Blue Light Flash [power cable voltage drop]

pauljpaulj
edited February 2018 in Problems and Issues
Hi All,

I'm having an issue with my KiwiSDR Kit.  

I have it setup as shown in the instruction paper.  GPS / Antenna / USB Micro / Power Adaptor.  With the cape on, the blue light on the BeagleBone flashes once, then nothing.  With the cape removed, the board starts up appropriately.  I've tried a 5V, 6V, and 12V Power Adaptor.  I've double checked to make sure the pins are lined up correctly.

I'm guessing there is a problem with the cape. 

Any suggestions?

Thanks
Paul

Comments

  • jksjks
    What do you mean 6V and 12V power adaptor? You didn't feed more than 5V into the Kiwi or Beagle power inputs did you?

  • pauljpaulj
    First one I tried was a 5v 1A supply.
  • jksjks
    Okay, but if you put more than 5v into it the outcome is uncertain. If the Beagle alone comes up using a 5v supply then you probably didn't fry it. Did you mean 6 and 12 WATT 5 volt supplies? The 5v input on the Kiwi goes though the headers into the Beagle for power management before coming back to the Kiwi. So hopefully the worst that can happen from any over-voltage is you have a dead Beagle.

    I can tell you what's probably wrong because this identical issue happened to me recently: the cable between your power supply and Kiwi has too much voltage drop because the wire gauge isn't large enough (or it's something besides decent copper). I sent a BBG + Kiwi to a guy here in NZ recently. He asked if I could include a USB/A-to-2.1mm adapter cable. I didn't have any, but I found some on the web and had one drop shipped to him and some sent to me.

    He had the same problem as you. Except that the Beagle power LED didn't even flash when the Kiwi board was installed. But with the Kiwi removed, and the BBG powered through its micro-USB input, the Beagle alone worked fine. So dead Kiwi board, right? He sent the whole thing back to me. I plugged it in to my standard 5v supplies and everything worked fine! I tried the adapter cable and NADA. Even though power at the Kiwi measured 5 volts!

    Using a BBB (not BBG) which has a 2.1mm input connector like the Kiwi I noticed this: With a multimeter at the BBB input connector it would briefly drop from 5v to 4v when power was applied and the Beagle's power management IC (PMIC) tried to startup. Then it would jump back to 5v because the PMIC rejected the 4v input and stopped drawing current.

    So this probably explains your situation. With the Beagle alone drawing 300 - 400 mA your cable must not drop below 4.75v which is about the PMIC lower limit. But with the Kiwi board the 1000+ mA draw is too much and the PMIC refuses to startup. But it's enough that the power LED still manages to flash.

    paulj
  • pauljpaulj
    Thanks for the great reply jks!  I'll mess with it later.
  • jksjks
    edited February 2017
    Yeah, so this whole question of USB charging cables is a thing apparently: http://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/

    It is very difficult to find a USB/A-to-5.5/2.1mm adapter cable where the wire gauge is specified. The only one I found is from Adafruit that specifically mentions 22 AWG: https://www.adafruit.com/product/2697

    If you use an online voltage drop calculator, a 1m 22 AWG cable will drop 5v to 4.8v, which is acceptable. See here:
  • pauljpaulj
    Wow.  Thanks again!
  • jksjks
    edited February 2017
    I had a an old 3.5 foot USB/A to Apple 30-pin adapter cable that was fairly "thick". After whacking off the 30-pin connector I found the red/black power wires were 22 AWG.  I used one of these to add a 5.5/2.1mm round connector to the cable end:

    These are nice because no soldering is required. You can also tie the outer braid and drain wire together with the black wire at the connector to increase the effective gauge of the ground wire. This is because in most USB power supplies the USB shielding shell and ground pin will likely be connected together.

    This new cable powered a Beagle + Kiwi with no problems and only about a 0.3v drop.


  • pauljpaulj
    Hey jks, just wanted to follow up, that 22 AWG cable you shared worked great.  Thanks a lot!
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