Some years ago when testing a 24vdc relay, I noticed a healthy spark
whenever I disconnected the 24v PS from the coil. Curiosity led me to
use my oscilloscope and a 10x probe to measure reverse potential as
current through the coil stopped and the magnetic field collapsed. To
my surprise, the reverse potential was c. 19x the PS voltage - which is
way above the DC rating on common toggle switches. This readily
explained the fireworks. It also suggested a possible reason why the
adjustable thermostat on my Presto® 120v quartz, space-heater had
failed after only one Winter's use. As an experiment, I went back to
Kmart, purchased another quartz heater, took it home, removed the cover
plate, and soldered a 120v MOV across the thermostat's switch contacts.
That was c. 20-years ago and since the second quartz heater still
works like the day I bought it, I am declaring the experiment over and
publishing the results.
So, even though the L of a quartz heater element is only about 0.15%
of the L in the 24v relay's coil, reverse potential when the current
stops in any circuit appears to be something that one should not
ignore. In the case of Apollo 13, the problem turned out to be a
thermal switch on an oxygen pressure-vessel's 70v heater element that
arced and welded the contacts closed as the switch tried to open. This
caused the vessel to overheat and eventually explode in space, spoiling
the mission and nearly resulting in the deaths of the three astronauts.
My guess is that a $1 MOV would have prevented this glitch.
- end
Rich Measures, 805.386.3734, AG6K, www.somis.org
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