Need advice on making 12V+ tolerant in/out to a micro - Page 2 - Electronic Circuits Projects Diagrams Free

iso9001 · 16th October 2007, 05:13 PM

As I was aware this topic would be a debatable one, I'll just go ahead and say I think you are both right. For something mission critical it probably doesn't hurt to double up, for a blinking teddy bear anything goes

As I fear I have only a basic understanding of what I'm doing I'll probably trust the internal clamping diodes and leave pcb space for papabravo's opamps in case I feel like getting redundant in here.

Two questions about those though.... One, since the inputs are open collector's they are going to absorb quiet a bit of power from each wire ? I was under the impression this was true for all open collectors. Two, if I feed the supply of the opamp with 5V, I can still tie my 12V signal to the + pin without damage (assuming correct) but for the - pin wouldn't I want to tie that to ground ? Rather (as I am reading it) it seems I am being recommended to tie it to ~1.6V, why is this ??

Then for the outputs I'm not 100% understanding the advantages between mosfet and npn in this instance but I'm leaning towards mosfet.

Cool, I feel much better about this project now

Papabravo · 16th October 2007, 11:55 PM

Be careful - a comparator is NOT an opamp. They are entirely different animals. The LM393 and LM339 which I recommended need a V+ supply that is at or above the signal input. At least one of the inputs must be within the devices "common-mode" range. Most choices of reference voltage will satisfy this requirement.

"Open collectors" are not inputs. They are outputs, and they only sink current when they are low. When they are high, the external pullup resistor pulls the output up to Vcc, which could be 5V or 12V or whatever.

iso9001 · 17th October 2007, 04:51 PM

Ok, I see now. My mistake on using the word opamp. You were saying use the 12V to power the package and on its output to my module drop the voltage, well, that makes sense. At least that way I'm not effecting the input signal at all with resistors. I'll definitely make a note of that.

So, for now it seems my input scheme is taken care of. Anyone want to give me some feedback on this (pic) for output ? I want to try and use a single pin for 12V digital input and 5V or 12V output. I know this method would be very trusting of the pic's internal hardware (debatable) but I figured I'd ask here,

http://tinypic.com/view.php?pic=un6tg&s=2

The problem I can see right away is that any input signal will also turn the mosfet on. So... That rules out this being used as a 5V input. I would think that if I have a common 12V source as the signal then it should be O.K. no ?

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16th October 2007, 05:13 PM	(permalink)
iso9001	As I was aware this topic would be a debatable one, I'll just go ahead and say I think you are both right. For something mission critical it probably doesn't hurt to double up, for a blinking teddy bear anything goes As I fear I have only a basic understanding of what I'm doing I'll probably trust the internal clamping diodes and leave pcb space for papabravo's opamps in case I feel like getting redundant in here. Two questions about those though.... One, since the inputs are open collector's they are going to absorb quiet a bit of power from each wire ? I was under the impression this was true for all open collectors. Two, if I feed the supply of the opamp with 5V, I can still tie my 12V signal to the + pin without damage (assuming correct) but for the - pin wouldn't I want to tie that to ground ? Rather (as I am reading it) it seems I am being recommended to tie it to ~1.6V, why is this ?? Then for the outputs I'm not 100% understanding the advantages between mosfet and npn in this instance but I'm leaning towards mosfet. Cool, I feel much better about this project now

16th October 2007, 11:55 PM	(permalink)
Papabravo	Be careful - a comparator is NOT an opamp. They are entirely different animals. The LM393 and LM339 which I recommended need a V+ supply that is at or above the signal input. At least one of the inputs must be within the devices "common-mode" range. Most choices of reference voltage will satisfy this requirement. "Open collectors" are not inputs. They are outputs, and they only sink current when they are low. When they are high, the external pullup resistor pulls the output up to Vcc, which could be 5V or 12V or whatever.

17th October 2007, 04:51 PM	(permalink)
iso9001	Ok, I see now. My mistake on using the word opamp. You were saying use the 12V to power the package and on its output to my module drop the voltage, well, that makes sense. At least that way I'm not effecting the input signal at all with resistors. I'll definitely make a note of that. So, for now it seems my input scheme is taken care of. Anyone want to give me some feedback on this (pic) for output ? I want to try and use a single pin for 12V digital input and 5V or 12V output. I know this method would be very trusting of the pic's internal hardware (debatable) but I figured I'd ask here, http://tinypic.com/view.php?pic=un6tg&s=2 The problem I can see right away is that any input signal will also turn the mosfet on. So... That rules out this being used as a 5V input. I would think that if I have a common 12V source as the signal then it should be O.K. no ?