Electronics Calculator

Multiply the resistance in ohms by the capacitance in farads. A 10 k-ohm resistor with a 100 microfarad capacitor gives a time constant of 1 second. The capacitor reaches about 63.2% of the supply voltage after one time constant and 99.3% after five time constants - so this RC pair settles in roughly 5 seconds.

The resonant frequency equals 1 divided by (2 pi times the square root of L times C). A 10 mH inductor paired with a 100 nF capacitor resonates at about 5033 Hz. At resonance the inductive and capacitive reactances cancel out, leaving only the parasitic resistance to limit current flow.

Energy equals 0.5 times capacitance times voltage squared. A 1000 microfarad capacitor charged to 50 V stores 1.25 joules. This is why large capacitors in power supplies must be discharged before servicing - a 450 V, 680 uF cap stores over 68 joules, enough to deliver a dangerous shock.

Divide the battery capacity in milliamp-hours by the average current draw in milliamps. A 2000 mAh battery powering a circuit that draws 50 mA lasts roughly 40 hours. In practice, expect 80-90% of rated capacity due to voltage cutoff thresholds and internal resistance losses at higher discharge rates.

The cutoff frequency of a first-order RC low-pass filter is 1 divided by (2 pi times R times C). For a 1 kHz cutoff, a 10 k-ohm resistor needs a 15.9 nF capacitor. The signal is attenuated by 3 dB at the cutoff frequency and rolls off at 20 dB per decade beyond it.