Re: [CAD_CAM_EDM_DRO] Analog Opto Isolator?

At 05:36 PM 9/24/2007, you wrote:

>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?

Roger,

An LM339 will drive an optoisolator nicely. Use it to compare to a
given voltage and you are set.

Sorry I can't draw it here, but if the "+" input goes to a pot and
the "-" input has Schottky diodes to ground and VCC and a series
resistor of a few dozen K ohms (or a voltage divider) and a small
capacitor to ground, the opto has the cathode to the output and the
anode to a resistor to VCC. Okay, I omitted bypass capacitors. Set
the pot where you want, and the opto will trigger when the input
voltage gets too high. It DOES need power, it IS cheap and
reliable. Maybe I should build a simple "up/down" circuit to demonstrate.


David G. LeVine
Nashua, NH 03060