EXPERIMENT V.docx

EXPERIMENT V CURRENT CONTROL OF AN SCR I. OBJECTIVE: After completing and performing the laboratory experiment, you will able to: 1. Demonstrate the effect that gate current has to turn on an SCR, and to determine the minimum holding current to keep the SCR conducting. 2. Define parameters associated with SCRs, such as gate trigger current, holding current, forward ON-state voltage and give the approximate range of values expected for these parameters. INTRODUCTION: The experiment shows that sufficient gate current must flow in order to turn on the SCR and that the minimum holding current can be found with the addition of large-value potentiometer in anode circuit. II. MATERIALS NEEDED: 1 Fixed + 12-V power supply 1 Standard or digital voltmeter 1 Standard or digital ammeter 1 C106Y1 SCR or equivalent 1 100- resistor at 0.05 W (R1) 1 22-k resistor at 0.5W (RB) 1 100-k resistor at 0.5W (RA) 1 50-k potentiometer (RH) 2 DPST switched (S1 and S2) 1 Breadboard for constructing circuit III. PROCEDURE: 1. Construct the circuit shown in figure 5.1. 2. Set both switches as indicated and then apply power to the circuit. 3. Calculate the gate current IRA, flowing through RA and record in table 5.1. 4. Measure VAK and record in the place indicated next to IRA. Is the SCR on or off? 5. Move S1 to position B. 6. Calculate the gate current IRB flowing through RB and record in the place indicated. 7. Measure VAK and record in the place indicated next to IRB. Is the SCR on or off? 8. Remove the power supply voltage from the circuit. 9. Modify the circuit by adding the ammeter and 50-k potentiometer (RH) in series with load resistor RL. 10.Set the wiper RH so that the resistance is completely “shorted out”. 11.Make sure the S1 and S2 are set as indicated and then apply power to the circuit. 12.Momentarily move S1 from position A to position B and back again. 13.Recording the reading of VAK and IA in table 5.2. 14.Slowly adjust RH so that the current IA begins to decrease. 15.Remember the reading of IA when VAK increases to +VAA. Record this value in table 5.2 indicated for the minimum holding current of the SCR. (Perform steps 10 through 15 a few for a more accurate reading).

Figure 5.1 Current control of an SCR.

DATA SHEET: Table 5.1 Figure 5.1:Without potentiometer. S1

S2

A B

A A

I RA =

I RB =

V AA RA

V AA RB

V AK

Condition(On or Off)

0A 0A

Table 5.2 Figure 5.1:With potentiometer Potentiometer setting(%) 0 5 10 20 30 40 50 60 70 80 90 100

V AK

IA

FILL-IN QUESTIONS: 1. If the gate resistor is too large, not enough gate current will flow to conduct/trigger the SCR. 2. When the gate resistor is sufficient enough to latch, the proper value of gate current will flow to trigger the SCR. 3. Sufficient anode current is required to keep the SCR conducting. 4. If the load resistance in series with the anode is too large, not enough current flows from cathode to anode and the SCR will turn off .

ANALYSIS OF RESULTS:

QUESTIONS AND PROBLEMS: 1. How much gate current is needed to trigger a medium-power SCR? More likely 10mA because the SCR will not conduct if the gate current is less than 10 mA 2. After an SCR has fired, what effect does the gate signal have on the SCR? The effect of a gate signal on the firing of an SCR is shown in the breakdown of the center junction achieved at speeds approaching a microsecond by applying an appropriate signal to the gate lead, while holding the anode voltage constant. After breakdown, the voltage across the device is so low that the current through it from cathode to anode is essentially determined by the load it is feeding. 3. What effect does an increase in anode current have on anode cathode voltage? There will be a Depletion layer. 4. Explain why an SCR is superior to a series rheostat for controlling and limiting current through a load. Rheostats are used in many different applications, from light dimmers to the motor controllers in large industrial machines. However, it has largely been replaced by the triac, a solid-state device also known as a silicon controlled rectifier (SCR). A triac does not waste as much power as a rheostat and is more reliable due to the absence of mechanical parts. Rheostats commonly fail because their contacts become dirty or the coil wire corrodes and breaks. 5. Explain the difference between an SCS and an SCR. The silicon controlled switch (SCS) is the next step beyond the silicon controlled rectifier (SCR). It is still a four-layer diode, but this time all four regions are accessible to the external circuit. The basic construction of the SCS is the same as for the SCR, with the addition of a second gate lead while Silicon-Controlled Rectifier, or SCR, is essentially a Shockley diode with an extra terminal added. This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage.