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I've read on forums that OBD port can provide max 5A current at 12V. What is the source of the power? Is it coming from battery directly or is it regulated power supply from somewhere else, maybe ECU? Also if the source is battery, what is the maximum transient voltage? I have read this and it says a maximum of 120V. Can anyone confirm this limit?
Thanks in advance for help :)
[Edit]
The reason for asking this question is that many open source obd readers I've seen do not implement over voltage protection or offer very basic one with just zener. Is zener protection enough or should I go with over voltage shutdown with automatic reset method?

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    \$\begingroup\$ Look at your car's schematic. The ODB port will most likely be directly connected to the battery via a fuse and maybe a relay or switch. Current determined by fuse. \$\endgroup\$ – Passerby May 26 '16 at 7:41
  • \$\begingroup\$ I'm talking about cars in general. Is it different for each car or make? And what do you mean by car schematic? I don't understand. Can you link a sample one here so that I can look further? \$\endgroup\$ – Lalit Kumar May 26 '16 at 7:50
  • \$\begingroup\$ If you want to be at safe side, then consider to use galvanic separation betwen PC and OBD. You can put some zener and LDO at OBD side, keep in mind that ECUs are also MCUs and have same problematics, so lokk for some automotive design. \$\endgroup\$ – Marko Buršič May 26 '16 at 9:42
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99% chance it's straight from the battery via a fuse, regardless of make or model. 80 V is the most common peak load dump voltage I have run across in the auto industry.

EDIT: 99 % of all aftermarket electronics, your average USB charger, car stereo and what have you, will simply blow up in case you loose the battery and the alternator runs of wildly. This does however happen so very rarely that few take notice. If you have low enough current consumption and you can afford to have a resistor in series, any zener diode will work wounders. If you want to meet any automotive specification, there are special clamp diodes/TVS which can withstand kW for the ms of duration at 80 V in the standard. It have very litte to do with real cars though.

EDIT2: Came to think of something. Mechanially, that 12 V may be drawn from or via the car MCU, but electrically there is nothing there to regulate the voltage, like a DC/DC converter keeping the OBDII-port at 12 V even in a 80 V load dump condition, and tha's what I assumed OP asked about.

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  • \$\begingroup\$ What would be a better circuit for protection from over voltage? Please see the edit. \$\endgroup\$ – Lalit Kumar May 26 '16 at 7:58
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Toyota Yaris and Peugeot 307 : it is taken from the battery (don't know about the fuse), but is switched off if the key in the off position.

If the car is okay, then the OBD device will live happily (the battery is a good filter itself), but if you disconnect the battery while the engine is running you will get nasty voltage spike (and most likely fry other parts of electronics inside the car).

I did not have any problems with cheap OBD adapters (both Bluetooth and USB).

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