Understanding AC Capacitance in Your Circuit Simulations

Key Takeaways

• Understand what  AC capacitance is

• Learn how capacitors work in an AC circuit

• Explore applications of AC capacitors in real environments

AC capacitance defines how the capacitor store charges.

Have you ever blown up a capacitor? I did, and it was not a pretty sight. One of my early days in the electronics industry I wondered what would happen when a capacitor is connected to an AC source. The problem was that I was connecting a polarized electrolytic capacitor instead of an AC capacitor.

A loud popping sound is quickly followed by a whiff of mushroom-like smoke whatever remains of the capacitors are splattered around. My team leader wasn’t amused by such a careless mistake done by a newbie. However, that incident sparked my interest in capacitors and I started adding terms like AC capacitance into my vocabulary.

What Is AC Capacitance 

Every electronics designer is likely to be familiar with capacitors. A capacitor is made of two conductive plates separated by a layer of dielectric. The construction of a capacitor allows it to store electric charges when voltage is applied to its terminals.

The amount of charge stored by a capacitor compared to the voltage applied is termed capacitance. Capacitance is measured in Farad (F) and is given by the formula:

C = Q/V. 

A higher value means the capacitor is able to store more electric charges when a voltage is applied. Capacitance is a value that electronics designers pay attention to in both DC and AC circuits. However, the behaviour of the capacitor differs in both types of circuits.

How a Capacitor Reacts in an AC Circuit

When a non-polarized capacitor is connected to an AC source, it acts as a closed-circuit but changes the phase of the voltage. As the AC power is turned on, the positive cycle will charge the capacitor from one terminal until the charge is equal to the voltage. 

Once the AC voltage reaches the peak and drops, the capacitor will start discharging to maintain a constant voltage. During this stage, the current flows from the capacitor in a different direction. 

As the AC voltage swings into the negative, the capacitor is charged from the opposite direction. The process continues until the capacitor is fully charged by the negative voltage. Once the voltage reaches its negative peak, it will change into an upward swing. This also starts the discharge cycle of the AC capacitor. 

The cycles will continue as long as the AC source is applied. When values of the voltage and current are plotted, you’ll find that the current is leading the voltage by a 90° phase. 

When you’re designing with capacitors on an AC circuit, capacitance isn’t the only variable that you’ll need to be concerned with. Besides AC capacitance, you’ll also need to understand what is capacitive reactance.

Ideal capacitors store charges without leakage. However, real capacitors do have some amount of leakage due to a certain amount of resistance. The resistance in capacitors is termed as capacitive reactance with the formula:

X_C = 1/2πfC 

The capacitive reactance has an inverse relationship with the frequency of the AC voltage and the capacitance. A higher value on both parameters will lead to smaller reactance and thus, higher leakage. 

Applications of Capacitors in AC Circuit

AC capacitors are quite common in electronics design. They are used in split-phase induction motors to provide better efficiency and phase shift on the secondary winding. Usually, such applications can be found in water pumps and air conditioners.

AC capacitors are used in air conditioners.

You’ll also find AC capacitors in power supply designs. The capacitor helps to suppress noises and regulate the voltage. The ability of an AC capacitor to transfer store and release charges improves the efficiency of power transfer. Systems with inductive loads also benefit from AC capacitors. The AC capacitance helps with power factor correction as it compensates for current lag introduced by the inductive loads. 

When designing with an AC capacitor, it helps if you’re able to simulate and analyze its response with reliable software. With its accurate models and simulations, PSpice Simulator will help you to ensure the chosen capacitors are responding correctly in an AC circuit. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.