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Triac tester enables manual or automatic operation

05 Nov 2015  | Abel Raynus

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Triacs are bidirectional ac switches that can control loads with currents as high as 25A rms at voltages as high as 600V. They find wide use in motor-speed, heater, and incandescent-lamp controls. Logic triacs are especially attractive for microcontroller-driven devices. You can activate a triac directly from microcontroller-output ports because of the triac's trigger current of only 3 to 10 mA. As with any electronic device, triacs can have some internal problems that you can detect before using them in a design.

Figure 1: A triac tester uses a switch to reverse the polarity of the test signal.

Figure 1 shows a simple and inexpensive test fixture that tests the L2004F31, L2004F61, L2004L1, and L4004V6TP triacs from Littelfuse, but you can use it to test any other leaded triac because all the standard packages, including TO-220AB, TO-202AB, TO-251, and IPak, have the same pin layout. An IC socket provides easy insertion of a triac under test. You can also apply this idea to SMDs (surface-mount devices), provided that you can find or create an appropriate test socket. Polarity switch S1, a DPDT (double-pole/double-throw) device, lets you check conductivity in both directions. Trigger switch S2, a momentary SPST (single-pole/single-throw) pushbutton device, activates the triac under test by connecting the gate (Pin 3) with MT2 (Pin 2) through resistor R2 (figure 1).

The test takes less than 5 seconds and comprises four steps (table 1). An LED indicates the result of each step to the test operator. A triac is good if it passes all four tests. You should perform another triac test during manufacturing to ensure that there is no problem with the subassembly board and that the triac works properly. This test saves time and labor in case you detect a problem after assembling the entire product. You perform this test with the triac soldered into place on the board. You use the nominal power-supply voltage of 120/220V ac. The test should have minimal influence on the DUT and should use minimal time and labor. This test uses the triac tester in place of a load. The connection from the tester to the DUT can vary, and be sure to take some safety measures when connecting 120/220V ac.

Figure 2: With a resistive load, the tester uses two LEDs to indicate pass and fail in both directions.

You use a different test fixture for triacs that drive a resistive load, such as an incandescent lamp or a heater (figure 2). Each LED checks conductivity in one direction. When the triac is closed, both LEDs should be off. When it is open, both LEDs should be on. In the case of an inductive load, such as a motor, use an RC snubber circuit comprising C1 and R1 in parallel with the triac (figure 3). Unfortunately, the snubber circuit introduces a small current leakage into the test circuit even when the triac is closed. The circuit in figure 3 shows you how to avoid this problem using resistor R2 and a neon lamp with an ac breakdown voltage of 95V.

Figure 3: For an inductive load, add a neon lamp to minimize leakage current.

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