltage, simply press it again. The one used here has a slight deviation, 0.12V too high. This seems common unless it's an expensive LED voltmeter. If you want to avoid noise and permanently display the output voltage, you might consider using an analog voltmeter.

Required components

1 x LM350T Variable Voltage Regulator (TO-220 Package)

1 x Heatsink of any size (38x34x12.8mm TO-220 finned heatsink)

1 x TO-220 Insulating Thermal Pad + Plastic Sleeve

1 x 3mm nut + bolt

A drop of thermal paste

1 LED voltmeter (optional)

1 x Instantaneous push-button switch (optional)

2 x 2 port screw terminals

1 x 50x70mm perforated plate

1 x 150 ohm resistor for R1

A 1 x 10 ohm resistor is used for R2.1 (in series with R2.2, minimum output => 1.33V).

One 5-10 kΩ trimmer potentiometer for R2.2

1 x 0.33uF capacitor (50V) for C1

1 x 1uF electrolytic capacitor (50V) for C2

Follow the schematic; don't be shy. The LM350T can withstand 300 degrees Celsius for up to 10 seconds, which tastes like freedom. Ensure proper electrical insulation between the heatsink and the LM350T. After installation, test the continuity from the heatsink to the tab. If your multimeter doesn't have a continuity test, use a 200-ohm setting. Of course, if you don't mind having Vout on both the tab and pin 2, you can skip this step.

You may have noticed the silicone spots on the sample board. There's a reason for this—especially since trimmer potentiometers and capacitors can't withstand slight movements for extended periods. Your solder joints may be strong, but even the malleable metal used for the pins will eventually succumb to metal fatigue. When they break, you have to repair it. This board sits in my special "everything box," which I frequently shake, rummage through, etc. By applying some unsightly glue spots, you ensure nothing falls apart. The higher the temperature, the better the bond!

If you followed the schematic, you now have a pretty decent little power supply. I retired my weak buck-boost power supply after I started using this. The device will benefit from a larger heatsink, but it's not designed for permanent installation—at most, I'll draw 500-750mA of continuous current. Remember: the output current is so clean it can even be used in RF applications. If your circuit is behaving erratically and you suspect noise is the culprit, try using this to power it—HopeRF 433/868MHz and longer-range LoRa SX1278 radio modules will benefit greatly.