What is silicon controlled rectifiers
Silicon controlled Rectifiers, as known as SCR, a four-layer solidstate current-controlling gadget. The name “silicon controlled rectifier” is General Electric’s exchange name for a sort of thyristor. SCRs are principally utilized in electronic gadgets that require control of high voltage and power. This makes them relevant in medium and high AC control activities, for example, engine control work. A SCR conducts when an entryway heartbeat is connected to it, much the same as a diode. It has four layers of semiconductors that structure two structures to be specific; NPNP or PNPN. Also, it has three intersections marked as J1, J2 and J3 and three terminals(anode, cathode and a door).
SCRs are unidirectional gadgets (for example can lead current just one way) rather than TRIACs, which are bidirectional (for example charge transporters can move through them in either heading). SCRs can be activated regularly just by flows going into the entryway rather than TRIACs, which can be activated typically by either a positive or a negative current connected to its door anode. A SCR is diagramatically spoken to as demonstrated as follows.
The anode associates with the P-type, cathode to the N-type and the entryway to the P-type as demonstrated as follows.
In a SCR, the natural semiconductor is silicon to which the required dopants are implanted. In any case, doping a PNPN intersection is subject to the SCR application.
How silicon controlled rectifiers works
The working principle of SCR
Let’s take an example here:
The SCR is a four-layer, three-intersection and a three-terminal gadget and is appeared in fig.a. The end P-locale is the anode, the end N-district is the cathode and the inward P-area is the door. The anode to cathode is associated in arrangement with the heap circuit. Basically the gadget is a switch. In a perfect world it stays off (voltage blocking state), or seems to have an interminable impedance until both the anode and door terminals have appropriate positive voltages regarding the cathode terminal. The thyristor then switches on and current streams and keeps on directing moving along without any more entryway signals. In a perfect world the thyristor has zero impedance in conduction state. For turning off or returning to the blocking state, there must be no entryway signal and the anode current must be decreased to zero. Current can stream just one way.
Without outside predisposition voltages, the lion’s share bearer in each layer diffuses until there is a worked in voltage that retards further dissemination. Some lion’s share bearers have enough vitality to cross the boundary brought about by the hindering electric field at every intersection. These bearers at that point become minority transporters and can recombine with larger part bearers. Minority transporters in each layer can be quickened over every intersection by the fixed field, but since of nonattendance of outer circuit for this situation the aggregate of lion’s share and minority bearer flows must be zero.
A voltage predisposition, as appeared in figure, and an outside circuit to convey current permit inside flows which incorporate the followâing terms:
The present Ix is expected to
Larger part bearers (openings) crossing intersection J1
Minority bearers crossing intersection J1
Openings infused at intersection J2 diffusing through the N-locale and intersection juncâtion J1 and
Minority bearers from intersection J2 diffusing through the N-area and intersection J1.
So also I2 is because of six terms and I3 is because of four terms.
The two straightforward analogs to clarify the essential activity for the thyristor are those of the diode and the two transistor models.
1. Diode Model. The thyristor is like three diodes in arrangement as there are three P-N intersections. Without entryway inclination, there is dependably at any rate one invert one-sided intersection to anticipate conduction independent of the extremity of a connected voltage among anode and cathode. In the event that the anode is made positive and the entryway is likewise one-sided emphatically as for cathode, the P-layer at the door is overflowed by the electrons from the cathode and loses its way of life as a P-layer. Likewise the thyristor winds up proportional to a directing diode.
2. Two Transistor Model. Envision the SCR cut along the specked line, as appeared in fig. a. At that point we can have two gadgets, as appeared in fig.b. These two gadgets can be perceived as two transistors. Â The upper left one is P-N-P transistor and the lower right N-P-N type. Further it very well may be perceived that the base of the P-N-P transistor is joined to the authority of the N-P-N transistor while the gatherer of P-N-P is joined to the base of N-P-N transistor, as represented in fig. c. The door terminal is brought out from the base of the N-P-N material. This development has been imagined simply to clarify the working of SCR, generally fit as a fiddle the SCR has four strong layers of P-N-P-N type as it were.
Presently we can see that the two transistors are associated in such a way, that the authority of Q1 is associated with the base of Q2 for example the yield gatherer current of Qt turns into the base current for Q2. In the comparable manner the authority of Q2 is joined to the base of Q1 which demonstrates that the yield gatherer current of Q2 is encouraged to Q1 as info base current. These are consecutive associations of transistors so that the yield of one goes into as contribution of other transistor and the other way around. This gives net addition of circle circuit as Î²1 x Î²2 where Î²1 and Î²2 are current increases of two transistors individually.
At the point when the entryway current is zero or the door terminal is open, the main current available for use is the spillage current, which is little if there should be an occurrence of silicon gadget speciallyâ and the complete current is somewhat higher than entirety of individual spillage flows. Under these conditions P-N-P-N gadget is said to be in its forward blocking or high impedance ‘off state. When a limited quantity of door current is given to the base of transistor Q2 by applying forward predisposition to its base-producer intersection, it creates the gatherer present as Î²2 times the base current. This gatherer current of Q2 is nourished as information base current to Q: which is additionally duplicated by Î²1 times as ICl which structures info base current of Q2 and experiences furtherâ enhancement. Along these lines the two transistors criticism one another and the authority current of each continues duplicating. This procedure is brisk and soon both the transistors drive each other to immersion. Presently the gadget is said to be in.on-state. The current through the on-state SCR is constrained by outer impedance as it were.
Silicon controlled rectifier working and characteristics
The working of thyristor can be comprehended by considering the three states methods of procedure of SCR (silicon controlled rectifier).
Current Flow and Voltage Bias in an SCR
Reverse Blocking Mode
On the off chance that we turn around the associations of the thyristors, at that point the upper and lower diodes are invert one-sided. In this way, there is no transmission way, so there is no progression of current. Consequently, this is called as invert blocking mode.
Forward Blocking Mode
For the most part, with no enacting heartbeat to the entryway terminal, SCR remains turned off, speaking to no progression of current the forward way. This is on the grounds that; the associated two diodes together will frame a thyristor. In any case, the association between these diodes is invert one-sided, which lessens the present stream through and through. Thus, this state is called as the forward blocking mode. In this mode, despite the fact that thyristor is including condition like a customary forward one-sided diode, it won’t execute as the door terminal isn’t initiated.
Forward Conducting Mode
In this mode, the anode voltage ought to be better than the cathode voltage and the door terminal must be actuated appropriately for the transmission of the thyristor. This is on the grounds that, when the door terminal is initiated, at that point the lower transistor will perform which switches ON the upper transistor and afterward the upper transistor switches ON the lower transistor and in this manner the transistors triggers one another. This method of inward positive input of both the transistors repeats until both get completely activated and after that the progression of current will from the anode to the cathode. Along these lines, this type of activity of SCR is called as forward conduction mode.
The figure demonstrates the silicon controlled rectifier attributes and furthermore speaks to the thyristor task in three distinct modes, for example, invert blocking mode, forward blocking mode, and forward directing mode. The V-I qualities of thyristor likewise speak to the invert blocking voltage, forward blocking voltage, switch breakdown voltage, holding current, break-over voltage, etc as appeared in the figure.
Silicon controlled rectifiers application
The Silicon controlled rectifiers applications is as following:
- Use of SCR incorporates into the circuits which manage huge flows and voltages like electrical power framework circuits with more than 1kV or bigger than 100A of flow.
- These are especially used to diminish the inside power misfortune in the circuit.using on-off exchanging control of the thyristors.
- SCRs are additionally utilized for adjustment reason, i.e., from rotating current to coordinate current. Normally, thyristors are utilized in cyclo converters (AC to AC converters) which is the most broad application.
How do you test a silicon controlled rectifier
A rectifier is a gadget that just enables electrical flow to stream one way. A silicon-controlled rectifier, otherwise called a SCR, is a rectifier where the forward obstruction can be controlled. Regularly a SCR won’t enable current to stream in either heading, yet on the off chance that you supply a sign to the entryway of the SCR, it will permit some measure of current (in light of the sign at the door) to stream in one course. An ohmmeter is an instrument that estimates electrical obstruction. An ohmmeter can be utilized to check that a SCR is carrying on accurately.
Set your ohmmeter to the R x 10,000 setting. Associate the negative lead of your ohmmeter to the anode of the SCR and the positive lead to the cathode of the SCR.
Silicon control rectifier advantages and disadvantages
Silicon control rectifier advantages advantages is as following:
- The silicon controlled rectifier (SCR) can deal with enormous voltage , current and power.
- It very well may be ensured with the assistance of wire.
- It is anything but difficult to turn ON.
- The Triggering circuit for silicon controlled rectifier (SCR) is basic.
- It is easy to control.
- It cost is low.
- It can control AC control.
Silicon control rectifier advantages disadvantages is as following:
- The silicon controlled rectifier (SCR) is unidirectional gadgets, so it can control just in DC control during positive half cycle of AC supply. Accordingly just DC power is controlled with the assistance of SCR.
- In AC circuit, it should be turned on each cycle.
- It can’t be utilized at higher frequencies.
- The door current can’t be negative.