78xx Voltage Regulator Calculator

The 78xx family are three-terminal fixed positive voltage regulators. The last two digits indicate the output voltage: 7805 outputs 5V, 7812 outputs 12V. They require only two external capacitors — 0.33 µF on the input and 0.1 µF on the output — for stable operation.

The input voltage must exceed the output by at least the dropout voltage, typically 2V for standard 78xx parts. Any excess voltage is dissipated as heat: P = (Vin − Vout) × Iload. This is why a 7805 powered from 24V gets extremely hot — it dissipates 19V × Iload as pure heat.

For applications where the voltage difference or current is large, consider an LM317 (adjustable) or a switching regulator (much higher efficiency). Low-dropout (LDO) versions of 78xx exist with dropout as low as 0.5V, useful when Vin is only slightly above Vout.

Key Points

• Dropout voltage: ~2V (Vin must be ≥ Vout + 2V)
• Maximum output current: 1.5A with adequate heatsink
• Always add 0.33 µF input and 0.1 µF output capacitors
• Junction temp must stay below 125°C — add heatsink if P > 1W

Applications

• Fixed 5V/12V power supplies for digital circuits
• Arduino and microcontroller power regulation
• Post-rectifier regulation in AC-DC supplies
• Replacing batteries with regulated wall adapter power