External 5V Power Supply

  • I have a BLTouch connected to the expansion header of a DuetWifi using the outline in the Duet documentation. I am using an external 5V power supply, which measures 5V. When I power on the board the BLtouch will not power up and run its self test.

    I measured the 5V pins at the expansion header and I'm only getting 4.6V. The BLTouch voltage operation range is listed as 4.8V-5.1V. I connected the 5V supply to another DuetWifi board and again I read 4.6V on the expansion header.

    I switched the jumper to the internal 5V and I measure 5V at the expansion header. The BLTouch then functions as expected. Unfortunately I can't utilize this mode as I have an external safety string to kill the 24V power to the board.

    Has anyone else run into this?

  • Moderator

    Which version of BLTouch is it?

  • This is not surprising, since there's a diode in the path of external 5v, you're seeing forward voltage drop.

  • administrators

    You could connect the BLTouch 5V power wire directly to your +5V supply, bypassing the diode.

  • I'm running version 3. A previous unit I built had no issues, but I'm guessing I got lucky with a BL touch that would work at a lower voltage.

    I'll have to add some terminal blocks and run it off the BL touch. Might not be a bad idea to update the BL touch section of the wiki, to warn about expansion IO not supplying 5V if connected to external power supply.

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