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HomeBlogUnderstanding the Fundamentals:Inductance Resistance, andCapacitance
on December 18th

Understanding the Fundamentals:Inductance Resistance, andCapacitance


In the intricate dance of electrical engineering, a trio of fundamental elements takes center stage: inductance, resistance, and capacitance. Each bears unique traits that dictate the dynamic rhythms of electronic circuits. Here, we embark on a journey to decipher the complexities of these components, to uncover their distinct roles and practical uses within the vast electrical orchestra. Inductance, with its magnetic flux wizardry; resistance, the steadfast gatekeeper of current flow; and capacitance, the agile custodian of electric charge, all converge to create the symphony that powers our electronic world.


Resistance—a conductor's innate defiance to electric current—is denoted by 'R'. Its magnitude hinges on the conductor's dimensions, material makeup, and the ambient temperature. Invoking Ohm's Law, we articulate this relationship: I = U/R, thus R = U/I. The ohm, symbolized by the Greek letter Omega (Ω), stands as the resistance's measure, with its kin: the kiloohm (kΩ), megohm (MΩ), and milliohm (mΩ).

A solitary ohm defines the resistance when one volt coaxes one ampere through the conductor.

Resistors serve as the guardians at the gates, curbing the electric current's rush. The term 'resistor' not only denotes a property but also christens the very components designed to uphold it.

Here's a snapshot of these components:

Fashioned from materials that balk at the flow of current, resistors adopt a form meant to reign in electrical chaos within a circuit. Fixed resistors stand their ground, immutable. In contrast, the potentiometer or rheostat—variable resistors—allow for a controlled variance in resistance.

An ideal resistor is linear and the instantaneous current through it is proportional to the instantaneous voltage applied to it. For some special resistors, such as thermistors, varistors, and sensing elements, there is a non-linear relationship between voltage and current.

The Basic Principle

The resistor consists of three parts: the resistor body, the frame, and the terminal (the resistor body and the SSR frame are combined into one). Only the resistor determines the resistance value.

Classification of Current and Voltage Characteristics

The resistance of a conductor is almost constant at a certain temperature. Above a certain value, this resistance is called linear resistance. The resistance value of some resistors changes greatly with current (or voltage), and the current-voltage characteristic shows a curve. This type of resistor is called a nonlinear resistor. These nonlinear relationships are often needed in electronic circuits.

(1) Fuse resistor: Also called fuse resistor, it generally plays the dual role of resistor and fuse. When a circuit fails and the power exceeds its rating, it burns like a fuse, breaking the circuit. . Fuse resistors typically have low resistance values (0.33Ω to 10kΩ) and low power.

(2) Sensitive resistors. Sensitive resistors are sensitive to certain physical quantities (such as temperature, humidity, light, voltage, mechanical force, gas concentration, etc.). When these physical quantities change, the resistance of the sensitive resistor also changes. Variability. It changes according to changes in physical quantities and represents different resistance values. According to the sensitive physical quantities, sensitive resistors can be divided into temperature-sensitive, humidity-sensitive, light-sensitive, pressure-sensitive, force-sensitive, magnetic-sensitive and gas-sensitive sensitive resistors. The materials used in sensitive resistors are almost always semiconductor materials. These resistors are also called semiconductor resistors.

The role of resistance

If the resistance of the resistor is close to 0Ω, then the resistor has no effect on preventing the flow of current. The circuit connected in parallel with this resistor is shorted and the current becomes infinite. If the resistance is infinite or very large, the loop in series with the resistor can be considered an open circuit and the current is zero.

Resistors commonly used in industry fall somewhere between these two extremes. It has a certain resistance value and can carry a certain current. Resistors are primarily used in circuits to regulate and stabilize current and voltage. They can be used as shunts, voltage dividers, and load matching circuits. Depending on the circuit requirements, negative feedback or positive feedback amplifier circuits, voltage-to-current converters, input overvoltage or overcurrent protection components can also be used, and the RC circuit can be used as oscillator, filter, bypass, differential, integrator and timing circuits, permanently configured components.


An inductor, also tagged as a reactive inductor, stands in defiance of current change—its electromotive force a shield against the ebb and flow of current. Structurally akin to a lone transformer winding, an inductor typically marries coil, shield, and core into a singular entity. In its quiescent state, an inductor resists current with stoic resolve, staunchly opposing flow upon the circuit's breach.

Symbol for inductance: L.

The inductance unit is the Henry (H), with its smaller kin the millihenry (mH) and the microhenry (μH). The conversion is crisp: 1H = 10^3mH = 10^6μH = 10^9nH.

Focusing on the core parameters:


this self-reflective trait gauges an inductor's magnetic prowess. Rooted in the coil’s turns, the winding strategy, the core’s presence and material, inductance is a telltale of magnetic induction capacity. More turns, more tightness—more inductance. A magnetic core further amplifies this effect, the core's permeability directly proportionate to the inductance ascension.

The basic unit of inductance is Hen, represented by the letter "H". Commonly used units are millihenries (mH) and microhenries (μH). The relationship between them is: 1H=1000mH, 1mH=1000μH.

(2) Rated current

The rated current is the maximum current that the inductor can handle under acceptable operating conditions. If the operating current exceeds the rated current, the inductor will change its operating parameters due to heat and may even burn out due to overcurrent.

Functional use

The inductor in the circuit mainly plays the role of signal shielding, noise filtering, current stabilization and electromagnetic interference suppression, as well as filtering, generating, delaying and suppressing functions. The most common role of an inductor in a circuit is to form an LC filter circuit with a capacitor. Capacitors have the characteristics of "blocking DC and blocking AC", while inductors have the characteristics of "passing DC and blocking AC". When a DC current containing a large amount of noise flows through the LC filter circuit, the spurious AC signal is absorbed by the heat in the inductor.


In the lexicon of direct currents (DC), "forward DC" signals an inductor's disengagement. Should the inductor's coil resistance be omitted, DC finds a path of least resistance, flowing unimpeded. Typically, the coil's resistance to DC is minuscule, almost negligible in analyses.

AC resistance is another story. Here, an inductor acts as a sentry, countering the flow of alternating current (AC) with its inductive reactance—a resistor in its own right.

Inductors are the antithesis of capacitors, champions of continuity for DC and barriers against the fickleness of AC. Through an inductor, DC encounters resistance equivalent only to the coil's wire, causing a trivial voltage drop. Introduce AC, and the coil retaliates, conjuring a self-induced electromotive force at its ends. This force aligns with the applied voltage, countering AC's attempt to pass. Inductors are conductive to DC, restrictive to AC, and as frequency ascends, so does their resistance. Paired with capacitors, inductors are instrumental in crafting LC filters, oscillators, and other circuit components like current loops, transformers, and relays.


Capacitance, the charge's haven, is measured in farads (F) and symbolized by 'C'. It encapsulates a capacitor's aptitude for charge storage, contingent on the potential difference's sway.

In the realm of circuits, capacitance is pivotal; it's the linchpin in functions ranging from power supply refinement to energy warehousing and even signal processing. The capacitor's charge (Q), divided by the voltage (U) spanning its electrodes, defines its capacitance. Thus, we have C, the symbol that heralds a capacitor's identity.

Here's the equation that binds them: C = εS/d = εS/4πkd (in vacuum) = Q/U.

Unit Conversion

Units morph across scales in the SI tapestry: the farad (F) branches into millifarad (mF), microfarad (µF), nanofarad (nF), and picofarad (pF), each a whisper or a shout in the choir of capacitance.

To navigate these scales, remember:
1 farad (F) equals 1000 millifarads (mF) or a staggering million microfarads (µF).
A microfarad (µF) translates to 1000 nanofarads (nF) or a million picofarads (pF).


If the potential difference between the two stages in a capacitor is 1 V and the charge is 1 coulomb, then the capacitance of the capacitor is 1 farad. per hour. C=Q/U. However, the value of the capacitor is not determined by Q (charge) or U (voltage). Hour. Capacity is determined by the formula: C = εS/4πkd. Where ε is a constant, S is the area facing the capacitor poles, d is the distance between the capacitor poles, and k is the electrostatic force constant. The capacitance of a conventional parallel plate capacitor is C = εS/d (where ε is the dielectric constant of the medium between the plates, S is the plate area, and d is the distance between the plates).

Find the formula:
The formula for connecting several capacitors in parallel is C=C1+C2+C3+...+Cn
The formula for connecting several capacitors in series: 1/C=1/C1+1/C2+...+1/Cn

The Role of Capacitors

(1) Bypass

Bypass capacitors are energy storage devices that balance regulator output and reduce load by supplying power to local devices. Like small batteries, bypass capacitors charge and discharge the device.

(2) Decoupling

This is a shunt, also known as a crossover. From a circuit point of view, when the load capacity is relatively large, the control circuit must charge and discharge the capacitor to complete the signal conversion. If the slope is steep, the current will be relatively large, affecting normal operation. The front stage is called the "clutch". The function of the decoupling capacitor is to act as a "battery", respond to changes in the control circuit, avoid mutual interference, and further reduce the high-frequency interference resistance between the power supply and the circuit reference ground.

(3) Filter

Theoretically, assuming that the capacitor is a pure capacitor, the larger the capacitor, the lower the impedance and the higher the frequency of the current flowing through it. But in reality, capacitors above 1 µF are mostly electrolytic capacitors with large inductive components, so the current frequency is high, but the resistance increases. Sometimes you will see large electrolytic capacitors in parallel with small capacitors. Large capacitors filter out low frequencies and small capacitors filter out high frequencies. The function of a capacitor is to convert alternating current to direct current and to block high frequencies from low frequencies. The larger the capacitor, the easier it is to conduct high-frequency current.

(4) Energy Storage

The storage capacitor collects charge through the rectifier and transfers the stored energy to the output of the power supply through the converter circuit. Typically, aluminum electrolytic capacitors are used with a voltage rating in the range of 40 to 450 V DC and a capacitance in the range of 220 to 150,000 μF. Depending on the power requirements, these devices are sometimes connected in series, in parallel, or in combination. For power supplies greater than 10 kW, larger screw-terminal capacitors are typically used.

That covers all the content of this article. If you have any questions, feel free to contact us. ARIAT will reply to you promptly.
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