Part A. Testingscrs Withanohmmeter

Part A. TestingSCRs withanOhmmeter

In Chapter 2 you learned how to test diodes with an ohmmeter. Remember that the meter has an internal battery which can forward bias the diode if the positive lead of the meter is connected to the anode and the negative lead is connected to the cathode. The diode then reads a low resistance. When the meter leads are reversed, the diode reads a high resistance. In this experiment, you'll learn how to test an SCR or TRIAC using an ohmmeter in a similar fashion.

EQUIPMENT

Multimeter

(1) low-power SCRs – SK3954

Jumper wire

BACKGROUND INFORMATION

You have learned that an SCR is a PNPN "sandwich," as shown in Fig. E13-1B. If you connect an ohmmeter from anode to cathode as shown, you will read a high resistance. Even if you reversethe leads of the meter, you will still read a high resistance. Likewise, if you connect

E13-1 Measuring resistance of a PNPN "sandwich

an ohmmeter from gate to anode, as shown in part C, you will read a high resistance in both directions. However, if you connect an ohmmeter between gate and cathode, as in part D of the figure, you will read a high resistance in one direction and a low resistance in the other direction. This gives you a simple way of identifying the gate lead and cathode lead. The anode lead is usually mounted to the stud or heat sink of the SCR and is easy to recognize.

PROCEDURE

1.Obtain a low-power SCR and make a sketch of it in the space below. With your ohmmeter identify the anode, cathode, and gate leads and label them on your drawing.

2.You will now perform a simple test to see whether the SCR is in working condition. With your ohmmeter set to the R x 1 scale, connect the meter from anode to cathode, as shown in Fig. E13-1E. Be sure to have the positive lead of the meter connected to the anode. The meter should read a high resistance (open circuit) with the gate lead unconnected.

Do you read a very high resistance?

3.Now connect a jumper lead from anode to gate. The resistance of the SCR should drop to a low value because the SCR fires when the gate is made positive with respect to the cathode (If you don’t get this result try a different scale on the meter). The battery in the meter makes the gate positive.

Do you read a low resistance?

4.Now remove the clip lead from anode to gate, but keep the meter attached from anode to cathode. Does the SCR remain on?

Here's an important point. Touching the clip lead from the gate to the positive terminal triggered the SCR. Once the SCR was triggered, the signal at the gate was no longer needed. The current from the meter keeps the SCR conducting. However, there is a minimum amount of current (called holding current) which must flow through the SCR to hold it in conduction. The amount of holding current is usually small, on the order of milliamps, but to supply this current the meter must be set on the low resistance scale (use the 2K ohm scale on the Fluke meter).

This test only works for low- to moderate-power SCRs (up to 20 A or so), because the gate drive and holding currents for a high-power SCR are more than the meter can supply. This test works for TRIACs as well as for SCRs.

Part B. SCRs in DC and AC circuits

You will continue to work with a simple SRC and it’s equivalent circuit that uses PNP and NPN transistors.

EQUIPMENT

Oscilloscope, Bench AC power supply

(1) low-power SCRs – SK3954

(1) 2N3904 Transistor

(1) 2N3906 Transistor

Resistors: (1) 160 Ω, (2) 1.0 kΩ, (1) 10k Ω.

(1) capacitor, 0.1 µF

 (1) 12.6 VACpowersupply

 (1) 10kΩ potentiometer

1. Build the circuit of below, but do not apply power yet. A piece of wire can be used for the switched. Adjust R2 for maximum resistance. Apply power and close the switch, the LED should be off.

Table 1

2. Measure the voltage across the transistor Latch (a representation of a SCR); from the emitter of Q1 (Anode of represented SCR circuit) to the emitter of Q2 (Kathode of represented SCR circuit) and record in Table 1 under Latch in the VAK –off state row.

3. Slowly decrease the resistance of R2 until the LED comes on. Re-measure the voltage across the Latch and record in Table 1 under Latch in the VAK –on state row. Measure voltage across R3 and record in the Gate Trigger row under Latch.

4. Open the switch, the LED should stay on because of the latching. Connect a meter across R4. Slowly decrease the VCC while monitoring the voltage across R4. Record the smallest voltage across R4 with the LED on. Then calculate the current through R4 and record as IHolding, this is the minimum value.

5. Replace the transistor Latch with the SCR as shown below. Repeat steps 2 – 4 and record the data in Table 1 in the column under SCR.

6. The only way to turn off an SCR is to drop the current through it below the minimum holding current. The following circuit is an example of how this can be accomplished for DC control circuits; called capacitor commutation.

7. Test this circuit operation by closing S1, adjust R2 so the SCR is switched on, open S1, and then closing S2 momentarily. Your observations:

8. SCRs in AC circuits are typically used to control motors. Control can come by triggering the gate on either the positive or negative transition of the AC signal (depends upon circuit configuration (the above circuit would work on the positive transition). The SCR losses conduction on the opposite signal transition and commutation isn’t necessary. Remove the commutation circuit from the above circuit. Replace the DC power supply with a 12.6 VRMS (use the plug-in Lab 12.6 VRMS). Record and plot the voltage across R4and the SCR (use the difference function on the O-Scope for VR4) and compare to the Voltage across the SCR.

VR4 vs Time

VAK vs Time

Lab Quiz Questions

1. How do you turn off a conducting SCR in a DC circuit?

1. To what does commutation in an SCR circuit refer?

1. For the SCR circuits in this lab, explain why directly connecting the Anode of the SCR to VCC would burn the SCR out.

1. In the SCR circuits of this Lab, what would be the symptoms of a short of the Anode to Kathode pins of the SCR?

1. How would the voltage waveform across R4 change if the holding current for the SCR was higher?