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1. 

   fugalster
   Member

   I've searched to forum and found conflicting answers to my question, so I post it anew.

   When is it (not) appropriate to have a jumper across both usb and ext?

   I have built a custom shield for the maple native beta, but neglected to bring the power selector jumpers up top. I'm afraid to put a jumper across both ext and usb for fear of damaging the board, requiring me to remove the shield to switch jumpers every time I need to upload (very annoying). Now I'm beginning to realize that removing and installing the shield repeatedly is wearing out the headers and will likely cause its own damage sometime soon (this is due to the large number of pins that have to be aligned and things never going in or coming out exactly square).

   The details:
   I'm using a 12v battery, regulated down to 3.3v before it ever reaches the Vin pin.
   There are also 9v and 5v regulators on the shield to run other hardware with current requirements in the low amperes (2 dc motors, 12 range finders, 32 LED's), so testing with only usb is not possible. Power has to be switched to battery before installing the shield and testing the program.

   Any concrete advice one way or the other would be much appreciated. Rules for when (if ever) multiple jumpers is allowable would be even more appreciated.

   Posted 4 years ago #
2. 

   poslathian
   LeafLabs

   First off, you probably should not be regulating your battery before Vin. the regulators on Maple Native expect something a bit higher than 4V, and they will regulate down to 3.3V for you. If you supply them with 3.3V, you will probably get 2.6 or so on Vcc, the chip will still run, but its not what you expect!

   Second, you can leave the ext and the USB jumpers both closed iff and only iff you never apply a voltage to USB and to Vin/Vext at the same time! you might also be able to get away with putting a diode across the jumpers instead of just a jumper. If you end up with:

   USB 5V -> diode -> Vout
   External power -> diode -> Vout

   you should also be OK. Keep in mind Vout is the output of the jumper bay and the input to the system regulators.

   You might also be able to do some surgery. I have some maple minis that I want to both steal the USB power as well as power externally from a battery. This is not possible with the default configuration, so I put a 3.3V regulator on my shield, and removed some diodes on the mini so that the onboard regulators were not being fed at all. I then tapped the USB power off of Vin (which is annoyingly renamed Vout on Maple Native).

   Have a look at the schematics. There is almost certainly a way to do what you want, but replacing the jumper bay with diodes is probably the easiest thing to do.

   Posted 4 years ago #
3. 

   fugalster
   Member

   Thanks for the reply.

   If I understand you correctly the problem to be avoided is damage to the USB port? If you supply >5v to the EXT pin and simultaneously apply USB power, then you're forcing current backwards through the USB, damaging it. If you have <5v on EXT you now create a short between the USB's 5v and the EXT pin, drawing too much current out of the USB port, damaging it.

   That makes a lot of sense to me, and that would explain why the diodes will do the trick. Just want to check that I'm thinking this through correctly.

   Posted 4 years ago #
4. 

   4rdu1n0
   Member

   What sort of diode would you use?

   Posted 3 years ago #
5. 

   gbulmer
   Moderator

   The Maple power supply is designed on the basis that it gets pretty close to 5V.

   So diodes should have a small forward voltage drop (VF or Vf), say 0.5V or less.
   It needs to carry at least 0.5A (If), so say 1A, but over 2A is overkill.
   It should resist an accidental reverse voltage of more than 5V (I'd pick 10V upwards)

   So this would be a starting list, using Farnell (I know how to use their search): VF <= 0.5V, If>=1A+, Vrrm>=10V

   (Full disclosure: I am not a member of LeafLabs staff)

   Posted 3 years ago #

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