Some help with a circuit I'm working on...hopefully

[kevlar129bp]

Hello all,

Long time, no post. Hope everyone is well. I'm sure hoping one of you brilliant minds can assist or critique an analog circuit I'm working on.
This circuit will scale the NEGATIVE dc voltage (ground referenced) of my plasma cutter, down to a usable ADC level. I've been simulating it
with the Falstad circuit simulator and it seems to work, but I would much rather have a second, third, fourth, etc. opinion on it. If you would like,
I can post the Falstad file in a code block as well.

Thank you all for your continued help in my crazy projects,
Chris

[Charlie]

What is the range of voltage you expect from the torch? (-300 V +/-??)

1) What is the relay for? Is there some function you did not mention?
2) The inductors are not necessary.
3) You will need positive and negative supplies for your op-amps in this configuration. You could avoid that by connecting your divider resistors to +12 instead of GND.

[mark_s]

Using an opto-coupler could make things simpler. A series resistor from the torch node to the LED, then to ground clamp node. You did not say what the function of your circuit is, and accuracy required?

[kevlar129bp]

Thanks for the replies guys,

As far as the voltage expected from the torch, it will range from a maximum of 0 VDC to a minimum of -300 VDC... or visa-versa, depending on how you want to look at it. The relay is implemented to delay the activation of the scaler due to crazy stuff happening on ignition of the plasma. I'll also use it in the PIC code as an interlock for the scaler itself. I realized, after I posted this, that my son had a copy of ISIS from school. We ran a simulation on it as well, with the same results... -300 VDC in, 2.048 VDC out. I am no analog guy, can you explain why I need a split supply on the differential amp? Love to be educated on this.
In regards to the accuracy, the ADC will be 10 bit so... 300/1024=0.29296875 (1/4 volt or so).
Thanks a bunch guys,
Chris

[mark_s]

I found this page doing what I suggested. Opto-couplers are safer for high voltage and isolate noise.

In this diagram you would need to change the series resistor to some higher value. Where it say's AVR connect to your PIC adc.
Try simulating it

http://www.ustudy.in/node/7519

[amgen]

and.... lookup linear opto couplers, plus your code can correct for some non-linearity.