Quick Question about External Power For Teensy++ 2.0

Status
Not open for further replies.
D

debrad

Active member
Hello again everyone!

My 25-note "foot"board project is coming along nicely (the code is working great, just building my enclosure) but I wanted to ask a quick question or two about powering everything via an external source.

At this point I am hoping to be able to run my Teensy++ 2.0 via USB and external power but NEVER at the same time...I will ONLY use external power when using the standard 5-DIN MIDI jack INSTEAD of USB. I was thinking that I would need to follow the advice on the Teensy pages where the 5v pads are cut and a diode is installed; however, when I thought about this, there shouldn't ever be an occassion for me to have both power sources connected so, am I right in thinking that I don't have to worry about doing any of three mod options (i.e. cut the link, cut the USB cable, or cut link and install diode)?

In terms of power, the second thing I have been thinking about is whether my project will consume more than the 5v available...

Obviously the Teensy itself needs some power, I'm going to have a slide potentiometer for "volume control" that needs some power, and I'm hoping to a) illuminate an LED (blue or green) when the 25 keys are shifted below my base octave and b) illuminate a different LED (red) when the 25 keys are shifted above my base octave.

Knowing that the LEDs may require 2-6v themselves, am I going to run out of available power? I know the LEDs will both have current limiting resistors on them but, to maintain enough power for everything else, would those resistors have to be large enough that the LEDs won't be bright enough to even make them worth including?

- brad -
 
If you promise yourself that Teensy will never ever be connected to both simultaneously (not even accidentally when you have been drinking or something) then you can probably get away with leaving the pad uncut - if there is even the slightest chance that ANYBODY might accidentally connect both at once then you need to mitigate the problem(s) this can cause.

Use 2.1V 20mA LEDs with correct value resistors (google: LED resistor calculator) and reconsider your power supply if able to determine that just two of these is too much for it.
 
What I can't understand is why Paul hasn't simply added two Schottky diodes like the BAT20J, one from Vusb and one from the Vin pin towards the internal +5V rail. Or at least one from Vusb to Vin, so that cutting the Vin-Vusb bridge would not longer be necessary in most cases and allow alternative powering without worries, though.
 
Last edited:
The VIN-VUSB jumper was done in anticipation of (eventually) supporting USB host mode. Current would need to flow in the other direction, to power USB devices.
 
robsoles said:
...if there is even the slightest chance that ANYBODY might accidentally connect both at once then you need to mitigate the problem(s) this can cause.
Click to expand...

Thanks for the replies.

Just to clarify...the Teensy site reads that cutting the trace and cutting the USB cable "always run the Teensy processor from the external power". In my mind, that means if I cut the trace (to be safe) and ONLY connect a standard USB cable, my Teensy processor will NOT receive power (i.e. I would ALWAYS have to have external power connected...is this correct?
 
Yes this is correct. BTW it should read "...the Teensy site reads that cutting the trace OR cutting the USB cable..." since cutting one of both is sufficient, but that doesn't change the result.
 
Status
Not open for further replies.
Back
Top