Part A. Testing SCRs with an Ohmmeter
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 reverse the leads of the meter, you will still read a high resistance. Likewise, if you connect
E13 1 Measuring resistance of a PNPN "sandwich
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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
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Oscilloscope, Bench AC power supply
(1) low-power SCRs – SK3954
(1) 2N3904 Transistor
(1) 2N3906 Transistor
Resistors: (1) 160 Ω, (2) 1.0 k Ω, (1) 10k Ω.
(1) capacitor, 0.1 µF
(1) 12.6 VAC power supply
(1) 10k Ω 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.
VCC
Table 1
15V
Latch
LED1
R1
1.0kΩ
R4
VAK (off state)
1.0kΩ
R2
10kΩ
42 %
Key=A
Q1
2N3906
VAK (on state)
S1
Q2
2N3904
VGate Trigger
Key = S
C1
0.1µF
R3
160Ω
VR4
I
Holding
SCR
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.
VCC
15V
LED1
R1
1.0kΩ
R2
R4
1.0kΩ
10kΩ
32 %
Key=A
S1
S2
Key = S
R3
160Ω
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.
VCC
15V
R1
1.0kΩ
LED1
R5
10kΩ
R2
R4
1.0kΩ
10kΩ
50 %
Key=A
S1
SCR
C1
0.1µF
S2
Key = S
Key = O
R3
160Ω
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 R4 and 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?
2.
To what does commutation in an SCR circuit refer?
3. For the SCR circuits in this lab, explain why directly connecting the Anode of the SCR
to VCC would burn the SCR out.
4. In the SCR circuits of this Lab, what would be the symptoms of a short of the Anode
to Kathode pins of the SCR?
5. How would the voltage waveform across R4 change if the holding current for the SCR
was higher?