RS232 and RS485 circuit protection


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  1. #1
    Join Date
    Feb 2009
    Location
    Southern California
    Posts
    86

    Default RS232 and RS485 circuit protection

    I have a product that uses RS232 or RS482 and wanted to add better circuit protection for lightning strikes n an upcoming release. Right now, all I have is a MOV on the RS232 line. I wanted to see what everyone else would recommend. I don't need a perfect circuit protector, just looking for better ideas. I figured I would ask the community how they protect their circuits other than buying someone's product to do it. I had a vendor recommend resettable fuses as one option, but not sure if that is the best way to go. We often have lightning strikes and overall our products hold up well, with usually just the max232 or SN65176 being killed.

    Thanks
    David

  2. #2
    Join Date
    Dec 2010
    Posts
    400

    Default Re: RS232 and RS485 circuit protection

    This is one of the nastiest problems in applied electronics to solve, and many people have made a career out of solving it.
    Don't waste your efforts on fuses. By the time they get around to doing anything, the circuit is already fried.

    The problem breaks down into several pieces - limit the voltage excursion as quickly as possible, limit the current flow, and direct it somewhere safe. You will be challenged to do this effectively with a MAX232 because it has a built in switching power supply to enable driving the outputs to double the supply voltage, so there is no solid power rail to clamp to.
    Board Layout will be absolutely critical too. Honestly, this won't be solved by a couple clues on a user forum. If you want to avoid the cost of external protectors, you need someone to go over the complete design from this perspective, including silicon technology choices, reference points, trace widths and spacing, application specs (cable lengths and types) and so on. You are as likely to make it worse as better without a holistic approach.

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