SILICON CONTROLLED RECTIFIER (SCR):-
A Silicon controlled rectifier (SCR) is a four layer PNPN device having three terminal anode (A), cathode (K) and gate (G).It is solid state current controlling device. SCR is used as a switching device in high power applications and also used as a controlled rectifier to convert ac into dc. it is advanced version of Shockley diode with an extra terminal added, this terminal is known as gate which is used to trigger the component into conduction using small potential.
SCR Mode of Operation:-
It has three modes and three junctions.
- Forward blocking mode
- Forward conduction mode
- Reverse conduction mode
Before understanding the modes first let’s have a look at what biasing means.
Forward bias: – When the higher potential (+) is connected to P junction and lower potential (-) is connected to N junction.
Reverse bias: – When the higher potential (+) is connected to N junction and lower potential (-) is connected to P junction.
Forward blocking mode:-
In this mode Anode is connected to positive, cathode is connected to negative and gate is at zero potential. When these terminals are connected to the source then the junction 1, 3 develop as forward biased junction and junction 2 develop as reversed biased junction. As the j2 is in reverse bias. And we know that in reverse bias the current does not flow easily so the breakdown will occurs when potential is supplied through it. Here the current value increase but voltage decreases. The region where the conduction of electron starts is called holding current. Here the voltage drops because the current is increasing and resistance is low.
Forward conduction mode
In this mode Anode is connected to positive, cathode is connected to negative and gate is also positive (a battery is connected to the gate terminal of the SCR). When these terminals are connected to the source. Due to gate potential breakdown will occur a bit earlier because the negative end push the electrons with high kinetic energy and the depletion region become thin will allow current flow easily from anode to cathode.
The Gate is used as a trigger once current start to flow then if we remove the gate then there will no effect on the conduction of electrons and flow of current. The region where we remove the gate potential and still there is no effect on the flow of current is called latching current.
Reverse conduction mode
In this mode Anode is connected to negative, cathode is connected to positive and gate is at zero potential. When these terminals are connected to the source then the junction 1, 3 develops as Reversed biased junction and junction 2 develop as forward biased. As the j1 and j3 are in reverse so the current does not flow only small amount of leakage current will flow through when higher potential is supplied the breakdown will occures which will break the junction3.The current value increase suddenly but voltage decrease.
SCR is a fast switch and it is difficult to cycle a mechanical switch several hundred times a minute. Several SCRs can be switched 25000 times a second which takes just microseconds to turn on or off these units. Changing the time that a switch is on as related to the time that it is off regulates the amount of power flowing through the switch. Therefore most devices can operate on pulses of power ac is a special form of alternating positive and negative pulse. The SCR can be used readily in control applications like
- Motor speed controllers
- Inverters remote switching units
- Controlled rectifiers
- Circuit overload protectors
- Latching relays
- Computer logic circuits
SCR V-I characteristics:-
The V-I characteristics in forward blocking mode start when we supply a potential to the anode and cathode of the scr. The small current start to flow at first through reversed biased junction2 (j2). Increasing potential will breakdown the junction which will start holding current where the voltage will decrease due to small resistance and suddenly the current will increase rapidly.
The V-I characteristics in Forward conduction mode is that the breakdown region will develop earlier then forward blocking mode because of the gate potential and once breakdown occures there will be no effect on current even if the gate potential is removed.
Similarly in V-I characteristics of Reverse conduction mode the small leakage current flows and after breakdown of j3 will suddenly increase the current and lower the voltage.
During testing an SCR for a moment connection b/w the gate and the anode is sufficient in polarity intensity and the duration to trigger it. The SCRs may be fired by an intentional triggering of the gate terminal excessive voltage breakdown b/w anode and cathode or an excessive rate of voltage rise b/w the anode and cathode. This Silicon controlled rectifier maybe turned off by anode current falling under the holding current value low current dropout using reverse firing the gate providing a negative voltage to the gate. The Reverse firing is only sometimes effective and always involves high gate current.
The variant of SCR known as a Gate Turn off Thyristor (GTO). This is specifically designed to be turned off by means of reverse triggering. Even this reverse triggering requires fairly high current naturally 20% of the anode current. The SCR terminals may be identified by a continuity meter the only two terminals showing any continuity b/w them at all should be the gate and cathode. The Gate and cathode terminals connect to a PN junction inside the SCR. hence a continuity meter should obtain a diode like reading b/w the 2 terminals with the positive lead on the gate and the negative lead on the cathode. Several large SCRs have an internal resistor connected between gate and cathode which will affect any continuity readings taken by a meter.
The SCRs are rectifiers they only allow current through them in one direction which means they cannot be used alone for full wave AC power control. Diodes in a rectifier circuit are replaced by SCRs for the makings of a controlled rectifier circuit while in DC power to a load may be time proportioned by triggering the SCRs at various points along the AC power waveform.
Break over voltage:-
This is the minimum forward voltage gate being open at which SCR starts conducting heavily e.g. turned on. As if the break over voltage of an SCR is 200 V which means that it can block a forward voltage e.g. SCR remains open till the supply voltage is less than 200 V. As supply voltage is more than this value then the SCR will be turned on. Normally the SCR is operated with supply voltage less than break over voltage and turned on by means of a small voltage applied to the gate. The available SCRs have break over voltages from about 50 V to 500 V.
Peak reverse voltage (PRV):-
This is the maximum reverse voltage cathode positive with respect to anode applied to an SCR without conducting in the reverse direction. The Peak reverse voltage (PRV) is an important consideration while connecting an SCR in an Ac circuit. Throughout the negative half of ac supply reverse voltage is applied across SCR. As PRV is exceeded there may be fall breakdown and this SCR will be damaged as the external circuit does not limit the current. The available SCRs have PRV ratings up to 2.5 kV
Forward current rating:-
This is the maximum anode current that an SCR is capable of passing without destruction. Each SCR has a safe value of forward current which it can conduct. When the value of current exceeds this value the SCR may be destroyed due to intensive heating at the junctions. E.g. When an SCR has a forward current rating of 40A this means that the SCR can safely carry only 40 A. The effort to exceed this value will result in the destruction of the SCR. The available SCRs have forward current ratings from about 30A to 100A.
Circuit fusing (I2 t) rating:-
The Circuit fusing is the product of square of forward surge current and time of duration of the surge Circuit fusing rating = I2 t
This circuit fusing rating indicates the maximum forward surge current ability of SCR. E.g. Lets us consider an SCR having circuit fusing rating of 90 A2 s. when this rating is exceeded in the SCR circuit the device will be destroyed by excessive power dissipation
Conduction of SCR
When SCRs gate is disconnected it act similarly as a Shockley diode which may be latched by break over voltage or from exceeding the critical rate of voltage rise between anode and the cathode just as with the Shockley diode. The Dropout is accomplished by reducing current until one or both internal transistors fall into cutoff mode similar to the Shockley diode. Though because the gate terminal connects directly to the base of the lower transistor it may be used as an alternative means to latch the SCR. By providing a small voltage b/w gate and the cathode the lower transistor will be forced on by the resulting base current which will cause the upper transistor to conduct which then supplies the lower transistor base with current that it no longer required to be activated by a gate voltage. This essential gate current to initiate latch-up, of course will be much lower than the current through the SCR from cathode to anode and SCR does achieve a measure of amplification.
The method of securing SCR conduction is known as triggering or firing and the most common way that SCRs are latched in actual practice. The SCRs are usually chosen so that their break over voltage is far beyond the greatest voltage expected to be experienced from the power source to make it possible to turn on only by a planned voltage pulse applied to the gate.
SCR Reverse Triggering
The Silicon controlled rectifiers SCR can also be turned off by directly shorting their gate and their cathode terminals through reverse triggering or by the gate with a negative voltage in reference to the cathode so the lower transistor is forced into cutoff. Sometimes it is possible because it involves shunting all of the upper transistor collector current past the lower transistor base. The current may be large making triggered shut off of an SCR difficult at finest. A change of the SCR called a Gate Turn off Thyristor makes this task easier. But as with this gate current required to turn it off may be as much as 20% of the anode load current.
Sensitive Gate SCRs
The SCR missing this internal resistor are sometimes mentioned to as sensitive gate SCRs due to their ability to be triggered by the slightest positive gate signal.
By the test circuit for an SCR is both applied as an analytical tool for checking supposed SCRs and also an excellent aid to understanding basic SCR operation. The DC voltage source is used for powering the circuit and the two push button switches are used to latch and unlatch the SCR respectively.
Triggering the normally open ON pushbutton switch connects the gate to the anode allowing current from the positive terminal of the battery through the load resistor through the switch through the cathode gate PN junction back to the battery. The gate current should force the SCR to latch on allowing current to go directly from anode to cathode without further triggering through the gate. As the ON pushbutton is released, the load should remain energized. By Pushing the normally closed OFF pushbutton switch breaks the circuit forcing current through the SCR to halt so forcing it to turn off low current dropout.
SCR Application in Normal Operation:
TO operate the SCR in normal operation the following points are in view.
- Voltage supply is generally much less than break over voltage.
- The SCR is turned on by passing a suitable amount of gate current a few mA and not by break over voltage.
- The SCR is functioned from ac supply and the reverse voltage which comes during negative cycle should not surpass the reverse breakdown voltage.
- Once SCR is to be turned OFF from the ON state anode current should be reduced to holding current.
The gate current is increased above the required value SCR will close at much reduced supply voltage.
SCR Application as a Switch
The SCR has only two states namely ON state and OFF state and no state in between. When appropriate gate current is passed. The SCR starts conducting heavily and remains in this position indefinitely even if gate voltage is removed. It corresponds to the ON condition. When the anode current is reduced to the holding current SCR is turned OFF.As behavior of SCR is similar to a mechanical switch. The SCR is an electronic device it is more appropriate to call it an electronic switch.
SCR Advantages as a switch
The SCR has the following advantages over a mechanical or electromechanical switch relay.
- No moving parts.
- It gives noiseless operation at high efficiency.
- Switching speed is very high up to 10^9 operations per second.
- SCR permits control over large current 30–100 A in the load by means of a small gate current a few mA.
- SCR has small size and gives trouble free service.
Some advantages of silicon controlled rectifier (SCR) are given below.
- The Silicon controlled rectifier is easy to turn ON.
- Triggering circuit for silicon controlled rectifier (SCR) is simple.
- SCR is simple to control.
- SCR is able to control AC power.
- The cost is low.
- The SCR can handle large voltage current and power.
- Silicon controlled rectifier can be protected with the help of fuse.
Some disadvantages of silicon controlled rectifier (SCR) are given below.
- SCR in AC circuit needs to be turned on each cycle.
- SCR cannot be used at higher frequencies.
- SCR)is unidirectional devices
- It can control power only in DC power during positive half cycle of AC supply. And only DC power is controlled with the help of SCR.
- Gate current cannot be negative.
Power Control SCR Application
As voltage across the capacitor reaches the trigger point voltage of the device. The SCR turns ON current flows in the Load for the rest of the half cycle. The current flow stops when the applied voltage goes negative.
Relay Control Using SCR
As the switch S1 is pressed for a moment Relay will turn on and it can be turned off by pressing S2. AS the switch S1 is replaced with an LDR and resistor R1 with 4.7K. This relay will turn on when light falls on LDR. Preset adjust the triggering point and this switch S1 is replaced with a 4.7 K NTC (Negative Temperature Coefficient) Thermistor and R1 with a 1K preset the relay turns on when the temperature increases. Preset adjust the triggering point.