78xx Voltage Regulator Calculator
Calculate power dissipation, heatsink needs, and capacitor values for 7805/7809/7812/7815 regulators.
Component Values
Results
⚠ P > 1W — heatsink recommended
⚠ Junction temperature too high — reduce Vin or Iload
How do 78xx regulators work?
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.
Power Dissipation
P = (Vin − Vout) × IloadKey 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
Practical Examples
7805 regulator with 12 V input, 5 V output, 500 mA load. Calculate power dissipation and efficiency.
Pdiss = (12 − 5) × 0.5 = 3.5 W · Efficiency = 5/12 = 41.7% → heatsink required
LDO regulator (Vdropout = 0.3 V) providing 3.3 V to a microcontroller at 200 mA from a USB 5 V rail.
Pdiss = (5 − 3.3) × 0.2 = 0.34 W · Efficiency = 3.3/5 = 66% · SOT-223 package sufficient
Did you know? The 7805 fixed voltage regulator was introduced in the 1960s and is still in production today. Its rugged TO-220 package and simple 3-pin interface made it the most widely used linear regulator for over 50 years, despite its low efficiency.