Capacitor Size Calculator

A smoothing capacitor holds up the output voltage between rectified peaks.

How the Math Works

The Capacitor Size Calculator uses the fundamental formula C = I·t ÷ ΔV to determine the required capacitance for smoothing a power supply output. This equation relates four key electrical parameters: capacitance (C) in farads, load current (I) in amperes, time interval (t) in seconds, and voltage ripple (ΔV) in volts. The physics behind this formula stems from the relationship between charge, current, and time - specifically, that a capacitor stores charge Q = C·V, and current is the rate of charge flow I = dQ/dt. When a capacitor discharges through a load, the voltage decreases linearly over time, and the formula calculates exactly how much capacitance is needed to limit this voltage droop to an acceptable amount.

Practical Applications

To use this calculator in practice, first determine your circuit's maximum load current in amperes and decide on the maximum acceptable voltage ripple (typically 1-2% of the DC output voltage). Calculate the time interval by finding the period of your AC input (for 60Hz, this is 1/120 seconds for a full-wave rectifier). Input these three values into the formula to get the required capacitance in farads. You can then select the nearest standard capacitor value from available components. For example, a 5V power supply delivering 2A with 0.5V ripple at 120Hz would require C = 2·(1/120) ÷ 0.5 = 0.033 farads or 33,000µF. Always verify your selection can handle the expected ripple current and voltage ratings.

Day-to-Day Use

This calculation helps you understand and improve the performance of everyday electronic devices that use wall adapters, phone chargers, and computer power supplies. By properly sizing the smoothing capacitor, you eliminate the 'humming' noise that occurs when voltage ripple is too high, ensuring your devices operate quietly and efficiently. This knowledge helps when repairing electronics, building DIY projects, or simply choosing the right replacement power supply - you can verify whether a device's power brick is adequate for your needs or if it's causing instability in sensitive electronics. Understanding this fundamental concept also helps you make informed decisions when purchasing devices, as properly designed power supplies run cooler, last longer, and don't interfere with audio/video equipment.

Worked example

1 A, 10 ms period, 1 V ripple → 10,000 µF.

FAQ