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Unread 22-06-2005, 20:21
Unsung FIRST Hero
Mike Betts Mike Betts is offline
Electrical Engineer
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Join Date: Dec 2001
Rookie Year: 1995
Location: Homosassa, FL
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Re: Robot Cart Switches

Quote:
Originally Posted by Sparks333
...and a few even are rated for AC...
Dane,

Any mechanical, non lighted, switch rated for DC is, by default, rated for AC but the reverse is not true.

AC current passes through zero at some defined period (for example, every 120 Hz for 60Hz current). For DC current, the spark caused by a switch opening must be extinguished which is much harder to do.

Think of water flowing through a pipe... If you were able to instantaneously turn off a valve, the "water hammer" effect will blow the valve apart as the water has mass and you have attempted to stop it... F=ma or F=m*dv/dt so that F approaches infinity as dt approaches zero.

Likewise, any wire has inductance and V=L*di/dt such that the opening of any switch in a DC circuit instantaneously will cause the voltage to approach infinity and the switch will arc over.

The difficulty in switching DC was one of the factors in the great Nicola Tesla/Thomas Edison "war of the currents" debates in the 1880's.

Edison lost...

Look back in these fori archives and you will find spirited debates among engineers when FIRST first introduced AC rated 60A circuit breakers to be used for a main battery disconnect (about 6 or 7 years ago).

Bottom line: Please do not use an AC rated product for DC unless you are sure...

Regards,

Mike
__________________
Mike Betts

Alumnus, Team 3518, Panthrobots, 2011
Alumnus, Team 177, Bobcat Robotics, 1995 - 2010
LRI, Connecticut Regional, 2007-2010
LRI, WPI Regional, 2009 - 2010
RI, South Florida Regional, 2012 - 2013

As easy as 355/113...