Newsgroups: comp.risks
X-issue: 13.77
Date: Fri, 21 Aug 92 23:55:04 -0700
From: karn@servo.Qualcomm.COM (Phil Karn)
Subject: Spontaneous appliance operation

There's a known problem with the BSR X-10 home automation system whereby the
"appliance modules" can spontaneously turn themselves on.  This happens with
certain types of loads, particularly electronic loads such as computers and
compact fluorescent lights, and it is due to a misfeature called "local on".
(The X-10 is a carrier current appliance control system widely marketed under
other names, including Radio Shack, Heath and Stanley. The "appliance modules"
are relay boxes that plug into the wall between the AC line and a load to be
controlled.)

The "local-on" feature is intended to allow a user to turn on an appliance
locally, without having to go to the control box. If the appliance is a lamp,
you just flick its regular switch on and off several times, and the appliance
module turns on.

This feature apparently works by trickling a small amount of current through
the load whenever the appliance module relay is 'off' and watching for sudden
voltage swings across the load that would indicate that the user is cycling the
power switch. It works great for simple resistive loads like incandescent
lamps, but nonlinear electronic loads (especially those that directly rectify
and filter the AC power line) will often draw almost no current until some
voltage is reached across the load's internal filter capacitor. Then the load
conducts, discharging the capacitor and causing the input voltage to drop
suddenly. A few cycles of this "relaxation oscillator" simulates a user
flicking a switch well enough to trigger the appliance module's "local on"
feature.

The problem usually occurs right after you switch the load off -- several
seconds later, it comes back on again. But it's possible that the spurious
turn-on could occur much later. This problem drove me crazy until I realized
from the description of "local on" in the manual what was going on. There was
no specific warning about this possibility. Indeed, the manuals take great
pains to point out that "lamp modules" (which contain dimmers) are not to be
used with fluorescent lamps and electronic loads; appliance modules should be
used instead. The instructions do warn about controlling appliances that could
cause damage if they were turned on inadvertently (e.g., an empty coffee pot)
but this doesn't really address the issue.

It turns out that cutting an undocumented jumper inside the appliance module
defeats the "local on" misfeature. It's obvious that someone anticipated this
problem since the jumper wasn't essential in the PC board layout, so it's
doubly annoying that there is no mention of this problem or its solution in the
manual.
                                      Phil