I'm wondering if there is a combined power distribution transistor that I could feed at least two power sources, and decide via logic which power source to use. My initial thought is to use it to simplify debug/use scenarios where I want to program the chip via USB, but power the chip via an external battery that can provide higher amps for running LED strings (or possibly higher voltage/amps that is fed into a voltage regulator for 5v).
Now, on the Uno, you could have two power sources (USB and external 7-12v power), but I don't use the Uno as much these days, and I'm looking for something that I could use on any microprocessor, having a small board that sits between the microprocessor and the computer. When I'm using it in the field, I probably would power it on batteries directly, so I don't have to worry about 2 different power sources.
Now, on some chips, it is possible to cut a trace so that you have to power the chip via VIN and it doesn't use power from USB at all, but I'm wanting a general solution, that I could use for whatever chip I'm currently working on.
In theory, it would be possible to do this with several components (2 diodes, 2 resistors, NPN and PNP transistors, capacitors), but it would be nice if it already existed. As a software guy, I don't have the grounding to know that such and such transistor already exists.
Case 1, select power based on signal. It is likely that the signal
is just power A. So if power A is available, it will use power A,
otherwise use power B. Both A/B are protected against
reverse current with diodes.
common ground ------> +-------------------+ ---> ground --->
| If signal, then | |
power A --> diode --> + output is power A | |
| else output is | filtering caps.
power B --> diode --> + power B | |
| | |
signal -------------> +-------------------+ ---> power ---->
Case 2, select power based on voltage.
common ground ------> +--------------------+ ---> ground --->
| If power A voltage | |
power A --> diode --> + is higher than | |
| voltage B, pass | filtering caps.
power B --> diode --> + voltage A, else | |
| pass voltage B | |
---------------------+ ---> power ---->
Now, since my initial use is USB, an alternative question is whether there is a USB cable that has a normal male A plug on one end, and a standard plug on the other end, and a separate 2 cables coming out that would allow me to tap into the USB power before hooking it up to the microprocessor (presumably you would have diodes on this power wire). I can see how to create such a cable, but I was wondering if somebody was already making it. The use case would be connect the two USB ends to the computer and micro-processor, but allow me to power LEDs directly without going through the VIN port. On one of my microprocessors (Digispark), I burned out the processor trying to connect 16 or 32 neopixels LEDs to the input pin.