Electronic Design

Power interruption tester for restart circuit

Compressors are susceptible to brief power interruptions as short as 30 ms or 2 cycles of ac power. Even these short “glitches” may cause a pump to stall when it’s under load. To alleviate this potential hazard, a restart circuit was designed. It controls a valve that opens momentarily to relieve the load after the power interruption. The powerinterruption tester presented here was designed to test the operation of the “restart circuit.” It also has been used to characterize pumps and determine which ones require a restart circuit.

The power interruption circuit uses a 555 timer to create the variable width pulse that momentarily turns off a triac. The triac functions as an electronic switch supplying ac power to the pump under test. The circuit is activated by pressing a momentary contact switch. This supplies a ground to the positive end of C2, which has been charged to +5 V. By setting the value of C2 to 1 µF, the contacts of the switch are debounced and won’t retrigger the 555 one shot. The leading edge of the pulse is transferred to the trigger in-put of the one shot.

The 555 timer circuit is configured as the standard one-shot design. C3 is the timing capacitor and chosen to be 1 µF, while R3 is a variable 50k potentiometer. Having it in series with R5, a 33k resistor, allows the pulse duration to vary from 20 to 50 ms. Changing these values can easily provide alternative pulse widths and adjustment ranges.

The pulse is coupled to the optoisolator through R4. Resistor R6 assists in wave-shaping the pulse. The output of the one shot is low until it fires. This condition maintains the isolator and the triac in the conducting state. When the one shot fires, the output switches to 5 V, which causes the isolator to remove the drive signal to the output triac. The triac remains open or off for the duration of the pulse.

The triac chosen for this application is a 2N6071. This part provides plenty of current capability for the largest motors that were tested. The 800-V breakdown voltage offers plenty of margin if the tester is to be used on 230-V motors. It was mounted on a heat sink to assure that the load of larger motors didn’t cause the triac to overheat. Using the power interruption tester makes it convenient to simulate the effect of power-line glitches that can occur with momentary failures due to storms or maintenance operations.

See associated figure

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