Wide-input power for Indian sites: surges, brown-outs and generator changeovers

A power input that works on the bench supply may not survive a pump factory, a shop on a long feeder or a site with a diesel generator. The mains is nominally 230 V, but sites see long sags, short spikes from switching loads and a changeover to a generator whose voltage and frequency are rough for the first seconds.

What the supply sees

• Brown-outs: the voltage sits well below nominal for minutes. A converter that cannot regulate at low input will reset or oscillate.
• Surges and spikes: switching of motors, capacitor banks and lightning-induced transients. IEC 61000-4-5 describes the test, with a 1.2/50 µs open-circuit voltage wave and an 8/20 µs short-circuit current wave.
• Changeover: when the mains drops and the generator starts, the device sees an outage from a fraction of a second to several seconds, then a restart where the voltage can overshoot before the governor settles.

Mains input stage

We start with a fuse sized to the load and rated for the mains voltage, followed by a varistor (MOV) across line and neutral, an inrush limiter such as an NTC, and X and Y capacitors with a common-mode choke when emissions need it. For an AC-DC stage we choose a module or converter rated for a universal input, typically 85 to 264 VAC, so a deep sag does not collapse the output. We check the data sheet’s behavior at low line, because output power often derates there.

DC input stage

On a 12 V or 24 V industrial rail we add a TVS rated just above the highest normal rail voltage, a reverse-polarity protection element and a fuse or resettable part. A series diode or ideal-diode controller stops a back-fed supply.

Hold-up and recovery

A bulk capacitor gives the device milliseconds to tens of milliseconds of ride-through, enough for a small dip but not for a changeover. For that, the board needs a supervisor or brown-out detector that holds the processor in reset until the rail is stable, and firmware that saves state to non-volatile memory when the input drops, so the device resumes cleanly. A supercapacitor or small battery can carry the save and a final network message.

Test with a programmable source or at least a variac: ramp the input down, interrupt it for 100 ms, 1 s and 5 s, and watch for latch-ups and resets.

What this does not fix

Protection parts have energy ratings, and a direct lightning strike or a miswired 415 V phase will exceed any of them. The device also cannot ride through a long outage, so if the process must keep running, the answer is a UPS or battery backup at system level, not a larger capacitor on the board.

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