# G6EJD

(Home) David Bird 2024

# 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. When a load increases and demands more power, the power supply charge referred to as Q (in Coulombs) required from the capacitor is Q = I * t, where I is current and t is time.
2. Also, Q = C * V, where C is the capacitance and V is the voltage drop as current flows out to the load.
3. So rearranging and converting Q into energy-time, then C * V = Q = I * t and rearranging for C gives C = I*t/V
4. Lets assume you need no-less than 0.2v drop on the DC supply when a 0.5Amp load is applied, which lasts for 0.05sec (50mS), then the value of C required is found as follows:

C = 0.5 * 0.05/0.2 = 0.125F = 125,000uF

It’s a big capacitor for this example, but that’s what’s required. A more typical example might be:

Lets assume you need no-less than 0.2v drop on the DC supply when a 50mA load is applied, which lasts for 0.005sec (5mS), then the value of C required is found as follows:

C = 0.05 * 0.005/0.2 = 0.000125F = 125uF and practically we’d choose 100uF

Large tubular often referred to as axial capacitors are very inductive, and so do-not lend themselves to radio frequency decoupling/filter. Always use ceramic or tantalum types.

Also, capacitors tend to have a finite life, the life will vary with quality of the materials used, but don’t expect a life of more than 10-years before they begin to fail.