Hi everyone,
I want to use ELM327 (a microcontroller used as an interface to allow reading data from any vehicle ECU that use the OBD-II protocols) to work with all types of vehicles, including those with 24-volt batteries.
Diagrams shown in Figures 9 and 12 of the ELM327 datasheet 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 Elm Electronics 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 interface for the ISO 9141 protocol as I don't know weather to use a high voltage chip interface like L9637 or leave the discrete components. I think the issue with the existing components would be that the resistors R17 and R19 values should be at least double otherwise the power dissipation would be too high for 24V supply. I read that the ISO 9141 require 510 Ω pullup resistors but I assume that value could be increase with a reduction of the communication baud rate. Also the resistors will drive current only in communication not in idle.
- 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.
I want to use ELM327 (a microcontroller used as an interface to allow reading data from any vehicle ECU that use the OBD-II protocols) to work with all types of vehicles, including those with 24-volt batteries.
Diagrams shown in Figures 9 and 12 of the ELM327 datasheet 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 Elm Electronics 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 interface for the ISO 9141 protocol as I don't know weather to use a high voltage chip interface like L9637 or leave the discrete components. I think the issue with the existing components would be that the resistors R17 and R19 values should be at least double otherwise the power dissipation would be too high for 24V supply. I read that the ISO 9141 require 510 Ω pullup resistors but I assume that value could be increase with a reduction of the communication baud rate. Also the resistors will drive current only in communication not in idle.
- 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.