Re: [CAD_CAM_EDM_DRO] Analog Opto Isolator?

vrsculptor wrote:

> Hi,
> I'm messing with a very basic RC EDM (Jon Eleson's) and want to
> measure the voltage drop across a resistor using a micro (coridium
> Armite) with an analog 5 volt input to sense the spark gap. The EDM
> voltage is adjustable so I can't use a fixed resistor network to keep
> the sense voltage within 5 volts. I need some form of isolation to
> protect the micro from over voltage on the sense line after adjusting
> the EDM voltage. I don't need anything automatic as I can manually
> re-adjust a pot on the sense line to get the voltage back into range
> after adjusting the EDM voltage. I'm looking for something simple.
>
> Any suggestions? Is there such a thing as an input voltage tolerant
> analog opto-isolator? An i2c A/D with isolation and over voltage
> protection?

The simplest thing to use and guarantee isolation is an
opto-isolator. I use H11A817D optos that are quite sensitive
and reasonably fast, in a bunch of stuff. It will turn on with
less than 2 mA on the LED. The output is a transistor that you
can wire as you need, but the simplest is to connect the emitter
to ground and the collector to the chip input with a 4.7 K Ohm
pull-up to 5V. If you give it a resistive circuit such that 2
mA flows in the LED for the max voltage condition, then it
should give a crude analog reading safely. There are more
complicated devices, usually featuring two matched sensors that
you hook up in a closed-loop control, one sensor for feedback
and one sensor for output on the isolated side.

Really, putting all this sophistication on an R-C EDM is a major
waste, you should rig up an electronic switch to control the EDM
current.

Jon