Electronics > Beginners
Sensing 12v line with 5v MCU
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David Hess:

--- Quote from: floobydust on April 24, 2019, 07:04:19 pm ---Opto-couplers aren't considered great for automotive environments, I'm not sure exactly why. Very few achieve automotive qualifications like AEC-Q100, 101.
I recall it was aging and CTR degradation from temperature swings. I don't see them used in cars. Electric cars will surely change this.
--- End quote ---

Opto-couplers are not very environmentally robust for whatever reason.  I have occasionally seen photo-interrupters used in their place.
MadScientist:
I don’t see the point of an opto if the two grounds are ultimately connected as all the issues from load dumping will be seen on the ground anyway

Surely a either a zener or a simple clamping diode to the micros Vdd along with a suitable series resistor is the simplest most effective solution .
vmallet:
What an interesting thread! From a seemingly innocuous question came a very informative discussion :)

Floobydust's suggestion looks like a winner: voltage-safe, draws no current in the common case (i.e. when the switch is open, even when the car is "off"), and bonus points for using components I already have in my bin :) (even though I now have PIC817s on order).

@Floobydust:

--- Quote from: floobydust on April 24, 2019, 07:04:19 pm ---Typical levels are 12V vehicles +100V and -150V spikes max., 24V trucks are +200V and -600V spikes max.

--- End quote ---
I understand the 600v PIV in your design to be super safe on the spikes, but why the 1A rating?

Like @MadScientist suggested, my MCU power supply takes its ground from the battery so in the end I have a shared ground with the rest of the system.
digsys:

--- Quote from: floobydust --- Opto-couplers aren't considered great for automotive environments, I'm not sure exactly why. Very few achieve automotive qualifications like AEC-Q100, 101 ....
--- End quote ---
In higher end vehicles / trucks etc, I see them used a lot. Often, it is a case of "minimize cost" and "if an alternative works ok, go with that".
Like in any field, not every design is "most optimal", works ok is acceptable. Just giving the OP options.

--- Quote from: MadScientist --- I don’t see the point of an opto if the two grounds are ultimately connected as all the issues from load dumping will be seen on the ground anyway  ...
--- End quote ---
Similar reason we use star grounds in vehicles / building etc etc The FEWER Inputs / outputs that can pass through a single device, especially
something like a CPU, greatly reduces risk. Similar to only "earthing" 1 end of a shield etc etc
It completely eliminates that Input path via the CPU.
PS: I'd still go for the opto option, but that is up to you. You have options.
magic:

--- Quote from: digsys on April 24, 2019, 11:43:21 am ---Sure ... what you can "get away with" and what is ADR compliant (ADR=Australian vehicle design rules) are completely "different' animals !! All countries have vehicle safety / wiring standards that are similar, especially IF they want to export.
They don't have electrical safety standards for no reason.
--- End quote ---
Well, the circuit I talked about was not an amateur job but a "smart" fuel pump relay from a pre-ECU Volkswagen car. I also peeked inside two LPG controllers for similar ECU-less engines and they too used nothing but resistors and diodes for input protection, going straight to some 40 series logic chips. All those components were built professionally and (hopefully) to standards, though standards may have been different 20-30 years ago. And they worked in practice.


--- Quote from: digsys on April 24, 2019, 11:43:21 am ---Sure, the opto could "blow up", but it will cause a LOT less damage than "blowing up" a micro, that "may" also be in charge of OTHER safety functions !!

--- End quote ---
I think it likely will because LEDs have poor reverse voltage ratings and a series resistor helps nothing in that regard. At least I would add some series or antiparallel diode for protection.
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