Q&A
years ago, I wired up a large metal
working machine with all DC motors
and control. The motor that moved a
table back and forth had to stop and
change direction about every three
seconds. I tried several relay arcing
fixes — including using capacitors.
But what finally cured it was connecting a light bulb across the motor
leads. When the motor was energized,
the lamp came on; when the motor
was stopped, the lamp brightened
considerably as it absorbed the inductive decay. All I had to do was try different wattage lamps until the arcing
was suppressed.
I was about 23 at the time, out of
college with an EE, and working for
my father-in-law. One thing he pushed
into my head was that simpler was
better. So that was why I tried the
light bulb trick. The owner of the
machine shop was quite impressed.
About five years ago, I went back to
my home town and visited the owner.
He was close to 90 and still active. He
had sold the business but retained the
right to use the machinery. He took
me on a tour of the shop and said
that he wanted to show me something. He fired up the old planning
mill and there was the same light
bulb going on and flaring up before
going off.
— Robert E. Robinson
Dear TJ,
Your answer to the question about
relay contact resistance measurement
(in the May 2005 issue) was interesting, but I believe has a serious flaw. If
you had computed the power dissipated at the contacts under the specified
conditions [ 50 milliohms], you would
have realized that 1.5 to 3 volts across
contacts carrying 30 amps dissipates
45-90 watts. That much power dissipated in such a small space would
raise the temperature of the contacts
higher than the temperature of the
filament of a light bulb, destroying the
relay and likely starting a fire.
— Howard Mark
Suffern, NY
Response: I didn’t invent these
numbers. They come from
datasheets. But your advice is well
taken, and a reason why relays fail
when contact resistance exceeds a
certain limit. And, yes. I have seen
my fair share of melted relays. — TJ
Dear TJ,
About your “Needs Stereo Chips”
answer in the April 2005, your answer
is perfectly sound. But your reader
must not know about Google,
because a search was not “fruitless”
for me when I was looking for the
exact same thing he was seeking.
http://electronickits.com/kit/
complete/ampl/ k100.htm is a kit
for a pre-amplifier that does the exact
same thing as the LM1036 does, but
is based on the Philips TDA1524A.
www.ramseyelectronics.com/
cgi-bin/commerce.exe?preadd=
action&key=UAM2 is the Ramsey
UAM2, a “class D” audio power amp
that works like gangbusters. You CAN
still build what your reader wanted
from kits.
— Kenneth Tindle
Elect. Technician
Univ. of Kentucky Language Lab
Cool Websites!
NASA Deep Impact will rendezvous
with comet Tempel 1 on July 4, 2005.
The objective is to shoot a “bullet”
into the comet and analyze the debris.
http://link.abpi.net/l.php?20050505A9
Are you fast on the draw? Simply “shoot”
five darting sheep with a tranquilizing
dart and score your reaction time
in milliseconds.
www.bbc.co.uk/science/humanbody/
sleep/sheep/
NASA World Wind — satellite view of
the big blue marble.
http://worldwind.arc.nasa.gov/
features.html
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