[Amps] Hotswitching

R at contesting.com R at contesting.com
Sat Apr 8 03:44:57 EDT 2006

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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