Understanding Capacitance
Capacitance measures a capacitor's ability to store electrical charge when voltage is applied across it. Formally, it is the ratio of stored charge (Q) to the voltage (V) across the device. The unit of capacitance is the farad (F), though practical capacitors are typically rated in microfarads (μF), nanofarads (nF), or picofarads (pF).
In circuit design, capacitance determines how much energy a component can hold and release. A larger capacitance value means the capacitor can store more charge at the same voltage. This property is crucial when selecting capacitors for motor start circuits, where a burst of energy is needed to overcome initial inertia.
The Capacitor Size Formula
The capacitor size calculation stems from the relationship between stored energy, capacitance, and voltage. Rearranging the energy storage equation gives us the formula for finding capacitance:
C = (2 × E) / V²
or equivalently:
C = E / (0.5 × V²)
C— Capacitance in farads (F)E— Start-up energy in joules (J)V— Voltage in volts (V)
Practical Example
Consider an electric motor requiring 16 volts and 64 microjoules (64 × 10⁻⁶ J) of start-up energy. Using the formula:
- C = 64 × 10⁻⁶ / (0.5 × 16²)
- C = 64 × 10⁻⁶ / 128
- C = 0.5 × 10⁻⁶ F
- C = 0.5 μF
This means you would select a 0.5 microfarad capacitor rated for at least 16 volts. In real applications, you might choose the next standard value available, such as 0.47 μF or 0.56 μF, which are common commercial ratings.
Standard Capacitor Codes and Values
Electronics manufacturers use a standardized coding system to mark capacitor values. A three-digit code indicates the capacitance in picofarads before conversion to larger units. For example:
- Code 100: 10 pF or 0.01 nF
- Code 102: 1,000 pF or 1 nF
- Code 152: 1,500 pF or 1.5 nF
- Code 472: 4,700 pF or 4.7 nF
- Code 333: 33,000 pF or 33 nF
When shopping for capacitors, voltage rating is equally important. The voltage rating represents the maximum potential difference the capacitor can safely withstand. Always select a capacitor with a voltage rating equal to or greater than your circuit's operating voltage.
Common Pitfalls When Sizing Capacitors
Avoid these mistakes to ensure reliable circuit operation and component longevity.
- Underestimating voltage headroom — Never select a capacitor rated exactly at your circuit voltage. Environmental factors, voltage spikes during start-up, and component aging all reduce the safety margin. Choose a capacitor rated at least 25–50% higher than your nominal voltage.
- Ignoring energy transient demands — The start-up energy value must account for actual motor inrush current, not theoretical minimums. Measure or consult datasheets to find realistic energy requirements. Undersizing creates insufficient charge storage and poor motor acceleration.
- Confusing capacitance units — Mistakes between picofarads, nanofarads, and microfarads are common. A 100 pF capacitor is 0.0001 μF—vastly different in function. Always confirm the unit and convert carefully before purchasing components.
- Overlooking capacitor series and parallel effects — When multiple capacitors are connected in series, total capacitance decreases. In parallel, it adds directly. If your circuit uses multiple capacitors, recalculate the combined value rather than assuming a simple addition.