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 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.

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.

The indicators of the test result in figure 1, figure 2, and figure 3 are LEDs. Sometimes, the triac test is part of a multitasking test system that checks other components or parameters of the whole device, which includes the triac. This test involves a sequence of measurements, and a system operator gets only one of two possible signals: pass or fail. These tests use a microcontroller-based system. Thus, all the interface signals should be in digital format: high or low.

You can also use analog signals by activating the microcontroller’s ADCs. This approach is less preferable, however, because of the limited number of ADCs in low-end microcontrollers and more complicated software. Interfacing the triac under test with the microcontroller creates no problem if the triac’s MT₁ pin is grounded. In most cases, MT₁ and MT₂ are isolated from the ground. When this scenario occurs, you can use an optocoupler, such as the PS2501-2 from California Eastern Laboratories (Figure 4). It comprises two optically coupled isolators containing LEDs and NPN phototransistors with a maximum voltage of 80V.

If the triac output comprises a sequence of pulses, such as a PWM (pulse-width-modulated) signal for motor-speed or lamp-brightness control, then use a lowpass RC filter before the microcontroller’s ADC inputs (Figure 5). The time constant of this filter, τ=R₆×C₂, depends on the PWM signal period and duty cycle. The measurement in the chain of tests should start no earlier than 3–5τ. Using the microcontroller’s ADC requires additional firmware. To avoid this requirement, you can compare the voltage after the filter with a reference voltage with a comparator, such as the LM393 from National Semiconductor, to produce a logic-high level for the microcontroller’s input. Reference 1 describes an alternative approach with minimal external components for the expense of the firmware complication.