sTEm–at-Work Puzzle #33: Analysis for Possible SCR Failure

sTEm Puzzle 33 (Published in the March edition of the FLATE Focus)

The technician’s examination of the voltage, current data from the SCR is straight forward. Following the
voltage values as they increase from left to right show the expected low current values when the SCR is OFF. The forward breakover voltage value is the voltage value as the current values take a quick upturn. At this voltage value, the SRC turns ON and voltage cascades back to a low value that corresponds to the holding current. Since the plotted data corresponds to the exact expectations for a properly operating SCR , the tech does not have to replace this circuit. The conversation with students can be expanded to explain the rest of the details on the operation of the SCR.

Under normal operation, the voltage applied across the anode and the cathode will be less than the forward breakover point. If a positive voltage is applied at the gate it causes the gate current to increase. The more the gate current, the lower the point at which forward breakover will occur. Once ON, the SCR stays ON and permits high current through the device. The SCR will turn off when the current between “A” and “K” is lowered below the “holding” current.

The SCR circuit the technician tested is not functioning properly and it needs to be replaced. NO