Re: [CAD_CAM_EDM_DRO] digitizing probe

Tom Eldredge wrote:

> Jon,
>
> Thanks for the advice. I guess I have been mixed up about the capacitor
> across the contacts. It seems that I have often seen some sort of an RC
> combination across some contacts in industrial equipment. How does this
> component operate?

You may remeber my questions a few weeks ago about "snubbers" for
Solid State Relays. This is the same thing. The trick is to put a carefully
selected resistor in series with the capacitor. Without this resistor, when the
contacts close, they short circuit the capacitor, and whatever charge is in
the capacitor surges across the contacts, developing very high currents,
dependant on whatever resistance the wires and the capacitor itself
offer. Hundreds of amps would be expected in many cases, quickly damaging
even heavy contacts. The snubber is to prevent a rapid rise in voltage
when the contacts open, due to inductance of motors and relay coils.
There is some art in selecting the capacitor and resistor voltages, but the
simplest one is if the contacts have a rating of, say, 10 Amps, and the line
voltage is 240 V, then a resistor of 240/5 = 48 Ohms might be the lowest
acceptable, limiting the current surge due to the snubber to about 5 Amps
worst case. Figuring out the correct capacitor value gets more complicated,
and involves knowledge of the load's characteristics.

Jon