EXPERIMENT IV SILICON-CONTROLLED RECTIFIER

EXPERIMENT IV
SILICON-CONTROLLED RECTIFIER

I. OBJECTIVE:

After completing and performing the laboratory experiment, you will able to:
1. Demonstrate a practical go/no go methods of testing an SCR with ohmmeter.
2. Show the turn-on (fire) and turn-off (reset) methods for an SCR.
3. Determine the electrical characteristics of a particular SCR.
4. Define the various SCR parameters.

INTRODUCTION:

The PN junction from gate to cathode of an SCR can be tested with an ohmmeter similar to a regular diode. However, testing from anode to gate will not indicate if an SCR is working properly, because one of the PN junctions is always reverse biased. The SCR can be tested with an ohmmeter by placing the positive lead on the anode and the negative lead on the cathode with the gate left open. The meter should read high or infinite resistance. Placing a clip lead from the anode or positive lead of the ohmmeter to the gate triggers the SCR and the meter should indicate low resistance. When the clip lead is removed, the meter continues to indicate low resistance if the power source is sufficient to produce the required holding current.

To conduct, the SCR must have its anode more positive than its cathode. When the gate voltage is made more positive than its cathode, the SCR turns on or fires and current flows from cathode to anode. When the gate voltage is again made equal to or more negative than the cathode, current continues to flow through the SCR. The SCR is turned off or reset by reducing the current through it below its holding current.







II. MATERIALS NEEDED:

1 Fixed + 12-V power supply
1 Standard or digital voltmeter
1 C106Y1 SCR or equivalent
1 100- resistor at 0.5 W (R1)
1 10-k resistor at 0.5 W (RG)
1 22-k resistor at 0.5 W(RA)
2 DPST switches (S1 and S2)
1 Breadboard for constructing circuit



PART I; TESTING AN SCR WITH AN OHMMETER

1. Set the ohmmeter to the midrange scale.
2. Connect the ohmmeter to the SCR as shown in figure 4.1 and record the meter reading in table 4.1.

Figure 4.1 Testing an SCR with an ohmmeter: (a) without clip lead; (b) with clip lead; (c) again without clip lead.





RESULTS:



NI Multisim Simulation



Actual


High resistance

PART II: OPERATION OF AN SCR

1. Construct the circuit shown in figure 4.2.
2. Set switches S1 and S2 as indicated and then apply power to the circuit.
3. In the first row of the data table 4.2, record the values of VG and VA.
4. Move S1 to position B and record the values of VG and VA in the second row of the data table.
5. Move S1 to position A and record the values of VG and VA in the fourth row of the data table.
6. Move S2 to position B and record the values of VG and VA in the fourth row of the data table.
7. Move S2 to position A and record the values of VG and VA in the fourth row of the data table.






Figure 4.2 Operation of an SCR.





NI Multisim Simulation


Fig 1a. Switch 1 is in position A and 2 in Position B


Fig 1a. Switch 1 is in position B and 2 in Position A






SILICON-CONTROLLED RECTIFIER
Experiment IV

DATA SHEET:
PART I : TESTING AN SCR WITH AN OHMMETER
Table 4.1: Ohmmeter Reading
GATE(G)
ANODE(A)
CATHODE(K)
READING(H or L)
None
Positive
Negative
-
Positive
Positive
Negative
-
None
Negative
Positive
-

FILL-IN QUESTIONS:
1. An SCR will have high resistance before being triggered.
2. An SCR will have low resistance before after triggered.
3. The gate -to- cathode resistance of an SCR can be checked like a normal diode.
4. An SCR is being tested with an ohmmeter. When the clip lead on the gate is removed, the meter indicates high resistance. This does not prove that
the SCR is defective, but that the power source of the meter is not sufficient to produce the necessary holding current through the device.

PART II: OPERATION OF AN SCR

Table 4.2
S1 Condition
S2 Condition
VG
VA
Condition of SCR(on or off)
A
A
1.2mV
11.996V
off
B
A
823.486mV
10.384mV
on
A
A
1.2mV
12V
off
A
B
1.2mV
12V
off
A
A
1.2mV
12V
off




FILL-IN QUESTIONS:
1. Before firing, the voltage from anode to ground of the SCR is equal to voltage source.
2. When the gate is made more positive the SCR fires and IAK flows.
3. Once the SCR fires, the gate gains control and the current starts to flow through the SCR.
4. When the SCR is conducting, the voltage from the anode to ground is equal to 10.384mV .
5. The SCR can be turned OFF by reducing the current through it below its holding current.


ANALYSIS OF RESULTS:

QUESTIONS AND PROBLEMS:

The letters SCR stand for silicon-controlled rectifier. Explain the use of the word rectifier in the name.
The SCR is a rectifier in that only when the anode is positive with respect to the cathode will it conduct with a positive gate trigger. The SCR is turned off when the anode is made negative with respect to the cathode. It's a controlled rectifier made of silicon. Controlled means it acts as a rectifier, conducts in only one direction, but that conduction can be turned on/off by a signal at the gate.

What two things must happen to cause an SCR to fire?
There are two ways to turn the SCR. The first method is to keep the gate open and make the supply voltage equal to the breakdown voltage. The second method is to operate SCR with voltage less than the breakover voltage and then turn it on by means of a small voltage applied to the gate.

What is the difference between triggering gate current and holding current?
Latching current is another name for gate trigger current -- it's a measure of the minimum current you have to apply to the gate of an SCR to be guaranteed it will turn on (given that there's a minimum voltage between anode and cathode). Holding current is the specified minimum current that must be flowing from anode to cathode (with no gate current) and still be guaranteed that the SCR will not turn off.


How much voltage across the anode-cathode terminals of a medium power SCR after it has fired?
It must be equal to it "Forward On " voltage, for 2N5060 is 1.7 V.

5. How can an SCR be made to conduct?
The thyristor is normally made to conduct by applying a gating pulse, while the main anode and cathode terminals are forward biased. When the device is reverse biased a gating pulse has no effect.

How can an SCR be turned off?
The simple way to turn off the SCR is to open the line switch. Also, To turn the thyristor off, the current flowing between anode and cathode must be reduced below a certain critical "holding current" value, (near to zero); alternatively the anode and cathode may be reverse biased.