| Electronics > Projects, Designs, and Technical Stuff |
| Automotive Diagnostic - 24v to 12v Converter for K-Line, L-Line, J1708, J1939 |
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| symp:
Hi Guys, I thought I would come to the electronics mecca for support. I am designing a 24v to 12v converter to be able to use 12v diagnostic adapters on 24v vehicles such as trucks. Calling all Automotive electronics gurus! The power conversion and logic conversion is taken care of. I have heard of high voltage J1708 (above 5v) but have not seen it anywhere and think it is a myth....I believe it is a case of the 12v diagnostic adapters not having a low enough resistance to pull or push the bus on a 24v vehicle. Which should be an easy fix if true. I am attaching a screenshot of my schematic, I can provide KiCad files, will get them up on github once I am happy with the design. A few things I am worried about besides the J1708 bus: Bus capacitance....Will my circuit add enough capacitance to take the bus out of spec for any of the Truck/Heavy Duty protocols? Rising and Falling edge time....Will my logic conversion be quick enough? I don't think it will be a problem with K-Line and L-Line as they usually operate at 5baud, although the K-Line standard (ISO-9141 physical layer) supports up to 10400 baud. Any help would be appreciated, My IQ is lucky to be above 70 and have not designed a circuit and pcb before, I usually deal with relays. Not sure if anybody here has any experience with Truck/HD diagnostic interfaces. But here is hoping! P.S. Thanks for the forum and channel Dave. I have a bunch of diagnostic goodies and others bits and pieces I would like to bring down to show you one day, am only a few hours north! Would be great to see you do a video talking about systems and buses used in vehicles of all types.....Most people are completely unaware most new cars use at least a 250Kbit/s bus, the usual reaction is "what the hell they do not need such speed"....CAN FD uses 1Mbit or more :rant: |
| patrick1:
FORGIVE THE SIMPLTON RESPONCE oops noob caps, but um, either have a smpt , or using a current burning device like in your diagram there are plenty lm78 series that are adjustable too 13.8, but you may even be better with a resistor voltage divider on a transistor base, feeding a cap, since its unlikley too use much current at all. .. this works very well, just heatsink it a little if it uses up too an amp |
| MosherIV:
Hi. I have not work on J1708, I am working with J1939. I am not aware of current vehicles using J1708. According to Wikipedia and kvaser, J1708 is just the electrical standard and data transmission standard. Both claim that the comms part is based on RS485. The kvaser site is quite detailed https://www.kvaser.com/about-can/can-standards/j1708/ (I alway thought RS485 was +/-12V differential. The kvaser site says in J1708 the data lines are +/-6V differential lines, with a 6 byte standard message length) I would look at RS485 transceiver chips instead of trying to roll your own tranceiver circuut. As far as I am aware, even goods vehicles obey ODB2 protocol for diagnostics. Like domestic vehicles, each manufacturer uses the manufacturer specific codes for useful diagnostic codes. The generic ones only indicate there is a problem. I have been shown that ODB2 can work along side J1939 CAN. The kvaser site indicates that there is another protocol layer on top of the J1708. |
| carl0s:
The K-Line itself can already be up to 36v, certainly for the L9637D chip anyway. e.g. https://www.ebay.co.uk/itm/223221235158 |
| carl0s:
I suppose that only helps if you want to make your own interface though :-/ |
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