lm350 calculator

What this LM350 calculator helps you do

The LM350 is a classic adjustable linear regulator capable of supplying up to 3A with proper cooling. It is popular in bench supplies, battery chargers, and analog projects because it is simple, robust, and easy to find. The challenge is that “simple” doesn’t always mean “foolproof.” You still need to choose resistor values correctly, leave enough input headroom, and manage heat dissipation.

This calculator focuses on the practical engineering questions:

• What R2 resistor gives me my desired output voltage?
• Is my input voltage high enough considering dropout?
• How much power will the regulator burn as heat?
• Will junction temperature likely exceed safe limits?
• What happens if I use the nearest standard resistor value?

Core LM350 equation

For the adjustable LM350 configuration, output voltage is set by a two-resistor divider:

Vout = Vref × (1 + R2/R1) + Iadj × R2

Where Vref is approximately 1.25V and Iadj is typically small (often around 50µA). In many quick designs, the Iadj term is ignored. This calculator includes it so your result is a little more realistic.

Why R1 is often 240Ω

Using 240Ω for R1 guarantees enough current through the divider network for stable regulation. It also matches common reference designs from regulator datasheets.

How to use the calculator

1. Enter your expected input voltage under load (not just no-load transformer voltage).
2. Enter your desired output voltage.
3. Add your expected load current.
4. Keep R1 at 240Ω unless you have a specific reason to change it.
5. Set thermal resistance based on your heatsink/case setup.
6. Click Calculate and review warnings, not just the resistor value.

Thermal reality: linear regulators are heaters

Linear regulators waste voltage difference as heat. The basic estimate is:

Pd = (Vin − Vout) × Iload

Example: If Vin = 18V, Vout = 5V, and load current is 2A:

Pd = (18 − 5) × 2 = 26W. That is a lot of heat for a small package and absolutely demands strong thermal design.

Then estimate junction temperature:

Tj = Tambient + (Pd × θJA)

If Tj rises too high, the regulator may enter thermal shutdown, degrade performance, or fail over time.

Common LM350 design mistakes

• Insufficient headroom: Vin must exceed Vout by dropout voltage plus ripple margin.
• Ignoring transformer/supply sag: “Nominal” input often drops under real load.
• No heatsink or undersized heatsink: thermal shutdown appears as unstable output.
• No protection diodes: output/input capacitor discharge paths can damage the regulator.
• Expecting switching-regulator efficiency: linear regulation is simple, but not efficient at high drop.

Practical resistor selection tips

Use standard resistor values

The calculator reports an ideal R2 and also a nearest E24 value. In most projects, the nearest standard value is perfectly acceptable. If you need tighter accuracy, use 1% resistors or combine two resistors in series/parallel.

Trim for precision

For adjustable bench outputs, place a potentiometer in series with a fixed resistor for safer adjustment range and better control.

Final checklist before building

• Confirm Vin(min) still satisfies dropout at full current.
• Verify thermal calculations for worst-case input and ambient temperature.
• Use proper input/output capacitors recommended in datasheets.
• Add reverse-protection diodes when large capacitors are used.
• Test with dummy load before connecting sensitive electronics.

Done right, the LM350 remains one of the easiest and most dependable ways to build a clean adjustable supply. Use this calculator as a first-pass design tool, then validate with real measurements on your hardware.