Re: hot switches [TANKS]

[Frank Pittelli <frank-at-rctankcombat.com>]

Chrysanthos Kanellopoulos wrote:
> 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.