Calculate Capacitor Values

When you need to smooth a DC power supply, you need to add so-called smoothing capacitors and you calculate the required capacitor value like this:

  1. The power supply charge Q (in Coulombs) required from the capacitor is Q = I * t, where I is current and t is time.
  2. Also, Q = C * ΔV, where C is the capacitance and ΔV is the voltage drop as current flows out to the load.
  3. So C * ΔV = Q = I * t and rearranging for C gives C = I*t/ΔV
  4. Lets assume you need no-less than 0.2v drop on the DC supply when a 1Amp load is applied that lasts for 0.05sec (50mS), the value of C required is found as follows:

C = 1 * 0.05/0.2 = 0.25F = 250,000uF