First Function: DC Isolation

The term “DC isolation” refers to the functionality of a high-pass filter. Here, “high-pass” means that high-frequency signals can pass through while low-frequency signals have more difficulty passing, and DC signals cannot pass at all.

This can be understood from Equation 3: The lower the frequency, the greater the capacitive reactance; conversely, the higher the frequency, the smaller the capacitive reactance. This is essentially the behavior of a high-pass filter.

It’s important to note the circuit conditions here: the capacitor comes before the load resistor. From Equation 2, the voltage across the resistor is:

Here, the RC time constant appears. What kind of effect does it have?

Second Function: Reactive Power Compensation

We know that the current of an inductor lags behind the voltage, while the current of a capacitor leads the voltage. This can be observed from Equation 2.

Let’s assume the voltage is [image]. Substituting this into Equation 2:

The current becomes a cosine function. When t=0, the sine value is zero, while the cosine value is 1. Therefore, when the voltage is zero, the current reaches its maximum value. Thus, the current passing through the capacitor leads the voltage by 90 degrees.

In power distribution systems, loads are generally inductive, causing the current in the system to lag behind the voltage, resulting in reactive power. Therefore, power distribution systems often incorporate parallel compensation capacitors to improve the power factor of the system and reduce the extent of current lagging behind voltage.

This use of capacitors is called reactive power compensation.

Third Function: Filtering

Filtering showcases the capacitive integration effect on signals.

How is a filtering capacitor designed? It’s quite simple. Let’s assume an output voltage of a certain regulated power supply is 12V with a current of 2A, yielding a load resistance approximation of 6 ohms (12/2 = 6).

When the load changes, we want the output voltage of the regulated power supply to remain relatively constant. Taking 5 times the time constant as 100 milliseconds, we have:

Choosing a nominal capacitance of 3300 microfarads for filtering is sufficient. However, the capacitance should have an adequate voltage rating.

Often, a 0.01 microfarad capacitor is also connected in parallel next to the filtering capacitor to eliminate high-frequency interference signals.

Fourth Function: Oscillator Construction

Oscillators are familiar to us, such as sine wave oscillators, square wave oscillators, sawtooth wave oscillators, etc. These oscillator configurations cannot do without capacitors.

Fifth Function: Voltage Reduction

Capacitive reactance is utilized to lower voltage. This is commonly used in chargers.

Reprinted from Connector Connection Wire Network WeChat Official Account.

Elaine Lin

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