DUEX5 PWM issue



  • I'm using the DUEX5 to control a small 9g servo through the PWM5 connector. I'm also using the E2 heater output (H3) to control a 24V LED strip. Something broke in my setup though and now whenever I turn on the machine all 5 red status LEDs next to heater terminal blocks on the DUEX are on. Servo shakes because of that and the LED strip is always on, if I use "M42 P3 S0" it will dim maybe 10% but it should go dark completely as it used to"
    I tried starting the machine without the microSD card, no change, if I disconnect the DUEX5-DUET ribbon cable and start the machine the DUEX heater status leds are off.
    I'm starting to think my DUET might be damaged, is there anything else I could try and test?


  • administrators

    My best guess is that the 74HCT02 chips on the DueX have failed due to over voltage on the +5V rail. How large is the servo? Large servos feed a big pulse into their power supply when the motor stops. I measured this at about 8V using a MG996R servo. The Duet 1.04 revision has a 220uF capacitor on the 5V rail to help absorb this, but it's better to connect a capacitor directly between the power and ground connections of the servo, especially for servos connected to the DueX.

    The DueX status LEDs are off when the ribbon cable is not connected because then there is no 5V supply to feed the 74HCT02 ICs, which drive the heater mosfets.



  • I'm using a tiny SG90 type 9g servo, all that together with a few weeks old DUET Ethernet 1.04 and a DUEX5.
    Is it possible that some pins of the microcontroller on the DUET board got damaged? I'm thinking of disconnecting the DUEX and connecting a servo directly to DUET expansion connector pins to check if it's working correctly.


  • administrators

    @mildw4ve said in DUEX5 PWM issue:

    I'm using a tiny SG90 type 9g servo, all that together with a few weeks old DUET Ethernet 1.04 and a DUEX5.
    Is it possible that some pins of the microcontroller on the DUET board got damaged? I'm thinking of disconnecting the DUEX and connecting a servo directly to DUET expansion connector pins to check if it's working correctly.

    It's unlikely that multiple servo pins on the Duet got damaged at the same time, although not impossible if the 74HCT02 chips on the DueX5 got damaged.

    The 74HCT02 chips can be replaced easily, given the right equipment.



  • Thank You for all the help, we do have hot air reworking stations at our disposal so we can definitely go down that route.
    I found 2x 74HCT02 chips on the DUEX5 board, both are labeled "HT02 82KG4 AP91", the closest I could find is a chip labeled "HT02 75KG4 CS41" found here https://www.tme.eu/gb/details/sn74hct02pw/gates-inverters/texas-instruments/

    Is that a proper replacement?


  • administrators

    Yes, 74HCT02PW is the correct part.



  • Replaced both of the chips, it took some skillful work with a soldering iron but it worked.
    Just a small followup in case anybody encounters the same issue with their DUEX 🙂


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