teensy 3.1 not working properly with 7805A power but works fine with USB power

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miknshla

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I'm having an issue with teensy 3.1 serial port as midi interface. It works fine with USB power but when I use my 7805 circuit my MIDI device doesn't respond. I can't see anything different about the waveforms with my scope. I have 1000uf and .1uf on input to 7805a and 10uf and .1uf on output of 7805a. The schematic for the Teensy 3.1 shows plenty of capacitors for filtering on the teensy circuit board.
I don't have these two power circuits on at the same time and I haven't cut the trace to isolate the two sources yet.
I tried a regulated wall power supply and it works too but I also tried a usb charger with the usb cable and it does not. USB only works when connected to my computer
 
Assuming you are using a hardware serial port on Teensy and it is not active when you use your own 7805A circuit, nor when you plug in the charger, but it works if you use a 'wall-wart' or just plug into USB port on PC.

1) what voltage are you feeding 7805A

2) Can you show us your wiring? (just one picture if you can all relevant connections (pretty much all of them) in one shot).



Your USB charger may not provide power if the current drain from the device plugged into it is very small.
 
Here's a couple cool tricks/tips that may help.

Cut the isolation tab on the board and place a diode in there. This will give you the ability to power it either way automatically, and will work fine as long as your power into the teensy is 5v.

Your 5v supply may lack high frequency filtering. Add a ferrite or common mode choke on both gnd and 5v from your 7805 regulator.

Add a common mode filter (single or double stage) on your mains supply.
 
Your trouble might be power released, or it could be one of the common startup issues (waiting for "while (!Serial) ;" or initializing external hardware before it's ready). Those startup issues can appear as if hardware doesn't work, even though they're entirely software-based.
 
well it is working now but I'm not totally sure what fixed it or if it is really fixed. the serial port was always transmitting but wouldn't be rec'd properly on the other end.
I replaced the 7805 since it had been shorted out once and I put some ferrite clamps on the output leads. When I powered up it had the same problem as before. I moved the ferrite clamp to different areas on the leads and it started to work and then I couldn't make it fail. So, then I looped the wire twice thru the clamp and tested some more and it worked fine. I also noticed that the voltage was a little low with both of the other power supplies that worked, 4.7V, so I added series diode and I'm now running at 4.2V and it is working fine.
 
Glad I could help. Many people do miss the "dirty power supply" problem.
Cheap Chinese switching power supplies often have this problem, especially Class II.
I've even seen cases of florescent lights, and even power supplies powering something else interfere with something totally unrelated.
Recently (yesterday as of this message) my daughter turned on the vacuum cleaner, and it actually caused one of my projects to reboot because of a class II supply.

Things to remember:
  1. Don't pollute the EM spectrum.
  2. You may have to handle and defend your equipment noise from devices that violate rule 1.
 
I plugged the USB in alone again and now the serial port is putting out a lot of garbage. I cut the trace from Vin to Vusb and all is good.
 
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