[Ovmsdev] OVMS v3 First Board Layout

Edward Cheeseman cheesemanedward at gmail.com
Fri Mar 31 04:07:23 HKT 2017

On 31/03/2017, at 3:33 AM, Collin Kidder wrote:
> I'm unsure of why the 3.3v transceiver would be a problem. CAN wiring
> is supposed to be differential

Totally agree. TI has an app note somewhere that argues that their 3.3V chips achieve the spec’d differential mode voltage, by slewing the offset from ground slightly (still well within specs)
But you don’t want to blow out cost.
Some of the TI chips will handle much more voltage, some cases +/-75V, which will make the device immune to almost anything in a car (keep away from the spark plugs or HV battery though!)

Another thing to consider in the power supply is Alternator “load dump”. Only applicable to cars with alternators, but what happens is when a large 12V load turns off, say airconditioner or powerful spotlights, the alternator field decays relatively slowly, blasting the same output current into the 12V supply. If the battery can’t accept the current, voltages as high as 80V, with whatever current the load was drawing can appear for a millisecond or so on the 12V. If your device happens to be the most susceptible device in the car, its the one that is going to wear it.

Something like a high surge rating zener or TVS and maybe a polyfuse might be enough to protect against it.


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