Silicon Controlled Rectifier (SCR) :

An atomic number 14 controlled rectifier or semiconductor-controlled rectifier could be a four-layer solid-state current-controlling device. physicist diodes square measure curious devices, however rather restricted in application. Their quality is also distended, however, by militarisation them with another suggests that of latching.


Silicon controlled rectifier Symbol

Fig: Silicon controlled rectifier Symbol


Silicon Controlled Rectifier (SCR) could be a unifacial semiconductor fabricated from atomic number 14 which might be accustomed give a specific power to the load by shift it ON for a variable quantity of your time. An atomic number 14 controlled rectifier or semiconductor-controlled rectifier could be a four-layer solid-state current-controlling device. primarily the operation of the thyristor / SCR is explained in terms of a latching switch. Once turned on by a current at the gate, it needs the voltage across the cathode and anode to be removed before it stops conducting.

Construction of (SCR) :

The semiconducting material controlled rectifier consists of 3 P-N junctions that are tagged and 4 semiconductor material layers, that are joined within the sort of PNPN. to seem at however a thyristor works it’s useful to use a simplified equivalent circuit. This consists of 2 back to back transistors.

P-N junctions-SCR

              Fig: P-N junctions SCR

In absence of external bias voltages, the bulk carrier in every layer diffuses till there’s a constitutional voltage that retards any diffusion. Some majority carriers have enough energy to cross the barrier caused by the retarding field of force at every junction. These carriers then become minority carriers and might recombine with majority carriers. If a semiconductor device and a thyristor do a similar job, what is the distinction between them? With a semiconductor device, once a little current flows into the bottom, it makes a bigger current flow between the electrode and also the collector. The semiconductor material layers are fashioned by the diffusion of gently doped semiconductor material into an extremely doped semiconductor material.


Working (SCR) :

The end P-region is that the anode, the tip N-region is that the cathode and also the inner P-region is that the gate. The anode to cathode is connected nonparallel with the load circuit. basically, the device could be a switch. By applying any low voltage between gate and cathode, the lower junction transistor is going to be forced on by the ensuing base current, which is able to cause the higher junction transistor to conduct, that then provides the lower transistor’s base with a current in order that it now not has to be activated by a gate voltage. For switch off or reverting to the block state, there should be no gate signal and also the anode current should be reduced to zero. Current will flow solely in one direction. Thyristors square measure a logical extension of diodes and transistors lets in short recap on those parts.

Forward blocking mode :

In this mode of operation, the anode terminal of atomic number 14 management rectifier is connected to positive terminal of the facility provide and the cathode terminal is connected at the negative terminal of the facility provide. whereas the zero voltage is AN applied at the gate terminal. during this case junction J1 and J3 square measure forward-biased, whereas J2 is reverse-biased, because of that solely little outflow current exists from the anode to the cathode till the applied voltage reaches its break over price, at that J2 undergoes avalanche breakdown, and at this break over voltage it starts conducting, however below break over voltage it offers terribly high resistance to the present and is claimed to be within the off state.


Forward conduction mode :

In this mode, the thyristor / SCR has been triggered into conductivity by a current on the gate. it’ll stay to conduct no matter the state of the gate. Then, once a current flows into the gate, it switches on the lower electronic transistor, that switches on the higher one, that switches on the lower one, and so on. Once it’s forward biased then there’s no would like of any voltages at the gate terminal. Either by increasing the voltage between anode and cathode on the far side the break over voltage or by applying a positive pulse at the gate. Once the SCR starts conducting, no additional gate voltage is needed to take care of it within the ON state.


Reverse blocking mode :

In this mode or state, the thyristor blocks this within the same approach as that of a reverse biased diode. The thyristor / SCR will solely conduct in one direction and blocks within the reverse direction. currently, you’ll be able to in all probability see that each the higher and lower diodes area unit reverse biased, therefore still no current flows through the thyristor. this is often known as reverse interference. Usually, the reverse interference voltage rating and forward interference voltage rating is that the same. the standard application for a reverse interference SCR is in current-source inverters.


Gate to cathode resistor :

The check circuit for associate degree SCR is each sensible as a diagnostic tool for checking suspected SCRs and conjointly a superb aid to understanding basic SCR operation. A DC voltage supply is employed for powering the circuit, and 2 pushbutton switches square measure wont to latch and unlatch the SCR, severally. With no current flowing into the gate, the thyristor is shifted and no current flows between the anode and therefore the cathode.

2 transistor using scr

        Fig: Two transistors using SCR

  • When a current flows into the gate, it effectively flows into the bottom (input) of the lower (NPN) electronic transistor, turning it on.
  • Once the lower electronic transistor is switched on, current will flow through it, activating the bottom (input) of the higher (PNP) electronic transistor, turning that on further.
  • Once each transistors square measure turned on fully, current will flow all the manner through each of them—through the whole thyristor from the anode to the cathode.
  • Since the 2 transistors keep each other switched on, the thyristor stay on even if the gate current is removed.

I-V Characteristics Curves :

I-V Characteristics Curves

        Fig: I-V Characteristics Curves

The application of any gate signals or pulses once regeneration is initiated can don’t have any impact in the slightest degree as a result of the thyristor is already conducting and fully-ON. The magnitude and period of the gate “turn-on” pulse have a very little impact on the operation of the device since the physical phenomenon is controlled internally.this is often as a result of with no gate signal applied, a semiconductor controlled rectifier blocks current in each direction of AN AC undulation, and once it’s triggered into a physical phenomenon, the regenerative latching action means it can’t be turned “OFF” once more simply by victimization its Gate. Thyristors employed in circuits fed from DC provides, this natural commutation condition cannot occur because the DC provide voltage is continuous thus other thanks to flip “OFF” the thyristor should be provided at the acceptable time as a result of once triggered it’ll stay conducting. throughout the negative cycle, the Anode becomes negative whereas the Cathode is positive. The thyristor is reverse biased by this voltage and can’t conduct notwithstanding a Gate signal is a gift.