2-22Q-15.When testing an SCR with an ohmmeter, the SCR will conduct if what two elements are shortedtogether?TRIACTriac is General Electric’s trade name for a silicon, gate-controlled, full-wave, ac switch, as shown infigure 2-13. The device is designed to switch from a blocking state to a conducting state for either polarityof applied voltages and with either positive or negative gate triggering. Like a conventional SCR, theTriac is an excellent solid-state device for controlling current flow. You can make the Triac conduct byusing the same method used for an SCR, but the Triac has the advantage of being able to conduct equallywell in either the forward or reverse direction.Figure 2-13.—Testing a Triac with an ohmmeter.To test the Triac with an ohmmeter (R 1 scale), you connect the ohmmeter’s negative lead to anode1 and the positive lead to anode 2, as shown in figure 2-13. The ohmmeter should indicate a very highresistance. Short the gate to anode 2; then remove it. The resistance reading should drop to a low valueand remain low until either of the ohmmeter leads is disconnected from the Triac. This completes the firsttest.The second test involves reversing the ohmmeter leads between anodes 1 and 2 so that the positivelead is connected to anode 1 and the negative lead is connected to anode 2. Again, short the gate to anode2; then remove it. The resistance reading should again drop to a low value and remain low until either ofthe ohmmeter leads is disconnected.Q-16.When a Triac is properly gated, what is/are the direction(s) of current flow between anodes 1and 2?UNIJUNCTION TRANSISTORS (UJTs)The unijunction transistor (UJT), shown in figure 2-14, is a solid-state, three-terminal semiconductorthat exhibits stable open-circuit, negative-resistance characteristics. These characteristics enable the UJT
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