The voltage passing through a switch is significant only when designing the
insulation.
The power that a switch dissipates is the current passing through the switch
times the voltage DROP across the switch. For example, if a switch has a
voltage drop of 0.01 volts and is passing ten amperes, it will be dissipating
0.1 watts. If the contacts get bad and the voltage drop is two volts with a
current of ten amperes, the switch is dissipating 20 watts. That's why a bad
switch will get worse very quickly.
Every time a switch opens or closes, an arc results. If the current is more
than an ampere, that arc will "clean" the contacts. However, with the new
solid state technology, we sometimes have switches that are passing
microamperes. There is no arc, so dust and oxidation can build up and cause
the switch to perform intermittently. That is why good quality electronics
equipment will use gold plated contacts to eliminate the oxidation.
A C switch ratings versus D C is a whole different ballgame. It is a function
of how quickly the arc is quenched when the switch is opened. As the
alternating current goes from positive to negative, it passes through zero. If
the switch has just begun to open and it is arcing, as the voltage goes through
zero, the arc goes out. Once the arc has shut off, it probably won't reignite.
When you open the switch on a D C circuit, the arc starts and keeps on going
until the switch contacts are far enough away to cause the arc to quench.
Switches designed for high D C current usually have a snap action so that the
contact is broken quickly. They also usually have more metal in the contacts
to dissipate the heat and to compensate for the metal lost to arcing.
When you see volt-ampere ratings on a switch, that switch is probably designed
to control a motor. A motor will draw several times as many amperes as it
starts than it will when running. Obviously, that takes a very strong switch.
If anyone has further questions or comments, please contact me off-line.
Jim Vance
va...@claflinwildcats.com
