24V ELM327 interface

[ismael]

Hi everyone,

I want to use ELM327 to work with all types of vehicles, including those with 24-volt batteries.
Diagrams shown in Figures 9 and 12 of the ELM327 datasheet (elmelectronics.com/DSheets/ELM327DS.pdf) are to work with 12 volt batteries. Although the changes to be made to adapt the circuit to work at 12 and 24 volts may seem obvious, I could not find any info on the rest of the documentation or others websites. I would like to be sure that any changes I make are correct.
I already contact to elmelectronics.com but they didn't help me very much. I thought maybe someone could give some advise in this forum.

For example, looking at the diagram in Figure 12 my thoughts are:

- LP2950 and LP2951 regulators could be used as they admit up to 30V at the input, although it seems a bit forced.
- The MCP2515 also appears to be suitable for the voltage levels at the CAN bus lines.
- My major concern is with the ISO 9141 interface as I don't know weather to use a high voltage chip interface like L9637 or leave the discrete components. But I think that the issue with the existing components is that the resistors R17 and R19 values should be at least double otherwise the power dissipation would be too high.
- Would have to change the values of R20 and R21 and R34 and R35 so the ELM327 input pins don't exceed 5V, as well as a possible zener for protection.

Any help will be appreciated.

Thanks in advance.