Fw: hot switches [TANKS]

["Chrysanthos Kanellopoulos" <xchrysk-at-otenet.gr>]

 

Thanks Frank for your your time and the expansive reply.  I'll try to find mechanical relays. You have to admit I am an amateur, but improving thanks to you guys!  (I must confess that I liked Mike Lyons' *fun* idea of dripping CO2 on the steel / brass crescent. That'd be a neat cooling device. AlRight, confront something as primitive as my system with something sophisticated ! ! ). Eager to experiment. I'll have it in my mind for the next item that gets overheated  Mike!. God , what else are we going to learn in this club??

And yes, I now seem to remember that there is a special gel -cream that should be used as grease for electronics, not just ordinary grease.

 

CHrys

 

> I am using this half moon relay, attached here. (also in my
> message of Dec. 24, 2006). The half moon is a metal plate, either iron
> or brass. The poles (switches) are two tiny ball bearings.

> I am using grease on the half moon plate , as in mechanical speed
> controls, so it can slip smoothly over the switches.

OK, I think I know what's happening.  The combination of the metal
plate, the grease and the tiny ball bearings is generating too much
resistance between the plate and the motor poles, effectively creating a
heating element.  To confirm this is the problem, connect an ohm meter
to the plate and one of the motor poles (with the plate touching the
pole).  If that resistance is greater than zero, you've got a problem.

The resistance can come from various sources.  First, the plate material
has resistance: iron bad, brass ok, copper better, gold best.  Second,
the grease may be causing resistance ... you need to use a grease
specially made for conducting electricity (usually a white paste).

Most importantly, using ball bearings as the contacts is a big no-no,
regardless of their size.  A sphere only touches a plate with a very
small surface area relative to its size.  For mechanical purposes,
that's fantastic because it reduces mechanical friction.  But, that's
not good at all for electrical purposes, because it significantly
increases electrical resistance, which in turn generates heat.  The
contacts on mechanical speed controls are usually flat "brushes" made
from wide strips of copper.  In such cases, the grease is actually used
primarily to ensure a uniform "contact area" for electrical purposes
between the two flat surfaces when moving.  This eliminates electrical
arcing, which also generates heat.  With ball bearings, you are not only
causing heat from a small contact area (electrical resistance), but you
are also causing heat from arcing because the grease can't effectively
fill the gap between the plate and the sphere.

If you replace the ball bearings by flat brushes, the heat should be
significantly reduced.  If you replace the entire assembly by two
inexpensive automotive relays controlled by two small micro-switches,
the heat will effectively drop to zero.

        Frank P.