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9.5.4 Advanced SCR Tests
Note: Steps using the battery should be performed quickly to
avoid draining the battery.
Test 1 - Check Gate Triggering
Gate triggering "on" can be demonstrated by using the 9V bat-
tery as a gate source and the megohmmeter as a high voltage
low current DC power source as indicated in figure 25. Use the
multimeter DC voltage range to determine the DC output volt-
age polarity of the megohmmeter if the megger is not marked.
Figure 25 - Gate Triggering Test Diagram
Note: Gate and one cathode wire must be disconnected.
1. Connect megohmmeter positive lead to SCR anode 'A' and
connect meter negative to the SCR cathode 'K.'
2. Connect 9V battery negative to the SCR cathode 'K.'
Connect the 100-ohm resistor to battery positive.
3. Energize the megohmmeter and note reading.
4. Touch the open end of the 100-ohm resistor to the appropri-
ate SCR gate 'TG' or 'LG.' Megger reading should go to
short.
5. Open the resistor connection to the gate. The megger read-
ing should rise to the reading noted in step 3.
6. Repeat connections and test sequence on the other SCR in
the module to verify its gating.
This test is for gating only since the megger cannot provide
enough anode current to latch or hold the SCR "on. " The SCR
turns off when gate current is removed.
Siemens Energy & Automation, Inc.
Test 2 - SCR Latching and Holding
If latching or holding current SCR characteristics must be vali-
dated, perform the following low voltage test using resistors,
battery (see note), and multimeter; see figure 26.
Note: For the largest sizes of SCR's, paralleled batteries and/or
heavy duty batteries may be needed to deliver sufficient current
to latch and hold.
Figure 26 - Latching/Holding Test Diagram
1. Connect battery minus to SCR cathode 'K.'
2. Connect battery positive to the 100-ohm resistor; leave
other resistor end open.
3. Also, connect battery positive to the 20-ohm resistor and the
other resistor end to the SCR anode 'A.'
4. Select DC voltage scale on the multimeter.
5. Connect the meter positive to the SCR anode 'A' and meter
negative to the cathode 'K.' Meter reading should be approx-
imately 9V DC.
6. Touch the open 100-ohm resistor end to the appropriate
SCR gate 'TG' or 'LG.' Meter reading should drop to approx-
imately 1V DC. Open the resistor connection to the gate;
meter should remain at 1V. (See note above re batteries.)
7 . Disconnect all wires and meter.
8. Repeat above sequence on the second SCR in the module.
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