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Checking a thyristor with a test lamp on table...
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Thyristor, when checked on
digital multimeter, should show "open circuit" in both directions.
Resistance between Anode and cathode is sometimes as low as 2 k but such
thyristors also work okay. Gate and cathode, when checked
on "diode test" range , sometimes show voltage drop of
approximately 0.5 to 0.7 volts in one direction and "open" in other
direction. Some thyristors show "diode" in both directions. But
"open" in both directions is not a sign for good thyristor.To check
if the thyristor ( rated at 60 amps or above ) is triggering okay with gate
pulses, the following set up on table can reveal the result. The switch
should be pressed for short time, for a second or so, to avoid damage to the
gate junction. If the lamp glows when the switch is closed, the thyristor can
be considered to be in good condition. Very small thyristor of ratings less
than typically 40 amps may require higher value of resistor to limit the gate
current. High values like 47 k can be selected to begin with and reduced if
the bulb does not glow. Gate resistor and a switch shown below may have to be
shifted before the lamp in some cases. Do not hold the switch down for long
time. Just a momentary trigger is enough. |
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Download PDF
version of these pages.
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Speed control not satisfactory
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