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MMD > Archives > January 1997 > 1997.01.08 > 09Prev  Next


Line Conditioner to Protect Electronics
By Robbie Rhodes

Michel Goffin writes that the music roll in his Welte Mignon piano is driven by a modern electric dc (direct current) motor which has failed. He notes that the mains voltage is uneven in Jakarta, but I think the more likely cause of failure is the voltage surge when the mains of a neighbor- hood (e.g., one square kilometer) is abruptly disconnected from the regional mains system during a storm.

Jakarta experiences violent lightning storms just like our American Southeast states have. When a fuse atop a power pole suddenly disconnects the neighborhood load from the regional supply, the energy stored in the big motors of the neighborhood (e.g., air-conditioners & refrigeration) is "dumped" into the neighborhood electric lines. Surges of more than 600 volts, lasting tens of milliseconds, are observed on American 120-volt mains.

The available current during this short period is monstrous: it's equivalent to the transformed current at the power-pole fuse before the disconnect event. Electric motors designed for 120- or 240-volt mains easily withstand the voltage surge; the mass of the windings absorbs the jolt of energy without damage. But inexpensive consumer electronics, including many computers, aren't designed to withstand these high-voltage surges.

The simple and ubiquitous "computer surge protector" in the home and office wilts instantly and provides no protection. It tries to limit the voltage surge but the input current is so large it overpowers the small varistors. Circuit breakers which protect the branch circuit wiring react far too slowly, and even most small fuses cannot rupture fast enough to limit the current.

The best protection methods employ a device which limits the fault current sent to the load, like the computer or electronic player piano. An example is the old-fashioned "line conditioner" unit, made by Sola and General Electric and others, which provides very good protection against mains surges. This device resembles a big electric transformer (which it is) with a large capacitor built-in. A loosely-coupled resonant "tank" circuit is thus created, and, much like a flywheel on a rotary machine, blocks the surges from appearing at the output.

Ten years ago a Sola 2- or 3-kva ferro-resonant line conditioner for 60 Hz cost about $2000; it will run indefinately with no maintenance, and protect all the computers and consumer electronics found in a home. I think it's very well-suited for places like Florida and Jakarta!

Robbie Rhodes


Key Words in Subject:  Conditioner, Electronics, Line, Protect

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