What is power dissipation in a linear regulator?

Power dissipation (Pd) is the heat generated by the regulator IC itself. It equals (Vin − Vout) × Iload. This energy is wasted as heat and must be removed by the package, a heat sink, or both. Exceeding the IC's thermal limit causes thermal shutdown or permanent damage.

How do I calculate the LM317 output voltage?

Vout = 1.25 × (1 + R2/R1) + Iadj × R2. Because Iadj is only ~50 µA, the simplified formula Vout ≈ 1.25 × (1 + R2/R1) is accurate enough for most designs. The standard reference resistor R1 is 240 Ω.

What is the minimum input-to-output voltage difference (dropout)?

The LM317 and LM78xx require roughly 2–3 V of headroom (dropout voltage) between Vin and Vout. If Vin − Vout falls below the dropout spec the regulator loses regulation. Low-dropout (LDO) regulators reduce this to 0.1–0.5 V.

When do I need a heat sink?

A rough rule: if Pd > 1 W a heat sink is recommended for a standard TO-220 package. The junction-to-ambient thermal resistance of a bare TO-220 is ~50 °C/W, so 1 W raises the junction ~50 °C above ambient. At 25 °C ambient that is 75 °C — acceptable, but near the limit for continuous operation.

Why is efficiency low in a linear regulator?

Efficiency = Vout / Vin × 100 %. The regulator turns the voltage difference (Vin − Vout) directly into heat. A 12 V supply stepping down to 5 V is only ~42 % efficient. Switching regulators (buck converters) achieve 85–95 % efficiency for the same conversion but are more complex.

What is the maximum output current for LM317 and LM7805?

Both the LM317 and LM7805 in a TO-220 package are rated for up to 1.5 A continuous output current, subject to thermal limits. At higher currents the power dissipation rises quickly — always check Pd against the allowed junction temperature with your heat sink.

Voltage Regulator Calculator

Name: Voltage Regulator Calculator
Availability: InStock
Author: Nham Vu

Enter input voltage, output voltage, and load current to get power dissipation, efficiency, and resistor values for LM317 or LM78xx linear regulators.

Regulator Parameters

Regulator Type

Output Voltage (Vout)

Input Voltage — Vin (V)

Load Current — Iload (mA)

Ambient Temperature (°C)

Enter regulator parameters and click Calculate

Power dissipation, efficiency, and resistor values appear here.

Summary

Enter input voltage, output voltage, and load current to get power dissipation, efficiency, and resistor values for LM317 or LM78xx linear regulators.

How it works

Select the regulator type: Fixed (LM78xx) or Adjustable (LM317).
Enter the input voltage (Vin), desired output voltage (Vout), and load current (Iload).
For LM317, set R1 (typically 240 Ω) and the calculator solves for the required R2.
Click Calculate to see power dissipation, efficiency, voltage drop, and junction temperature rise.
Review the heat sink guidance: if Pd exceeds ~1 W the IC will need a heat sink.
Adjust input voltage or current to explore the efficiency trade-off.

Use cases

Size a heat sink for a 5 V bench supply built around an LM7805.
Verify an LM317 resistor divider produces the correct output voltage.
Estimate regulator efficiency when converting 12 V down to 3.3 V.
Check whether a TO-92-packaged regulator can survive at a given load current.
Teach regulator basics: show students why low-dropout regulators save power.
Validate datasheet examples for LM317 or LM78xx circuits.
Compare dropout penalty at different input voltages for the same load.
Quickly find R2 when R1 is fixed at the standard 240 Ω reference resistor.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu