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COLLEGE OF ENGINEERINGELECTRONICS ENGINEERING DEPARTMENT

SILICON CONTROLLED RECTIFIER (SCR)

ECE513N-1FFRIDAY 0730am-1030am

BALBIN, MICHAEL RYAN R.20060150085

DAYAG, JOSE JR. J.

PELLOBELLO, BERNARD A.20070158633

QUEZON, JOHN PHILIP20060108778

DATE PERFORMED:JULY 01, 2010DATE SUBMITTED: JULY 09, 2010

GRADE

ENGR. ROMMEL G. RAMOSINSTRUCTOR

INTRODUCTION:


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A Silicon-Controlled Rectifier (SCR) is a four-layer (p-n-p-n) semiconductor

device that doesn't allow current to flow until it is triggered and, once triggered,

will only allow the flow of current in one direction. It has three terminals: 1) an

input control terminal referred to as a 'gate'; 2) an output terminal known as the

'anode'; and 3) a terminal known as a 'cathode', which is common to both the gate

and the anode.

SCR Component and Schematic Symbol for SCR

SCR's are generally used for switching and power control purposes in AC and high-powe

circuits. The SCR is a device that falls under a group of devices known as 'thyristors'

which refer to devices that have a 4-layer or p-n-p-n structure. The term 'silicon-controlled

rectifier' is a trade name used by General Electric in 1957 to refer to this type o

thyristor.

An SCR may be thought of as a rectifier whose ability to conduct current can be

controlled using a third terminal known as a 'gate'. While untriggered, an SCR wil

prevent any current to flow through it, except for a very small leakage current

caused by non-ideal conditions. The SCR is triggered to turn on if the voltage

across its gate and its cathode exceeds a certain threshold level.Once an SCR has been triggered, it will remain 'on' even if the triggering gate

voltage is removed, until the current flowing through it falls below a level known as

its 'holding current'. Thus, a conducting SCR will continue to conduct as long as the

current flowing through it is greater than the holding current. In normal AC

applications, an SCR is turned off automatically during the half-cycle wherein the

voltage and current are below zero.


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The p-n-p-n structure of an SCR may be modeled in terms of a PNP and an NPN

transistor, as shown in Figure 2. It can easily be seen from this diagram why an

SCR remains 'on' once triggered, even if the triggering gate voltage is removed.

Applying sufficient triggering voltage at the gate drives the NPN transistor to

conduct. This, in turn, pulls down the PNP's base voltage, causing the PNP to

conduct. The conducting PNP then supplies the base current to the NPN transistor

to keep it conducting. Unless the supply of current to the base of the NPN is cut

off, the circuit will continue conducting under this 'on' condition.

The Equivalent Circuit (left) and Structure (right) of an SCR

The four-layer construction shown above is known as a Silicon Controlled Rectifier

or SCR. To form it, we have added a connection to the p-type region next to the

cathode. This connection is known as the gate.

Characteristics curve of SCR

If we ground both the cathode and the gate, and apply a positive voltage to the

anode, no current will flow through this device. This is in keeping with the basic


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four-layer diode. In this case, however, we will not allow the applied anode voltage

to exceed the SCR breakover voltage. Thus, if nothing happens, the SCR wil

remain turned off indefinitely.

However, if we now apply a small positive voltage to the gate lead sufficiently to

forward bias the cathode junction, the device will immediately turn fully on. Again,

this is in keeping with the behavior of the basic four-layer diode. The difference is

that we can accurately control the timing and the applied gate voltage, ifnecessary. Thus, we can determine the conditions under which the SCR will fire

more accurately than we can for the basic four-layer diode.

APPLICATION:

SCR's, which can have voltage ratings of up to 2,500 volts and current ratings of

up to 3,000 amperes, are encountered in many AC and high-power applications.

Examples of applications for SCR's include: 1) power switching; 2) phase control;

3) battery charging; 4) power inverters; 5) motor switching and control; 6) high

voltage DC conversion.


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MATERIALS/EQUIPMENT:

The materials and equipment used in this experiment are:

1 12 Volts AC Power Supply

1 Dual Oscilloscope

1 Multimeter

10 Connecting wires

1 SCR (T106D1MEX)

1 Diode (1N4004)

1 Miniature lamp (12V)

1 47k resistor

1 500k potentiometer

1 Lamp holder


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EXPERIMENTAL PROCEDURE:

1. Set-up the experimental circuit shown (Gate Control Circuit for triggering

SCR).

VL

L1 D1 A

G Vak

12Vac K

R1 47kohms

R2 500kohmspotentiometer

Vg

2. In the above circuit L1 represents the load. Turn the 500k potentiometer

fully clockwise (R2=0 or 0%).

3. Using your multimeter, measure the voltages across the source (Vs), the

anode-to-cathode of the SCR (VAK), the gate voltage (VG) and the load

voltage (VL). Record the readings in the appropriate space provided.

4. Using your oscilloscope, monitor the voltage waveform across the source

(Vs), the anode-to-cathode voltage of the SCR (VAK), the gate of the SCR


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(VG) and the load (VL). Draw the waveform in the appropriate space

provided.

5. Repeat steps 3 and 4 for R2 (potentiometer) values of 25%, 50% and 75%

and 100%. Increase R2 resistance by turning the control counter clockwise.

Record and complete the readings in the appropriate table and space

provided below.

NOTE: To be more accurate, you can use the multimeter in changing the

resistance R2 (potentiometer). But dont forget to disconnect first the Vs before

changing and measuring the resistance of R2.

ANALYSIS AND COMPUTATION:We started analyzing from the waveform. After reading some references, the

increasing angle observed was the firing angle and with the degree was the firing

delay angle which is the angle on the sine wave where a switch turns on. The

switch turning on, like any switch, allows current to flow. As the gate current

decreases the firing delay angle goes up to 90 degrees and as the gate current

increases, the firing delay angle goes down to almost zero degrees. If R2 is low, the

gate current will be sufficient to fire the SCR when the supply voltage is low. Hence

the firing angle is will be small and the average load current will be large. If R2 is

high, the supply voltage must climb higher to deliver enough gate current to firethe SCR. Hence increases the firing angle and average load current. The purpose of

R1 is to maintain some fixed resistance in the gate load even if R2 is set to zero

which protects the gate from overcurrents, R1 also determines the minimum firing

delay angle.


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EXPERIMENTAL DISCUSSION:

When we performed the experiment and all the parameters were set up a certain

voltage soure of 12.6 volts ac and 0 ohm variable R2 (look at the schematic

diagram above). When we turned on the supply voltage (AC) the load or lamp also

turned on. The voltage across the anode and the cathode on the silicon controlled

rectifier (SCR) was 0.7 volts. Also, the voltage across the gate (Vg) on the SCR was

0.5 volts which seemed to have a very small voltage level. We also tried to turn up

the variable resistor (R2) from zero to maximum resistance and it showed that the

lamp turned off correspondingly. Hence the higher the resistance will result of no

current flow. The observation on the oscilloscope showed that when R2 was set to

zero resistance, the wave seemed to have a negative half cycle graph only it has a

little pulse at the beginning of the cycle. When we tried to increase the resistance

on the R2, the little pulse from before went up gradually to a 90 degree quarter

cycle and then down to zero to a negative half cycle.

The output on the oscilloscope at the last procedure was pretty obvious since we

already tried to increase and decrease the amount of R2 out of curiosity. At 25%

value of R2, there was a small increase of angle at the beginning of the cycle and

suddenly went down to zero then a negative half cycle. At 50% increase of R2, theangle at the beginning increased up to 45 degrees then again went down to zero

then a negative half cycle. At 75% and 100% resistance value, the angle increased

up to 70 degrees and 90 degrees respectively and repeated the same cycle just

like the others. The thing is, the increasing wave angle never went beyond 90

degrees.


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CONCLUSION:

After doing the experiment, we conclude that the operation of SCR issomehow comparable to an ordinary diode. The only difference of the two is thatthe SCR needs to be fired through its gate known as gate triggering current. Afterthe SCR has fired into its gate, then the SCR will just act as an ordinary diode.Maintaining the current flowing on the gate is not necessary after the SCR turnsON. In turning off this device, the current through Anode and cathode must beabove the holding current of the SCR.

On the circuit shown on this experiment, the triggering of the gate currentand its conduction level depends on the value of R2. If R2 is low, the current willpass through the gate because, based on Ohms Law, the current is indirectlyproportional to the resistance. And the current on the gate will not pass through ifthe value of the R2 is high.


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REFERENCES:

http://www.ecelab.com/scr.htm

http://www.play-hookey.com/semiconductors/scr.html

http://www.allaboutcircuits.com/vol_3/chpt_7/5.html


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ANSWERS TO QUESTIONS:

1. Described and interpret the waveform produce in VL, VG and VAK in all trials.

2. What is the function of the SCR in the circuit with respect to the load?

3. What is the effect of changing the value or R2 (potentiometer to the load)?

Answer:

The setting of R2 determines the firing delay angle. R2 can be in two possible

values. R2 can be low or high and of course it will have different effects since

current will flow through it. If R2 is low then the gate current will be

sufficiently large to fire the SCR when the Vs is low. Therefore, the firing

angle will be small, and the average low current will be large. If R2 is high,

the supply voltage must climb higher deliver enough current to fire the SCR

This increases the firing angle and reduces average load current.

4. What is the function of the gate in SCR?

Answer:

The gate in SCR is responsible for turning on the SCR which is the gate

triggering current (igt). Without this component the SCR will not be turned on

and in order for this device to be turned on, the gate current must be

triggered between 0.1 and 50mA to fire (turn on).


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5. In your own understanding, describe briefly the operation of the SCR.

Answer:

After doing the experiment, the operation of SCR is started by firing the gate

current (for about 0.1 to 50milliamperes) to start its operation. After the gate

has fired with few current, the SCR will function as an ordinary diode. The

current will flow now on anode going to cathode. It is not necessary required

to maintain the flow of current on the gate once the SCR has fired.

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