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    RS-422 using SN75176 running good but hot

    I have designed a prototype RS-422 using SN75176 with input opto isolated via H11L1. The H11L1 output is biased by 270 ohm resistor at its output pin and fed to a hex schmitt trigger inverter. The inverter output will then drive the SN75176 transceiver D input.. The transceiver out no bias needed since RS-422 is always transmitting.. The transceiver is hot around 50~60 degC. Also i used the Receiver oit pin to drive my LED indicator.. My voltage supply out is using Mornsun B0505S with output 5.34V.
    Need your idea on what caused the temperature rise.. My ambient is about 30degC tropical region.

    •   Alt18th January 2018, 04:24

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    Re: RS-422 using SN75176 running good but hot

    What do you mean "...receiver oit pin to drive my LED..."? Maybe post a schematic.

    BTW, your supply voltage is a little high. 5.25v is recommended max.


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    •   Alt18th January 2018, 07:01

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    Re: RS-422 using SN75176 running good but hot

    Hi,

    The SN75176 consume a lot of power...they will become warm, but they must not be hot. See datasheet.

    Please give a complete schematic. Your description is too vague.

    Please check all IC input levels. Are they all within recommended levels. Read datasheet.
    I see a good chance that your circuit doesn't give clean HIGH and LOW levels...this may cause increased power supply current.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. Thank you.


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    •   Alt18th January 2018, 08:26

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    Re: RS-422 using SN75176 running good but hot

    Quote Originally Posted by barry View Post
    What do you mean "...receiver oit pin to drive my LED..."? Maybe post a schematic.

    BTW, your supply voltage is a little high. 5.25v is recommended max.
    I acknowledged your question, it is unconventional and was thinking since Receiver Output is TTL level, I might as well use it to drive LED as output indicator via 10k resistor.

    - - - Updated - - -

    Quote Originally Posted by KlausST View Post
    Hi,

    The SN75176 consume a lot of power...they will become warm, but they must not be hot. See datasheet.

    Please give a complete schematic. Your description is too vague.

    Please check all IC input levels. Are they all within recommended levels. Read datasheet.
    I see a good chance that your circuit doesn't give clean HIGH and LOW levels...this may cause increased power supply current.

    Klaus
    My initial check was about 4V high and around 0.4V low level at SN75176 pins 4 and 1 for D input and R output respectively. Give me time to capture waveforms, thanks all for the reply. I did tried to test the circuit with ambient (room airconditioner running) 28degC and temperature about 45degC.



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    Re: RS-422 using SN75176 running good but hot

    Hi,

    According datasheet there us a no load supply current of typ 35mA.
    This makes 5V x 35mA = 175mW

    And the voltage at the load is typ 2.7V with 100Ohms.
    This makes a current of 2.7V / 100 Ohms = 27 mA.
    And thus a power dissipation of (5 - 2.7) x 27mW = 62mW

    A total of about 250mW.
    With the "D" package and it's thermal resistance of 171K/W makes a temperature rise of 43K.
    With the ambient temperature of 28°C this gives a chip temperature of 71°C.
    This is really hot.

    If you don't want it that hot, then use another chip, or use a heatsink.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. Thank you.


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    •   Alt18th January 2018, 22:15

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    Re: RS-422 using SN75176 running good but hot

    Quote Originally Posted by KlausST View Post
    Hi,

    According datasheet there us a no load supply current of typ 35mA.
    This makes 5V x 35mA = 175mW

    And the voltage at the load is typ 2.7V with 100Ohms.
    This makes a current of 2.7V / 100 Ohms = 27 mA.
    And thus a power dissipation of (5 - 2.7) x 27mW = 62mW

    A total of about 250mW.
    With the "D" package and it's thermal resistance of 171K/W makes a temperature rise of 43K.
    With the ambient temperature of 28°C this gives a chip temperature of 71°C.
    This is really hot.

    If you don't want it that hot, then use another chip, or use a heatsink.

    Klaus

    Apologies for my delayed respose.
    firstly, Thank you so much KlausST for the calculations.
    And as promised herewith the waveforms taken from transceiver Data pin4 and RS-422 out.

    I will test with another chip with same footprint and see the results.



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    Re: RS-422 using SN75176 running good but hot

    Quote Originally Posted by fishbowlfx View Post
    Apologies for my delayed respose.
    firstly, Thank you so much KlausST for the calculations.
    And as promised herewith the waveforms taken from transceiver Data pin4 and RS-422 out.

    I will test with another chip with same footprint and see the results.
    UPDATE:

    Perfect!
    This would be a fitting way to archive this thread.

    I have followed KlausST suggestion and replaced the transceiver with SN65HVD485EDR which has the same footprint as SN75176, though expensive by about 300% (from 0.25usd) but the goal was achievd: Temperatures were 38degC.
    Thanks guys hope this post can help other people. All the best to you all.

    --fishbowlfx



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