Electrical Projects Diodes and triacs are key components in motor-speed control devices such as those used in variable-speed electric drills. This tester enables the appliance repairman to efficiently troubleshoot such speed-control assemblies without removing the components from the circuit.

Using the clip leads connected to the top three terminals of the 8-terminal barrier strip, connect the lead from terminal 1 to the anode of the diode or triac, terminal 2 lead to the cathode; if testing triacs, connect terminal 3 lead to the gate. Switch SI, across terminals 4 and 5 of the terminal board, applies main power to the circuit and serves as a safety device that shuts off voltage to both sides of the diode to prevent shock. Low-wattage lamp II is merely to indicate when power is on. Switch S2 can be either a single-pole, single-throw or pushbutton switch (which operates from a normally open position). When closed, S2 shorts the diode.

If a diode is being tested, when S2 is closed lamp 12 should light with about half the brilliance of II since it is conducting only half the time. If it glows with equal brilliance the diode is not operating properly.

For triacs, R (100 ohms) provides a gate current. When S2 is closed, 12 should light—this time with greater brilliance than the diode since it is conducting during both the positive and negative half-cycles. It should not glow as brightly as II, however, since the voltage available to it has been reduced by R. In either instance, the lamp's failure to light indicates an open component.



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October 12, 2008