Inductor in a DC Circuit

| at 3:48 PM


Inductor in a DC Circuit

 

 

An Inductor is a passive device that stores energy in its Magnetic Field and returns energy to the circuit whenever required. An Inductor is formed by a cylindrical Core with many Turns of conducting wire. Figure 1 and Figure 2 are the basic structure and the schematic symbol of the Inductor.

 

Figure 1: Basic Structure of the Inductor  

 

When an Inductor is connected to a circuit with Direct Current (DC) source, two processes, which are called “storing” and “decaying” energy, will happen in specific conditions.
The Inductor is connected to the DC Power Supply, Figure 3. The sudden increase of current in the Inductor produces an Self Induced Electromotive Force, vemf, opposing the Current change, Figure 1. This appears as a Voltage across the Inductor, vL = - vemf. This - vemf will slow down the Current change, and in turn, the slow down of the Current change, will make vL become smaller. When the Current becomes stable, the Inductor creates no more opposition and vL becomes zero, the Storage Phase is over.

Figure 3: Inductor is Storing Energy 

An Inductor is equivalent to a Short Circuit to Direct Current, because once the Storage Phase has finished, the Current, iL, that flows through it is stable, iL = V / R, no Self Induced e.m.f. is produced and vL is zero. The Inductor acts like an ordinary connecting wire, its Resistance is zero. The Current iL through an Inductor cannot change abruptly.
When the Inductor is disconnected from the Power Supply, Figure 4, vL reverses polarity and drops instantaneously from zero to a negative value, but iL maintains the same direction and magnitude. The energy stored in the Inductor decays through the Resistor RD. vL rises gradually to zero and iL drops gradually to zero.

Figure 4: Inductor is Decaying Energy 


In Figures 3 and 4, the Resistance of RS and RD affects the storing rate and the decaying rate of the Inductor respectively.
The quotient of Inductance L and Resistance R is called the Time Constant τ, which characterizes the rate of energy storage and energy decay in the Inductor, Figure 5.

Figure 5: The Voltage VL and Current iL during the Storage Phase and Discharge (Decay) Phase


The larger the Resistance, the smaller the Time Constant, the faster the Inductor stores the energy and decays the energy, and vice versa.
Inductors are found in many electronic circuits. For example, two Inductors can form a Transformer that is used to convert between high and low Voltages, and vice versa.

 

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Capacitor

| at 3:22 PM

What is a Capacitor?



A capacitor is a passive two terminal component which stores electric charge. This component consists of two conductors which are separated by a dielectric medium. The potential difference when applied across the conductors polarizes the dipole ions to store the charge in the dielectric medium. The circuit symbol of a capacitor is shown below: 

The circuit symbol of a capacitor    

The capacitance or the potential storage by the capacitor is measured in Farads which is symbolized as ‘F’. One Farad is the capacitance when one coulomb of electric charge is stored in the conductor on the application of one volt potential difference.
The charge stored in a capacitor is given by 
Q = CV
Where Q - charge stored by the capacitor
            C - Capacitance value of the capacitor
            V - Voltage applied across the capacitor
Note the other formula of current, I = dQ/dt
Taking the derivative with respect to time,
dQ/dt = d(CV)/dt
From the above statement, we can express the equation as
I = C (dV/dt)
As you turn on the power supply, the current begins to flow through the capacitor inducing the positive and negative potentials across its plates. The capacitor continues to charge until the capacitor voltage equalizes up to the supply voltage which is called as the charging phase of the capacitor. Once the capacitor is fully charged at the end of this phase, it gets open circuited for DC. It begins to discharge when the power of the capacitor is switched off. The charging and discharging of the capacitor is given by a time constant.
 
The voltage across the capacitor is given by
 
Capacitors are widely used in a variety of applications of electronic circuits  such as
·         store charges such as in a camera flash circuit
·         smoothing the output of power supply circuits
·         coupling of two stages of a circuit (coupling of an audio stage with a loud speaker)
·         filter networks(tone control of an audio system)
·         delay applications (as in 555 timer IC controlling the charging and discharging)
·         tuning radios to particular frequencies
·         phase alteration.
 
The conductors offer a series resistance and if the capacitor is constructed using tubular structure then some inductance is also induced. The dielectric medium between the plates has an electric field strength limit and also passes a small amount of leakage current which results into a Breakdown voltage.  
 
There are different types of capacitors, they can be fixed or variable. They are categorized into two groups, polarized or non-polarized. Electrolytic capacitors are polarized. Most of the low value capacitors are non-polarized. The symbol of capacitors from each group is shown below:
 
 
 
 

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Limit Switches

| at 2:46 PM

Limit Switches

The switch, which is one of the most basic of all sensors, comes in two types? normally open and normally closed. Prior to advances in sensor technology, mechanical switches were used extensively in control applications. Due to improved reliability and performance, mechanical switches are still used for this purpose, but they are primarily used where switch actuation and wear are minimal. The standard limit switch is a mechanical device that uses physical contact to detect the target. A typical limit switch consists of a switch body and an operating head.

 


The switch body contains electrical contacts to energize or de-energize a circuit. The operating head incorporates a lever arm or plunger. This is also called an actuator. The actuator rotates when the target applies force. This movement changes the state of contacts within the switch body. Several types of actuators are available...
   The roller type actuator is most suited to applications where a sliding contact causing the rotary part to rotate would otherwise cause contact wear to take place over a period of time.

The fork-style actuator must be physically reset after each operation and is suitable for critical stop applications in movement control. i.e. where a limit of movement has been exceeded and a manual reset is required following an emergency stop.
    Flexible loop and spring rod actuators can be actuated from all directions, making them suitable for applications where the direction of approach is constantly changing.
    Plunger-type actuators are ideal where short, controlled machine movements are present, or where space or mounting does not permit a lever-type actuator. The plunger can be activated in the direction of plunger stroke, or at a right angle to its axis.
All switches use the following common definitions of contact type      
  Single Pole, Single Throw (SPST)
 
A switch that makes or breaks the connection of a single conductor in a single branch circuit. This switch typically has two terminals. It is commonly referred to as a "Single-Pole" Switch.


Single Pole, Double Throw (SPDT)
 
A switch that makes or breaks the connection of a single conductor with either of two other single conductors. This switch typically has 3 terminals, and is commonly used in pairs and called a "Three-Way" switch.

Double Pole, Single Throw (DPST)

A switch that makes or breaks the connection of two circuit conductors in a single branch circuit. This switch typically has four terminals.

Double Pole, Double Throw (DPDT)
A switch that makes or breaks the connection of two conductors to two separate circuits. This switch typically has six terminals and is available in both momentary and maintained contact versions.


The limit switch is used to determine the proximity of equipment (the Applications section illustrates a few examples). The limit switch can output only two signals: ON and OFF. It outputs a digital ON signal when the lever arm is depressed and a digital signal OFF when the lever arm is not depressed (see Figure 2 below). External forces, such as collisions, produce a digital ON signal, which remains ON until the lever arm is released.








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