teensy 3 external power

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barcan

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Hi,

Before searching the forum i thought teensy 3 ran on 3.3V, but now it seems that if connected to USB that the USB supplies 5V.
OK. Then i want to supply power from a computer power supply. I have found the answers to most of my questions already about USB and external power but just want confirmation before I do something I will regret.
I have the option of 3.3V or 5V from the power supply. I am leaning towards 5V because I need to put in a 1N5817 diode (which will use maybe .5V). But that will still mean around 4.5V for teensy 3.
So the question is when I power my teensy externally do I use the 5V or the 3.3V from my computer power supply? I am sure I have read somewhere that it is a little tolerant as to voltage.
Thanks.
 
OK that make sense. Thankyou.
So in that case the answer to my question is to use the 3.3V from the power supply to the VIN.
 
I think you want to give 5V to Vin or Vusb.

Consulting the schematic, http://www.pjrc.com/teensy/schematic.html, the USB pin 5 connects to the Vusb pin, goes through a 500 mA fuse, goes through the cuttable jumper between Vusb and Vin, then drops through two parallel diodes to the VREGIN input of the K20 processor, before coming out as regulated 3V3.

If you want to supply regulated 3V3, then it should go to one of the 3V3 pins. Otherwise you will be trying to drive your 3V3 input through a diode drop and into a 3V3 regulator.

-- rec --
 
If you apply power to VIN, at least 3.7 volts is needed, because of the on-board diode and the low-dropout regulator.

By default, VIN and VUSB are connected by a pair of pads on the button side with a cut-able wire. Normally you would cut that wire, so the USB voltage does not conflict with whatever power source you're connecting to VIN.
 
Thankyou for responses.
Went for option 3...2 power sources (both 5V) each with 1N5817 Diode. Was a bit nervous before switching on. But now works with just USB, or just computer power supply, or both on at the same time.
 
I have a similar problem : the teensy3 is connected (Serial1, reset, Vcc) to a panstamp (3V3 as well, so no 5V level issue here).
But if I connect the USB on the Teensy, the two 3V3 regulators will be in parallel. To avoid that, I could/should ? cut the connection between the two pads. But then I can't anymore feed the teensy with USB when it's not in the "shield". Any other solution ? For example, could I set a 15R between both 3V3 to avoid too high currents (at 20mA, it would only drop 0.3V). I unfortunately don't really have much space to put some kind of switch on the 3V3 wire, but if that's absolutely necessary, I could consider that as well.
 
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