Hi MIKE,

On 05-Oct-03, MIKE ROSS wrote to Greg Mayman:

 RJT>> The other thing that just comes to mind with those is that
 RJT>> underneath the mounting base were wire-wound
"resistors",  maybe
 RJT>> 2 or 3 of them.

 GM>> Usually wound on mica strips. IIRC they were connected across the
 GM>> "buzzing" contacts of the voltage and current control relays to
 GM>> minimize arcing.

 MR> That's not the right thing to do however because the capacitor
 MR> causes the spark energy to be dissipated into heating the contacts
 MR> thus making them burn or wear out faster. The proper way is to add
 MR> a certain amount of resistance in series with the capacitor to
 MR> safely dissipate the emf energy while keeping the voltage low
 MR> enough so there is no arcing. The value of resistance is found by
 MR> dividing the voltage limit by the peak reactive current. The other
 MR> advantage to adding a series resistor is that the capacitance can
 MR> then be a lot smaller than before. Sony equipment in the 70's
 MR> always had this combination of resistance/capacitance across
 MR> switching contacts.

No one has mentioned capacitors, and I've _never_ seen a capacitor in
a mechanical "trembler" regulator in a vehicle. They were indeed
resistors and they were wound on mica formers, as one of the few high
temperature insulation materials that could stand the temperatures
involved. In a regulator, having a DC path across the control
(trembler) contacts is not a problem.

However, for normal electronics use on DC you do need a capacitor in
series with the resistor to provide the DC isolation across the
contacts when they are open.

AC circuits don't need these extra components as the arc extinguishes
on the zero crossing...


George

--- Terminate 5.00/Pro