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THVD1450: Inquiry about the temperature of the RS-485 termination resistor.

DEUGKI SHIN
Prodigy 50 points
Part Number: THVD1450

Tool/software:

Hi.

I am currently using this product (THVD1450DRBR) and I have a question regarding the termination resistor for RS-485 A and B.

The temperature of the termination resistor increases when the power to the controller is turned on.

I'm only using the driver side and have set the pull-up for DE and nRE.

The termination resistor value is 120ohms and the size is 1608.

Should I increase the size (2012?) or am I using it incorrectly?

Please provide an answer.

Thank you.

  • 0
    Guru 292610 points

    The typical power dissipation in the resistor is about 0.1 W. This is right at the limit for typical 1608 resistors; so 2012 or even larger would be a good idea.

  • 0 in reply to Clemens Ladisch
    Prodigy 50 points

    Thank you for your response.

    I have a question about the current situation. In a state where the MCU controller program has not been downloaded and the RS-485 communication is not connected, with DE in Pull-Up and nRE also in Pull-Up state, can the termination resistors A and B still heat up in this situation?

    The temperature is currently rising to around 46 degrees. I am wondering if this is normal.

    Please provide an answer.

    Thank you.

  • 0 in reply to DEUGKI SHIN
    Guru 292610 points

    When DE is high, then the outputs are active and will drive about 30 mA through each resistor. If you want to save power, you need to pull DE low.

    Dissipating 0.1 W in a resistor rated for 0.1 W will make it hot (but 46 °C is not actually dangerous).