Re: [CAD_CAM_EDM_DRO] digitizing probe

Tom Eldredge wrote:

> Gentlemen,
>
> I am not trying to get things going on the digitizing thread again, but I have a question for you electronics geniuses ( I say that respectfully, and gratefully):
>
> I am using a digitizing probe made by Fowler. It is a copy of a Renishaw cmm probe that the patents have now run out on. It has no debounce electronics or anything like that, just a set of contacts configured as the Renishaw probes are. My question is:
>
> I am applying + 12 volts through the probe contacts then through a 100 kohm resistor then to ground to protect the contacts from too much arcing. Should I put a capacitor accross the contacts to further protect them from arcing? It seems as though a .1 ceramic might help.

NO! The capacitor will store energy, and when the contacts close, it will deliver the energy to
a short, causing a very high current. Given the typical wiring, it could be 100 Amps if the cap
is real close to the contacts. This will cause pitting even at 12 volts, and quickly ruin the probe.
You can probably go with a much lower resistor than 100 K Ohms.

If you need some debouncing, you can put the capacitor on the OTHER side of the resistor,
where it will slow down the change in voltage, but not deliver large currents to the contacts.

> By the way, I am using 12 volts, because when I tried to use 5 volts, the contact resistance sometimes gave me erroneous indications. I compare the signal coming from the probe to a reference of 8 volts using an LM339 to avoid erroneous indications produced by contact resistance. I also use software to debounce the contacts.

Yup, there can be a film of oils or oxides that won't break down without some rubbing of
the contacts or a higher voltage to break down the film. I think 12 Volts is fine, as long
as the current is limited.

Jon