Industrial Electronics N3 - Silicon Controlled Rectifier (SCR)

Summary

This educational video script delves into the Silicon-Controlled Rectifier (SCR), a crucial component in industrial electronics for high voltage and current applications. It explains the SCR's advantages, such as low on-resistance and high resistance, and its applications in electrical machines, power supplies, and car ignition systems. The script covers the SCR's construction from two transistors, its operation with a gate signal, and methods to turn it on and off. It also discusses SCR characteristics, transient suppression techniques, and various control methods including static switching, phase control, and cyclotronic control, providing a comprehensive understanding of this electronic device.

Takeaways

• 😀 Silicon Controlled Rectifiers (SCRs) are used in applications requiring high voltages and high currents.
• 🔌 Like diodes, SCRs allow current to flow in one direction, but they have advantages such as low on-resistance, high forward resistance, low control power, and long life.
• 🏗️ SCRs are utilized in various applications including electrical machines, relay controls, power supplies, car ignition systems, and light dimmers.
• 🔍 The basic construction of an SCR involves two transistors, specifically a PNP and an NPN transistor, forming three junctions (J1, J2, and J3).
• 🌐 The SCR can be turned on by applying a positive gate voltage, which forward biases J1 and J3, and then forward biases J2 when the gate is triggered.
• 🔄 To switch off an SCR, methods include reducing the anode-cathode current below the minimum holding current or using commutation techniques.
• 🔋 The operation of SCRs involves charging capacitors when they are on, and the SCR remains on as long as the anode-cathode current exceeds the minimum holding current.
• 📈 SCR characteristics resemble those of diodes, conducting when forward biased and blocking when reverse biased, with a small leakage current in the reverse direction.
• 🛠️ Transients, or unwanted voltages and currents, can be mitigated using LC filters, Zener diodes, and Varistors.
• 🔄 Control methods for SCRs include static switching, phase control, cycle control, and cyclotronic control, which combine phase and cycle control techniques.

Q & A

What is a Silicon-Controlled Rectifier (SCR) and why is it used in high voltage and high current applications?

A Silicon-Controlled Rectifier (SCR) is a semiconductor device that allows current to flow in one direction, similar to a diode. It is used in high voltage and high current applications due to its low on-resistance, high voltage resistance, low control power, and long life.

What are the advantages of using an SCR over diodes and transistors in certain applications?

The advantages of an SCR include low on-resistance, which allows for higher current flow, and the ability to handle high voltages and currents, making it suitable for applications that require these characteristics.

What are some common applications of SCRs?

SCRs are used in various applications such as electrical machines, relay controls, power supplies, car ignition systems, and lamp dimmers.

How is an SCR constructed in terms of its basic components?

An SCR is constructed from two transistors, specifically a PNP transistor and an NPN transistor, which together form three junctions labeled J1, J2, and J3.

How does the SCR conduct current when it is forward-biased?

When the SCR is forward-biased, junctions J1 and J3 are forward-biased, and J2 is reverse-biased. The SCR will only conduct current when a positive voltage is applied to the gate, causing J2 to also become forward-biased and allowing the current to flow.

What are the methods used to switch an SCR on?

SCRs can be switched on by applying a positive gate voltage, applying a positive gate current, or by increasing the anode-cathode voltage to reach the breakover voltage.

How is an SCR turned off once it is on?

An SCR can be turned off by reducing the anode-cathode current below the minimum holding current. This is the minimum current required to keep the SCR in its conducting state.

What is meant by 'breakover voltage' in the context of SCR operation?

The breakover voltage refers to the point at which the anode-cathode voltage is increased enough to turn on the SCR, even without a gate signal, once it has been reached.

How can a switch be used to control an SCR in a circuit?

A switch can be connected in series with the SCR to control its state. When the switch is open, the SCR is off, and when the switch is closed, the SCR is on.

What are the different control methods for SCRs mentioned in the script?

The script mentions static switching, phase control, cycle control, and cyclotronic control as different methods for controlling SCRs.

What is the purpose of LC filters, Zener diodes, and Varistors in relation to SCRs?

LC filters, Zener diodes, and Varistors are used to eliminate transients, which are unwanted voltages and currents that can occur during the operation of SCRs.

Outlines

00:00

🔌 Introduction to Silicon-Controlled Rectifiers (SCRs)

The video script begins with an introduction to industrial electronics, specifically focusing on Silicon-Controlled Rectifiers (SCRs). The speaker, Munya, from Middlebury Campus, explains that SCRs are used in high voltage and high current applications, functioning similarly to diodes by allowing current to flow in one direction. Advantages of SCRs include low on-resistance, high voltage resistance, low control power, and long life. Applications of SCRs are vast, including electrical machines, relay controls, power supplies, car ignition systems, and lamp dimmers. The basic construction of an SCR is also discussed, highlighting its formation by two transistors, a PNP and an NPN, creating three junctions (J1, J2, and J3). The script explains the forward and reverse biasing conditions for these junctions and how the SCR is turned on by applying a positive voltage to the gate, which overcomes the reverse bias of J2.

05:03

🛠 Methods to Turn Off SCRs and Their Characteristics

This paragraph delves into the methods of turning off an SCR, which includes reducing the anode-to-cathode current below the minimum holding current. The holding current is defined as the minimum current required to maintain the SCR in its conducting state. The script mentions two primary methods to turn off an SCR: commutation and reducing current below the minimum holding current. Additionally, it describes the use of switches in series or parallel to control the SCR's state. The characteristics of an SCR are compared to those of a diode, emphasizing the SCR's unidirectional conduction and its behavior in both forward and reverse bias conditions. The summary also touches on the gate's role in maintaining the SCR's on-state even after the gate voltage is removed.

10:04

🛑 Control and Elimination of Transients in SCR Circuits

The third paragraph discusses the control methods for SCRs, including static switching, phase control, cycle control, and cyclotronic control. It explains the concept of transients as temporary short-duration changes in voltage and current values, which are often unwanted. To mitigate these transients, the script mentions the use of LC filters, Zener diodes, and varistors. It also describes the operation of static switching, which is akin to a half-wave rectifier, and how SCRs conduct during the positive half cycle and not during the negative half cycle. The waveforms for the input voltage and the output after SCR are provided to illustrate this behavior.

15:05

🔄 Advanced SCR Control Techniques: Phase and Cycle Control

The final paragraph of the script introduces advanced control techniques for SCRs, such as phase control and cyclotronic control. Phase control is explained with the use of a timer to regulate the SCR's on and off states during the positive half cycle of the input waveform. Full-wave control is also discussed, where the load's connection affects the DC voltage output. The script further explains cycle control, which uses a specific number of cycles to switch the SCR on and off. Lastly, cyclotronic control is described as a combination of phase and cycle control, utilizing both a timer and cycle count to manage the SCR's operation.

Keywords

💡Silicon Controlled Rectifier (SCR)

The Silicon Controlled Rectifier, commonly known as SCR, is a type of semiconductor device that allows current to flow in one direction and can be turned on by applying a positive voltage to its gate. It is used in applications requiring high voltages and high currents. In the video, SCR is discussed as a replacement for diodes and transistors in such scenarios, emphasizing its role in industrial electronics.

💡Diode

A diode is a two-terminal electronic component that allows current to flow primarily in one direction. It is mentioned in the script to contrast with the SCR, highlighting that while diodes allow unidirectional current flow, SCRs also facilitate this but with additional control mechanisms.

💡Transistor

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is mentioned in the context of constructing an SCR, which is formed by combining a PNP and an NPN transistor, thus creating a device with three junctions.

💡Junction

In the context of semiconductors, a junction refers to the boundary between two types of material with different electrical properties. The script mentions three junctions (J1, J2, J3) in an SCR, which are crucial for its operation as they determine the flow of current and the device's switching behavior.

💡Gate

The gate in an SCR is a control terminal that, when positively charged, triggers the device to conduct current. The script explains that applying a positive gate current is a method to switch on the SCR, emphasizing the gate's role in controlling the device's operation.

💡Forward Bias

Forward bias is a condition where a diode or transistor is connected in such a way that its p-n junction is forward-biased, allowing current to flow. The script uses this term to describe the state of the SCR's junctions when they are ready to conduct current, specifically when J1 and J3 are forward-biased.

💡Reverse Bias

Reverse bias is the opposite of forward bias, where the p-n junction of a diode or transistor is connected in a way that opposes the natural flow of current. The script mentions reverse bias in the context of SCR operation, explaining that certain junctions must be reverse-biased for the SCR to function correctly.

💡Holding Current

Holding current is the minimum current required to maintain the conduction of an SCR once it has been turned on. The script discusses this concept as a method to turn off the SCR by reducing the anode-cathode current below this threshold.

💡Commutation

Commutation in the context of SCRs refers to the process of turning off the device by applying a reverse voltage or by reducing the current below the holding current. The script mentions commutation as one of the methods to control the SCR's operation, highlighting its importance in managing the device's conduction state.

💡Transient

A transient is a temporary change in voltage or current, often unwanted, that occurs in an electrical system. The script discusses transients as unwanted phenomena in electrical systems, which can be mitigated using filters like LC filters, Zener diodes, and varistors.

💡Control Methods

Control methods in the context of SCRs refer to the techniques used to manage the conduction and non-conduction periods of the device. The script mentions static switching, phase control, cycle control, and cyclotronic control as examples of such methods, detailing how they are used to regulate the SCR's operation.

Highlights

Introduction to Silicon-Controlled Rectifiers (SCRs) as a replacement for diodes and transistors in high voltage and high current applications.

Explanation of the unidirectional current flow characteristic of SCRs, similar to diodes.

Advantages of SCRs including low on-resistance, high voltage resistance, low control power, and long life.

Applications of SCRs in electrical machines, relay controls, power supplies, car ignition, and lamp dimmers.

Basic construction of an SCR composed of a PNP and NPN transistor, forming three junctions J1, J2, and J3.

Operation of SCRs with conditions for forward and reverse biasing of the internal transistors.

Turning on an SCR by applying a positive gate voltage, which forward biases J2 and activates the device.

Latching behavior of SCRs where once turned on, the gate loses control, and the device remains on even after gate voltage is removed.

Methods to turn off an SCR, including reducing anode-cathode current below the minimum holding current.

Use of switches in series and parallel to control the on/off state of an SCR.

Commutation method to turn off an SCR by applying a reverse voltage.

Characteristic curves of SCRs, comparing them to diodes and explaining their conduction behavior.

Transients in electrical circuits defined as temporary changes in voltage and current values.

Use of LC filters, Zener diodes, and Varistors to eliminate unwanted transients.

Control methods for SCRs including static switching, phase control, cycle control, and cyclotronic control.

Explanation of static switching in half-wave and full-wave rectifiers and their respective waveforms.

Phase control using timers to regulate the conduction period of SCRs.

Cycle control using a specific number of cycles to switch SCRs on and off.

Cyclotronic control combining phase and cycle control methods for SCR operation.

Transcripts

[Music]

good day all

my name is munya am from middlebury

campus

today we'll be looking at industrial

electronics entry

chapter 6 scr which is silicon

controlled

rectifier when diodes and transistors

are failing

they use scr in other words

it means when they need high voltages

and high currents

they use scr

remember in a diode current flows in one

direction

so in scr current also flows

in one direction scr also allows current

to flow in one direction

so for the advantages of scr it has low

on resistance high of resistance

low control power and long life remember

as i said

when they need high voltages and high

currents they use

as they are so obviously when it has low

on resistance it means

it has a higher current

so for the application of scr scr it is

used in electrical machines

and then applications such as relay

controls power supplies

car ignition and then lymph dimmers

and then for the basic construction of

scr

and scr it is formed by

two transistors so it is iron

it is formed by pnp transistor and

npn transistor remember a transistor it

is formed by

two diodes and then in scr

it is formed by two transistors

so in scr it has three junctions

which is j1 j2 and

j3 and then this is a simple office they

are

annoyed which is positive cathode

reaches

negative and then it has a gate

so the second it is forward biased when

j1 is forward biased

j3 is forward bias and j2 it is reversed

buys

and then the second it is reverse biased

when j1

is reverse buys j3 is reverse bias

and then j2 it's forward bias so the

second

even when it is forward but remember i

said when it is forward bias only j1

and j3 they are forward biased

and then j2 it does reverse by so when

the second even if it's forward biased

the second is not gonna be on so you can

only turn on the second

by applying a positive to the gate when

the positive is applied to the gate j2

will also be forward biased and then the

second it's gonna be

on

and then method used to switch the scr

on

we have applying a positive gate

applying a positive gate current and

then by increasing the android cathode

voltage to such an extended

the break over voltage is reached

so in other words when they say by

applying

a positive gate current remember i

said for a forward bias j1 and j3 they

are reversed by

zen then j2 it is four was biased and

then the second will only be on when the

positive is

applied to the gate then j2 will be

forward but

then the second is on and then when they

say by applying a positive gate current

they mean by applying a positive

clock to the gate then the circuit will

be on

j2 will be forward by the then the

second will be on is the one that i was

talking about

and then for by increasing the unknown

trick effect

which is a break over voltage turn on

so for this one when you apply more

positive to the anode the second will be

on so when the second it's on

even when you remove the positive

from the gate the circuit is not going

to be off

once the circuit is on the gate will

lose control and

indicate when it loses control even when

you remove the positive from the gate

the circuit will still be on

so we use the following methods

to turn off the scr

reduction of android cathode current

below the minimum

holding current a holding current is

defined is

minimum current required to hold the scr

in its conducting state

so by reducing a current

below minimum holding current is the

method that we use to turn off the

sdr circuit we have two methods remember

is commutation and by reducing current

to

below the minimum holding current so

when you connect a switch in series with

the scr

the second is gonna be off remember when

the switch is open

the second is gonna be off and then when

the switch is closed the circuit will be

on

and then again you can turn off the

circuit by applying

a switch in parallel to the scr

when the switch is in parallel with the

scr the second is gonna be

off when the switch is open and then

again by connecting a transistor

in parallel to the scr the circuit is

gonna be

off and then for commutation method

accommodation method

the scr is going to be off by applying

a positive battery today

i mean by applying a reversal bedroom to

the svr

when a reversal battery is applied to

the sdr

the second is gonna be it's gonna be off

reversal battery it means remember this

is a node which is the positive this is

calculated which is the negative

when positive is applied to the negative

and the negative is applied to the

positive the second this penalty is

going to be

off

and then let's check the second

for the operation of the circuit when

scr is connected

for this one it represents more

practical seconds of

scr and then for this one

switch one it's open

the sdr is gonna be off and then when

switch

one is closed the scr will be on and

then when the scr

is on the capacitor i will charge and

then when you check

s2 s2 is connected to the ground so when

you close

s2 the second is gonna be

off because

s2 is connected to the ground so it

means annoyed

it's

it's made negative with respect to the

half point

then the second is gonna be off switch

one when is closed scr is gonna be on

and then the capacitor will charge when

search two

is closed the second is gonna be off

because the anode is made negative

within respect to the cathode

and then scr characteristics

this one is the same as the

characteristic curve of

a diode remember for a diode the current

will conduct 20 days forward biased and

will not conduct when it is

reversed biased

when it is forward burst

i mean when it is reversed but it's only

a small leakage current flows in the

reverse direction

when it deserves spices only a small

leakage current will flow

in the reverse direction it means when

it is reversed by that means j1

and j3 they are forward by then j2 it

deserves spice

so it means the circuit will not conduct

until you

apply a positive to the gate

and then in the forward direction when

no gate has been applied to a small

leakage

it's more leakage current also flows is

the one that i was talking about the

circuit will conduct

in forward biased when a positive is

applied to

the gate when

gate current is applied the scr conducts

and then the scr will remain on for as

long as the android should cathode

current exit

the minimum holding current so the

second is going to be

on even when you remove the positive

from the gate the circuit will still be

on because

the gate loses control

so in other ways it means the scr will

only conduct when it is

forward biased and then when it is

reversed by the scr will

not conduct

for a transient they saying a transit

can be

defined as a temporary short duration

during which known voltage

and current values change from existing

values to new values

in other words they mean

a transient it's we can say transients

they are unwanted voltages and unwanted

currents

as we can simply say they are unwanted

voltages and unwanted

currents so in order to eliminate

the unwanted voltages

we use lc filters zener tires and

varistas so you must be able to explain

numerous knowledge along methods that

they are used to eliminate transients

and then for control methods of scr

we have static switching phase control

and then cycle control and cycle

trony control so for static switching

for static switching is the same as the

half wave rectifier

so for the operation of the circuit

you'll have positive then you'll have

negative

so during positive half cycle with

positive then it's negative

so jordan positive half cycle

the scr will be on when the switch is

closed the sdr will be on

and then during negative half cycle

remember after i cycle the polarity

changes it means during negative half

cycle

you'll have positive you'll have

negative

so in a second current flows from

positive to negative then the positive

is going to be here

and then when you start from positive

remember and with this positive cathode

it's negative when you start from

positive the scr it is reverse biased

and then when it is reversed by that

means

it will not conduct so during positive

half cycle

is positive is negative you start from

positive this one is positive

it's negative it means it is forward by

then then during positive half cycle

when switch is closed the scr

will conduct and then during negative

half cycle

the scr will not conduct this they are

the waveforms

this is the input voltage for hc this is

the output form

after scr this is the output during

positive half cycle will have a waveform

because

sdr conduct during negative half cycle

scr does not conduct

you'll have a space during positive half

cycle

will conduct you'll have a waveform

negative half cycle will have a space

because it does not conduct

positive half cycle you will have a

waveform cause it

conducts so for a full wave control

for a full wave control the first one

when the load

is increased with the supply it's for

shc voltage and then

when the load is connected in series

with the scr

is for a dc voltage

this is the input waveform for ac and

then

output waveform for dc for

a full wave control

and then for a face control a face

control they use

a timer to control as they are so

for the shaded area is for conduction

and then

unshaded area it's off delay

so they use a timer to control the scr

so of delay it means the timer is off

and then on delay

it means the timer it's on so they use

a timer to switch the scr

on and off

so for a half wave control

they've used a timer to control the

circuit

during positive half cycle the control

circuit

the timer will be on and then when it is

on the scr will also be on

and then during negative half cycle the

polarity changes after a cycle it means

you will have

positive will have negative during

negative half cycle

the sar the timer

the control circuit will not be on then

we have a space shuttle positive half

cycle is positive it's negative

and then when the control circuit is on

the scr will be on then we'll have a

waveform and then for a full wave

control

during positive half cycle we have

positive we have negative

so during positive half cycle control

circuit one

will be on and then when it is on will

turn on

scr1 during negative half cycle

we have positive we have negative

only scr2

will be on the control circuit two will

be on

and then as the r2 will be on

and then for cycle control

for a second control they use number of

cycle

to switch on scr

to switch scr on and off so it

means when they've used eight cycle to

switch on the essay are they gonna use

another eight cycle to switch off scr so

they use

number of cycle to switch as they are on

and

off for cyclotronic control

cyclotronic control is the combination

of phase control and cycle control

in other ways it means they've used a

timer and number of

cycle to turn on that second

[Music]

you