Application of RC circuit in PCB board

An RC circuit is a basic building block of modern electronics built on a PCB (Printed Circuit Board). It consists of integrated resistors and capacitors, among other elements. These circuits can be found in many electronic devices, such as LED modules, battery panels, computer motherboards, etc.

This blog will cover RC circuits in complete detail, including their basics, types, how an RC circuit charges a capacitor, its purpose, and more. So, keep learning!

1) What is an RC circuit?

“An RC circuit is an electrical circuit consisting of a resistor (R) and a capacitor (C) in series or parallel configurations.”

Such circuits are widely used in electronics because they can store and control potential and current over certain durations of time. The limiting resistor prevents excess current from flowing, while the capacitor provides electrical storage and discharge.

In an RC circuit, additional elements, such as the resistor in series and the capacitor, work together to create a time-dependent response to voltage (∆V). This increases their capacity for timing and voltage regulation techniques, among other applications. 

The operation of circuit boards is described by the “time constant τ (tau)”. It is the multiplicative combination of the resistance (R) and the capacitance (C). It relates to the response time of the given circuit to the applied change. Moreover, it is related to the rate of the capacitor charging or discharging practice.

2) Types of RC Circuit

There are generally two RC circuit board types;

• Series RC circuit
• Parallel RC circuit

i) Series RC circuit

“A Series RC Circuit can be described as a circuit in which the current flows through both the resistor and capacitor with the configuration such that these two components are joined end to end (one after another).” 

This type of circuit can be seen in some filtering applications such as the suppression and transmission of certain frequency ranges. As an illustration, it may be used as a low-pass filter where it allows low-frequency signals to pass through while those that are in the high-frequency range are suppressed. The phenomenon is also applied in timing applications to create a time delay since the capacitor will charge and discharge through the resistor at a certain rate. 

ii) Parallel RC circuit

“In the parallel RC configuration, the current flows through the circuit when both the elements (the resistor and the capacitor) are connected in such a way that they become parallel to each other.”

Such a design is also adopted when there is a need to make voltage regulation in power systems or filtering out high-frequency noises from various signals. It will practically behave as a high-pass filter, allowing high-frequency signals to pass while low-frequency signals are cut off or blocked.

Both series and parallel RC networks perform certain functions and have diverse applications in terms of detecting and utilizing AC and DC. Knowing these types is important because they help create some electronics and achieve effects in process tracking.

3) How does an RC circuit charge a capacitor?

The interaction between a resistor and a capacitor connected to a source of voltage, known as an RC circuit, helps to charge a capacitor. 

At first, when the circuit is open, the capacitor is uncharged and the potential difference across the two end of capacitor is zero. Now suppose the circuit is closed, there is a flow of current from the power source to the capacitor. Because capacitor has negatively charges electrons and you know negative attract positive (here positive= positive terminal of the power source). Current flows due to difference in potential difference. Remember!

“Current flows in the opposite direction of movement of electrons.”

So, the current flows from the power source to the capacitor. As the electron starts moving from the capacitor towards the power source, the capacitor starts gaining a positive charge. The electron will follow the path through the resistor, and power source, and finally will reach the other end of the capacitor. This phenomenon creates a positive charge at one end of the capacitor while the other end will be negatively charged. Clear!

One thing to consider here is that the current entering the capacitor is limited by the rate of current flow by the resistor. Okay! So the flow of current will fill the capacitor with electric charge over a period of the capacitor’s voltage upping. Notice that as the time will pass the rate of current will decrease because the potential difference between capacitor and power source is decreasing gradually (because the positive charge on capacitor is increasing).

Kirchhoff’s Voltage Law (KVL);                   

V=Vr​+Vc

Vr=I×R

Vc=CQ​

I=dQ​/dt

V=R(dQ​/dt) +Q/C

R(dQ​/dt)= V – Q/C

(dQ​/dt)= V/R – Q/RC

​dQ​/(V−Q/C)= (1​/R) dt

After iterating both sides and solving the integral, we get;

ln(V/(V−Q/C​))= t​/RC

Exponentiate both sides, and we will get;

q = CV (1- e-t/RC)

Finally, the time will come when the charge between power source and capacitor will be equal. Means that the potential difference between capacitor and power source will be zero. So, no current will pass through the circuit. At this point, the capacitor is referred to as fully charged.

It can be seen that the time of charging a capacitor is affected by the values of R and C known as the time constant (T = R * C). 

4) Impedance of RC Series Circuit

“The term impedance in the case of an RC series circuit is seen to mean the net total hinderance of the flow of alternating current.” 

It includes both resistance R provided by the resistor and some capacitor reactance Xc to the alternating current source. The reactance remains constant irrespective of the frequency of the input signal.

Impedance (Z) of an RC series circuit is defined as a complex quantity using the following expression:

Z= (R^2+(1/ωC)^2)^½

V/Z= V/ (R^2+(1/ωC)^2)^½

Xc​=1​/2πfC

Here Xc is capacitive reactance, f is the frequency of the ac signal, and C is capacitance. As the frequency rises, the capacitive reactance decreases consequently lowering the net impedance. However, at lower frequencies, the lower capacitance reactance increases the impedance.

Impedance has both ill and phase components. The ill is known as the total opposition which is impinged on the flowing of the current. The phase angle θ, on the other hand, depicts the angular displacement that exists between the voltage and current. It also depicts the lagging and or the leading of voltage with respect to current or vice versa. 

It is important to learn about impedance because it helps you design circuits that perform as needed for different working frequencies. It is therefore important in signal processing as well as ac circuit analysis.

5) What is the purpose of the PCB board RC circuit?

Now we know that an RC circuit can also be referred to as a capacitor circuit. It is used in a PCB board to regulate and control electrical signals within electronic devices. The circuit performs several functions depending on its design and the purpose for which it is to be used. 

• Filtering of signals: Most significant of these is filtering of signals. RC circuits can assist in suppressing noise and other disturbances by allowing only the required frequencies of the signal to pass whilst rejecting undesirable ones. Thus, ensuring that the intended signal gets through clean.
• Pulse generator: Time and delay can also be obtained by another foremost function. Wisely used these hours can even be incorporated in the process where delays are undesirable such as in oscillators, and timers, and in such devices as pulse generators. These circuits help in determining the time of operations of the different elements which are important to stimulate operations in either a digital system or an analog system.
• Rectifying voltage: Further, RC circuits also perform the function of rectifying or stabilizing the voltage. In power supply circuits, they are able to dampen oscillations and transients that could be present. Thus, making sure that the proper voltage is applied to sensitive elements. In this way, future damage is avoided and proper functioning is guaranteed.
• Coupling and decoupling: RC circuits mounted on the PCB boards to perform signal coupling and decoupling and provide isolation between sections of the circuit to eliminate unwanted coupling. Their multifunctional nature also makes them basic structural elements when it comes to the fabrication of efficient, reliable, and high performing electronic gadgets.