THYRISTOR-Part A: TESTING SCRs WITH AN OHMMETER
In the past 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
 VOM (ohmmeter) Best results when using either a Simpson 260 or RSR 7050 analog meters.
 (1) low-power SCRs – SK3954
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
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 (the procedure is
detailed above).
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
(use the 2K ohm scale on the Fluke 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 20A 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.
THYRISTOR-Part B: DIAC-TRIAC PHASE CONTROL
You will now work with a simple DIAC-TRIAC phase control circuit which can be used to
vary the brightness of a lamp or vary the speed of a small motor. For safety purposes, it is
recommended that you use an isolation transformer between your circuit and the a-c line or
that you connect your measuring instruments to the various test points before plugging your
circuit into the a-c line.
EQUIPMENT
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Oscilloscope, Bench AC power supply
Use TRIAC 2N6342 instead of the 2N6151
Use DIAC ST2 instead of 1N5761
capacitor, 0.5 µF, 200 V, paper or plastic
capacitor, 0.1 µF, 200 V, paper or plastic
potentiometer, 100 kΩ, 1 W
resistor, 1 kΩ 5%
resistor, 100 Ω, 5%
25-W, 120-V lamp
1. Build the circuit below, but do not apply power yet. A suggested layout of components is
shown in the pictures below the schematic.
Circuit for Part B
2. Connect the common of your scope to point D, which is terminal MT1, of the TRIAC.
Connect one probe to point A and the other to point C.
NOTE Sync your scope on waveform A.
3. Use the Bench AC power supply to apply 50 VAC. Vary the resistance of R2 back and
forth a few times. You should observe a waveform at point C similar to that of signal shown at
the top of the 20th lecture slide.. Does the brightness of the lamp change as R2 is varied?
________. With the light on dimly sketch the waveform at points C over-laid on the waveform
at point A (NOTE - use dashed lines for point A waveform) and mark the sketch with voltage
and time.
Shut off the a-c power, remove your probe from point C, and connect it to point G, the
gate of the TRIAC. Now reapply a-c power. Vary R2 back and forth a few times and observe
the waveform at point G. Observe the trigger spikes that fire the TRIAC. The spikes should
advance or retard as you vary R2. Note how the brightness of the lamp changes with the position
of the spikes. With the light on dimly sketch the waveform at point G over-laid on the waveform
at point A (NOTE - use dashed lines for point A waveform) and mark the sketch with voltage
and time.
4.
Shut off the a-c power, remove your probe from point G, and connect it to point E, the
voltage across the TRIAC. Now reapply a-c power. Vary R2 back and forth a few times and
observe the waveform at point E. Observe the time that the voltage across the TRIAC is near
zero varies with as you vary R2. Note how the brightness of the lamp changes with the amount
of the cycle that has the voltage across the TRIAC near zero. Below with the light on dimly
sketch the waveform at point E over-laid on the waveform at point A (NOTE - use dashed lines
for point A waveform) and mark the sketch with voltage and time.
5.
Power Control Devices 391
QUIZ
1.
In step 3, increasing the resistance of R2 caused C1 to charge (faster, slower).
2.
When C2 took a longer time to charge, the lamp burned (brighter, dimmer).
In step 4, increasing the resistance of R2 caused the trigger spikes to occur (earlier,
later) in the cycle.
3.
4.
When the spikes occurred earlier in the cycle, the lamp burned (brighter, dimmer).
If Cl became disconnected from point C, the trigger spikes (would never occur, would
always occur early regardless of the setting of R2).
5.
If R2 were replaced with a 10-KΩ pot, could you still adjust the lamp to maximum
brightness? (yes, no)
6.
Referring to question 6, could you still adjust the lamp to the same minimum
brightness? (yes, no)
7.