I have machine working in dry environment and as there's bunch of moving parts there is a build up of static electricity. I managed to insulate electronics pretty good from all external "interference" but I still have one nasty line coming in. The problem is in a NTC resistor that is sitting embedded in aluminium heater block that can collect some static from time to time. The worst problem is current design that goes like
Code:
3V3
|
600R
|
+-----ADC INPUT ON PIC32MX
|
NTC 200K
|
GND
so if the spark from the Al block hit the thermistor leg that is connected directly to ADC input of PIC32MX (that is not even 5V tolerant) I get all kind of weird issues (PC jumps to unknown location, PIC reset, PIC freeze). The WD makes sure no real damage can happen when this happens (except if once I get spark powerful enough to burn the PIC) but I want to solve this as fast and as simple as I can. I have 3 of these circuits going to 3 different ADC inputs.
As the PCB is already laid in and not cheap to change, mine idea was to add, inside the connector something like this:
Code:
3V3
|
600R
|
+----------[ferite bead]---[ + MCP6021 out]--+ ADC INPUT ON PIC32MX
| [ ] |
| +---[ - ] |
NTC 200K | |
| +---------Rx-----------+
GND |
10K (Ry)
|
GND
Now, questions .. the Rx shoud be 0 in order to have the same voltage on the ADC as there was before this change (but I was never very good with op-amps) anyhow, does anyone have idea if this is viable solution at all - it's not that I can test easily if spark will travel trough op-amp or not .. I also considered adding a 3V zener between +op-amp and GND
So - Any suggestions? Better calculation for Rx and Ry? (I know I suck with op-amps, the little I know I learned on this forum from nice guy's like audioguru, ericgibbs and others
) .. better way to filter the input and prevent nasties to kill my pic? all the digital lines go trough opto-couplers but I can't solve this 3 analog ones