Electrical Resonance

Resonance

What's Resonance?

resonance is the state at which any element is naturally oscillate at the frequency they like to oscillate 

when connecting an initially charged capacitor with an inductor we notice that energy which stored at capacitor as an electric field transfer to inductor which store energy as magnetic field.

so what happen there?
capacitor charge indutor until it fully charged so capacitor at this moment fully discharged then inductor charge capacitor back until it fully charged and indutor fully discharged. this cycle continues forever if there's no losses in the wiring system but in fact this energy gradually die out because of losses.
this circuit act like a pendulum, in pendulum kinetic energy transfer into potential energy then potential transfer into kinetic and so on until it stops because of losses so LC circuit and pendulum oscillate at frequency they like
 so this state is called resonance.

Condition Of Electrical Resonance Occurrence:

Resonance in electrical system occur when supply drives circuit at resonance frequency.
at resonance frequency capacitor reactance is equal inductor reactance.

So resonance  frequency is:

Series Combination Of RLC Circuit In Resonance:

when applying voltage source with frequency equal to resonance frequency the combination of LC will act like a short circuit. 
as circuit element are connected in series so the current is the same passing through all element RLC so let us consider the current as reference.

at inductor: 
the voltage leads the reference current by 90 degree.
at capacitor:
the voltage lags the reference current by 90 degree.

so when adding capacitor voltage and inductor voltage at resonance we notice that they have the same magnitude and out of phase by 180 degree and the resultant voltage become zero so that LC acting like short circuit and the series resistor have all supply voltage.




Parallel Combination Of RLC Circuit In Resonance:

when connecting element in parallel the voltage is the same at the capacitor and inductor so let us take it as a reference.
 
at inducor: 
the current lags the reference voltage by 90 degree.
at capacitor:
the current leads the reference voltage by 90 degree.

so when adding capacitor current and indutor current together at resonance we notice that they have the same magnitude and out of phase by 180 degree so the resultant current become zero and combination of LC act like an open circuit and all current is passing through the resistor only.

Types of Diode

Types of diode

Diode is a one way valve, it mean that it allow current on one direction and block current in the other direction.

it has two terminal: anode and cathode 

and has two state: 

1-forward bias: connect positive rail of supply to anode and negative rail to cathode in that condition diode allow current but has voltage drop across it depend on material of diode.

2-Reverse bias: connect positive rail of supply to cathode and negative rail to anode in that condition diode block current and it act like open circuit with very small leakage current.

Diode generally made of silicon or germanium , these three material have a voltage drop 0.7 , 0.3 respectively.

let's discuss types of diode and application of every type.

Normal Diode:
It also called general purpose diode. it have meduim to high volltage drop, low current rating , low speed rectifier and inefficient for square wave more than 15kHZ.

Power Diode:

It also called rectifier diode . it has high current rating in ampere depend on the required application it usually came on large size 
these diode used in power supply or voltage doubler circuit.

Small Signal Diode:
When dealing with high frequency and small current in mA, so this kind of diode is convenient
it has small size and enveloped with glass to protect it from contamination.

it characterized by low power consumption in mw
and widely used in radio and television application.

Light Emitting Diode (LED):
These diode isn't only allow current in one direction but also emit light .

it has high forward voltage rating compared to normal diode, also rating forward voltage depend on the color.

for example:V_F of red LED is around 2.2V and blue is 3.3V

some of these LED are invisible such as infrared LED which are commonly used in remote control application.

Photo Diode:

This type of diode consume light energy and convert it into current

sensing light in reverse bias operation is the main function of these diode

in addition of sensing , large area of this photo diode can generate electricity in that case it used as solar cell.

Shockley Diode and DIAC:

it's kind of power diode that supports hysteresis phenomena . you can see the whole topic here Thyristor | Shockley Diode and DIAC

Silicon Controlled Rectifier (SCR) and TRAIC:

it similar to Shockley diode with gate terminal added . you can see the whole topic here Thyristor | Silicon Controlled Rectifier (SCR) and TRIA

Transient Voltage Suppression Diode:

transient occur in circuit when a sudden change of voltage occur this transient may damage circuit component, this kind of diode can avoid that 

so it's kind of protection component.

this diode can provide high impedance in normal operation and doesn't affect the current flow of circuit but when sudden spike occur it has very fast response and starts to enter the avalanche breakdown state  providing  very low impedance and reduce voltage to normal. 

Zener Diode:

as mentioned before diode allow current in one direction but in case of zener diode it also allow the other direction.

in reverse bias there's a reference value of voltage called breakdown voltage as exceeding this value the diode conduct current.

there are many product of zener with different break down voltage. 

it used as over voltage protection and voltage regulation.

Schottky Diode:

It also called schottky barrier or hot carrier it formed by combination of semiconductor material and metal that's why this kind of diode has very low forward voltage rating reach 0.15 , high current rating , low power consumption and very fast switching action.

it can used as voltage clamping and high frequency rectifier application.

Silicon Controlled Rectifier (SCR) and TRIAC

 Silicon Controlled Rectifier (SCR)

schematic symbol

Today we will discuss SCR which is one of the types of Thyristor. Recently we discussed two other types, Shockley diode and DIAC. 

SCR is an enhancement device for shockely diode and there’s no much difference between them, SCR definitely like Thyristor family that it’s a type of 4-layer semiconductor and can be modeled as three diodes connected in series back to back or 2 transistors with feedback position but here there’s a third terminal added to cathode and anode and that is the difference between it and Shockely diode, SCR model is shown in the figure below.

(i) Basic Structure                          (ii) BJT Modeling                              (iii) BJT Equivalent Circuit

It supports hysteresis phenomena, the device doesn’t ‎return to its original state even the cause of the action has been removed and it back again under condition.

It can handle high voltage reach some kilovolts therefore it’s suitable in high power application.

Typical SCR Packages

some definition that will help us to read data sheet:

Breakover voltage (V_BO):

It’s the minimum farward voltage -with gate is open- at which SCR start conducting

It rang from about 50 V to 500 V.

Latching current (I_l):

It’s the minimum anode current which must be attained to turn on the device before removing gate voltage.

Holding current (I_H):

It’s the minimum anode current -gate is open- that switch the SCR off and it’s always lower than latching current.

Farward current rating:

The maximum anode current the SCR can pass without damaging it.

Peak reverse voltage:

It’s the maximum reverse voltage that SCR can maintain without damaging it.

Circuit fusing (I²t) rating:

It’s the maximum surge current that SCR can capable ( surge current occur at the time of switching operation and surge current is higher than normal full load current.

We will now move on to conducting condition: 

First, when the gate is open: it acts like Shockley diode exactly on its 3 state:

SCR modeling with gate open

   1) Forward conducting: the state when the SCR work and conduct current and it require sufficient voltage across anode and cathode higher than breakover voltage
V≥V_BO
once SCR starts conducting it conducts until the current decrease to holding current.

   2) Forward blocking: the voltage of anode is higher than cathode but not sufficient to make SCR to open because voltage across anode and cathode is lower than brakover voltage.
V<V_BO

   3) Reverse state: voltage of cathode is higher than anode and in that situation the SCR doesn’t work because the two-outer junction is reverse and the only inner is forward and there’s no current flow.

Second, when the gate is positive with respect to cathode.

SCR model with voltage across gate w.r.t anode

The importance of the gate is to reduce the voltage we must apply to anode and cathode in other words we the gate reduce the breakover voltage and hence we need lower voltage to apply to anode and cathode to make SCR works

And here the question what’s does gate do?

BJT equivalent circuit

it’s better to explain it on BJT equivalent circuit of SCR

when gate is positive w.r.t cathode current flow throw the gate of T2 and hence it works and flow current from collector to emitter and as shown collector of T2 is connected to the base of T1 hence T1 work then current flow from anode to cathode once the SCR work gate terminal doesn’t no longer important and SCR will continue work even if we open gate terminal so we usually apply a pulse across gate for completely turn SCR on.

SCR I-V characteristic:

As shown by increasing the gate voltage the breakover voltage is decrease           

TRIAC

SCR is a unidirectional device so if we need a device like SCR and conduct current in both direction we use TRIAC

TRIAC is a bidirectional device and consist of two SCR connected in parallel with different direction as shown

And I-V characteristic of TRIAC is:

Application of SCR:

     1.   Full-wave rectifier

    2.    Half-wave rectifier

    3.    Overlight detection

    4.    Power control

    5.    Speed control of DC shunt motor

    6.    Protection of voltage-sensitive loads (crowbar)

    7.    Lighting system for power interruption (backup system)

And we will discuss most of them later 

Shockley Diode and DIAC

Thyristor

Thyristor is a type of 4 layer semiconductor and can be modeled as three diodes connected in series back to back or 2 transistor with feedback position ‎

it supports hysteresis phenomena and hysteresis here in thyristor is that the device doesn’t ‎return to its original state even the cause of the action has been removed and it back again under condition

thyristor can handle high voltage reach some kilovolts therefore it’s suitable in high power application

types of thyristor :

    1.  Shockley diode

    2.  DIAC

    3.  Silicon Controlled rectifier SCR

    4.  TRIAC

Shockely diode

Like PN junction diode in the main concept of that it conducts with forward bias and cut off with reverse but unlike PN junction diode it can handle high power .

And here the models of thyristor .

thyristor can be modeled as 3 PN junction diode connected back to back as following .

And here all conducting condition :

Condition 1:

Forward blocking state (OFF) and here j1 and j3 are forward but the voltage between anode and cathode not enough to breakdown j2 to become conducting and breakdown here doesn’t mean that the junction will Permanently damage, because it fabricated to withstand excessive reverse voltage and conduct current like Zener diode in reverse state

Condition 2:

Small reverse leakage current of J1 or J3 only and this stat can be considered OFF state

Condition 3:

Forward conducting state (ON) and here J1 and J3 are forward and voltage between anode and cathode are enough to breakdown J2 and thyristor is fully ON and conduct current.

Then we can say that shockely diode conduct only if the voltage of anode is higher than cathode with a value called break over voltage V_BO  then V_AK  ≥ V_BO  

Once conducts it stay on as long as the current is above the threshold called holding current I_H  and we can explain that by BJT equivalent circuit , once we apply V_AK  greater than V_BO one of the BJT will switch on causing base current on the other BJT then the other BJT works causing base current to the first BJT , we here in positive feedback stat , this loop will continue as long as the current is greater than holding current , and that explain the hysteresis phenomena , once current is below I_H one of the BJT will turn off causing base current in the other BJT to stop then both BJT will stop and diode now OFF

And here the characteristic curve that explain what we have discussed.

The DIAC

It has the same characteristic of shockely diode but with one only difference

Shockely diode can allow current in one direction making it unidirectional device, then two shockely diode can connect in parallel facing different direction to make a device can allow current in two direction that device called DIAC

DIAC can now pass current in the two direction but first the voltage must be higher than V_BO  , so the input wave to the output wave will be shown in the following figure

 Also, the characteristic curve shown on the following figure

It’s the same as the characteristic curve of the shockely diode but in the two direction