Power drop issue

Discussion in 'The Projects Forum' started by Simon Czarnota, Jul 24, 2015.

  1. Simon Czarnota

    Thread Starter New Member

    Jul 24, 2015
    2
    0
    Hey guys, first let me say that I tried seeking help in the appropriate forums with no avail, so I am seeking help here, perhaps a fresh pair of eyes can shed some light on my issue.

    I am building a custom drone, the setup is very simple. I have a flight control board, 3 motors, a battery, and a 5V step-up circuit for the board (it needs 5V whereas everything else is happy at 3.7V). This is the gist of the setup (except one less motor):

    [​IMG]

    My problem is that I am seeing a drop in voltage between the positive junction of the battery, the board + all the motors, and one of the negative terminals of one of the motors (trying to measure the voltage which the motor is getting). What I'm seeing is that it starts around 3.3V, and gradually drops to below 2.8 within the span of 30 seconds.

    Through various tests I have confirmed that the setup is correct, the battery is fine (tried a bunch, they're all fine), the motors are fine and the board is fine. That only leaves the wiring. I am clueless as to what could be causing this drop in voltage. I have learned that an increase in current would cause a drop in voltage, so I came to the conclusion that I must have a current "leak" somewhere?

    Thank you in advance for any help and any insight anyone can provide.

    PS: If interested here are actual pics of the setup: http://imgur.com/a/1BXkM though you can ignore it if it seems like a mess, it is essentially wire for wire with the schematic above.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The negative terminals of the motors are being switched (PWM) by FETs inside the control board. My guess is that the FETs are under-rated for the current they are expected to handle, so heat up over time (there appears to be no heatsink to disperse the heat). Higher temperature results in higher Rds, hence a greater voltage drop.
     
  3. Simon Czarnota

    Thread Starter New Member

    Jul 24, 2015
    2
    0
    That makes a lot of sense, I was reading about FETs but never came to that conclusion. I ordered a few with a much higher rating so hopefully it works. Thanks for your help!
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,804
    1,105
    It's not only power/current rating which matters: low Rds(on) is also important for minimising heat generation.
     
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