Linear Voltage Regulator Calculator (78xx / LDO)

Power Dissipation & Efficiency

P_diss = (V_in − V_out) × I_out + V_in × I_Q • Efficiency η = P_out / P_in = (V_out × I_out) / (V_in × I_in)

7805: 12V→5V, 500mA

9V→3.3V, 250mA

LDO 5V→3.3V, 500mA

Enter values and press Calculate.

Junction Temperature & Max Current

T_J = T_A + P_diss × θ_JA • I_out(max) = (T_J(max) − T_A) / (θ_JA × (V_in − V_out))

TO-220 no heatsink (65°C/W)

TO-220 + heatsink (10°C/W)

Enter values and press Calculate.

How a Linear Regulator Dissipates Power

A linear regulator (like the 7805) works by dropping the difference between input and output across an internal pass transistor. That voltage difference × the load current becomes heat. Because the input and output currents are almost equal, the efficiency is roughly V_out/V_in — so a big input-to-output gap wastes a lot of power.

Quantity	Formula
Power dissipated	P_diss = (V_in − V_out) × I_out + V_in×I_Q
Efficiency	η = (V_out×I_out) / (V_in×I_in)
Junction temp	T_J = T_A + P_diss×θ_JA
Max load current	I_out(max) = (T_J(max)−T_A)/(θ_JA×(V_in−V_out))

To carry more current, add a heat sink (lower θ), reduce the input-output gap, or switch to a buck converter for high-power designs.

Real-World Applications

Microcontroller supplies: a 7805 turning 9–12 V into a clean 5 V for Arduino and sensor boards.
Low-noise analog/RF rails: LDOs post-regulating a switching supply to remove ripple for ADCs, op-amps, and radios.
Reference & bias rails: a steady 3.3 V or 5 V for logic and reference circuits.
Battery devices: LDOs with tiny dropout squeeze more runtime from a falling battery.

Worked example

Powering a 5 V Arduino drawing 200 mA from a 9 V battery with a 7805:
P_diss = (9 − 5) × 0.2 = 0.8 W, and efficiency is only 5/9 ≈ 56%. On a bare TO-220 (≈65 °C/W): T_J = 25 + 0.8 × 65 ≈ 77 °C — acceptable, but a small clip-on heat sink adds margin. At 500 mA it would overheat, so a buck converter would be the better choice.

FAQ

Linear or switching regulator?

Linear regulators are cheap, quiet, and simple but waste the voltage difference as heat. Switching (buck) regulators are >90% efficient but noisier and more complex. Use linear for small drops/low current, switching for big drops or high power.

Do I need capacitors on a 7805?

Yes — typically 0.33 µF on the input and 0.1 µF (often plus 10 µF) on the output to stay stable and quiet. LDOs are pickier and specify a minimum output capacitor and ESR range in the datasheet.

What is an LDO?

A Low-DropOut regulator works with a much smaller input-output difference (often <0.3 V versus ~2 V for a 78xx), so it wastes less power and suits battery circuits.

Can I parallel two regulators for more current?

Not directly — small differences make one hog the load. Use a single higher-current regulator, add a pass transistor, or use a design made for current sharing.

Why does my 7805 get so hot?

All the input-output voltage difference times the load current turns into heat. 12V→5V at 0.5A dissipates 3.5 W — that needs a heat sink.

What is dropout voltage?

The minimum V_in−V_out the regulator needs to keep regulating (≈2 V for a 78xx, <0.5 V for an LDO). Below it, the output sags.

When should I use a buck converter instead?

When efficiency or heat matters. A linear regulator wastes the voltage difference as heat; a switching (buck) converter can be >90% efficient.