Design Ideas March 3, 1997

Circuit tests power-outage susceptibility

Ron Tipton, TDL Electronics, Las Cruces, NM

Brief (1 sec or shorter) power outages are common in developing countries and seem to be increasing in the United States. They are generally nondestructive, so power-protection equipment should not trip and disconnect the load. Electric motors do not appreciably slow, and electronic equipment can usually ride through the outages on the energy stored in filter capacitors. As a point of reference, a 10-msec outage makes an incandescent light bulb flicker. At 100 msec, you can see the bulb go completely dark. Figure 1 shows a simple circuit for generating brief outages for equipment-susceptibility testing.

The PIC microprocessor is programmed as a variable-pulse-width single-shot (Listing 1). Click here to download  DI-SIG, #1993. You initiate an outage by pressing the momentary-contact pushbutton switch, S₁. The binary-coded toggle switches, S₂ through S₈, set the pulse width in increments of 10 msec. This design uses a PIC16C71, but any model with nine I/O lines should work. The 2N3904 transistor is unnecessary, because the µP can sink the relay control current. However, you can consider the transistor as a 10-cent "fuse" protecting a $5 IC. The crystal frequency is not critical, but it must match the software to keep the timing correct.

A solid-state relay has the needed microsecond switching time, but some relays are designed to switch when the line voltage goes through 0V. On average, this trait adds 8 msec to the programmed time (a half-cycle at 60 Hz). Random-switching models are available from such manufacturers as NTE Electronics (Bloomfield, NJ) and SSAC Corp (Baldwinsville, NY). Solid-state relays have a small leakage current in their off state. A voltmeter or other high-input-impedance instrument reads full line voltage even when the relay is open. To zero this voltage, you connect a load resistor or incandescent light bulb in parallel with the test outlet. A light bulb works especially well because of its low cold resistance: approximately 13 ohms for a 100W bulb. (DI #1993)

Figure 1

You can use this simple circuit to test the effects of brief power outages on equipment.

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