I'm running into an issue with an Arduino project. I have a power supply at 24V going into a DC DC converter (V7809-1000) which is supposed to have a 9V output. This 9V output is tied to the "Vin" pin on an Arduino Uno.
What I've noticed is that if I plug the Arduino Uno into my laptop USB port first, and then power on my 24V supply, the output of the DC DC converter is only 5V. Conversely, if I power on the 24V supply first and then plug the USB in, the output is 9V as expected.
I understand the Arduino has some type of automatic power source switching to determine when to use power from the USB and when to use power from an external source, but I don't think this is the issue here.
Does anyone have any suggestions on why this may be happening? Perhaps the DC DC converter is not functioning properly when it sees a voltage on its output without a voltage on its input when I first plug the USB in?