External DC DC converter issues with Arduino Uno Vin

Hi All,

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?


Could be that the regulator, when not loaded, is in sort of a 'hickup' mode. And your DMM is reporting it wrong. What happens if you unplug the USB lead. Does the regulator jump back to 9volt? Leo..

P.S. I would power through the DC socket. 9volt is plenty for that. DC socket has just a diode in series.

Hey Wawa,

If I unplug the USB, the regulator output does increase to 9V. The regulator output is tied to Vin, which I believe is the same as the DC socket input, but just after the diode. Also, it's tied to the 5V regulator on the Arduino which is acting as a load for the regulator.

I wonder if there is any issue with having 5V on the output of the regulator when there is nothing on its input (ie. when I plug the USB in but do not connect the 24V supply) which then continues to affect my circuit when I turn the 24V on.

Yes, Vin is after the polarity protection diode. Basically the same to inject 9volt at Vin or DC socket.

So basically the Arduino functions in all situations, but you just want to find out why you have this strange behavour.

I have a similar setup for my home lighting system. Pololu (24v in) DC/DC converter (modded to output 7.5volt) into the DC socket. And during occasional programming I plug in the USB cable.

The Mega I use has indeed circuitry that switches the USB supply off if Vin is above 6.6volt. Tomorrow I will see if I have the same problem. Leo..

Also some strange behaviour in my system. As said, the DC/DC converts 24volt into ~7.5volt that I feed into the DC socket. I measured 7.43v without USB plug connected. That voltage rises to 7.46v when I plug the USB cable in. A voltage rise would mean that the USB power takes over.... Not what I expected. But it works fine, so who cares. I have another project coming up. If I have time, I will track it down. Leo..

Hey Wawa,

Thanks for trying. I tested putting a series diode after my DC DC converter and between the Vin pin in order to prevent reverse biasing the converter. This seems to have solved my issues. Now even when I plug the USB in first and then turn 24VDC on, it seems to work as expected.

Can you try plugging your DC DC converter output directly to the Vin pin instead of the DC socket? The schematic shows that there is a diode for the DC socket input, but not the Vin input. I'm curious to see if you experience the same problems as me in doing so without a reverse bias protection diode for your DC DC converter.


Also, the reason why I am going straight to the Vin pin is because my circuit is on an Arduino shield so I'm avoiding the DC socket.

Good to hear you have found something.
I have this Mega permanently wired to my main home lighting system.
The Mega controls now about 18 12watt downlights on a whole floor of the house with web pages (Wifi), touch control RF remotes (wall units), daylight sensors and PIR sensors.
I will get in trouble with the family if I muck that up.
Soon I will have a project of adding voice control to the system with another Mega and the EasyVR shield.
This will be a separate unit that gives final commands via RF (433Mhz) to the main system.
I can experiment with that without turning the lights off.
Will keep you updated.

If it made you curious…
Here is a pic of the boards I made. 16x CC LED drivers for 3-18watt (down)lights per board.
Each light can be individually set to any 12-bit dim level.
Two boards now, but expandable to the silly amout of ~60 boards.
All controlled by the I2C bus from one Mega.
Each board has it’s own 24v/5A switchmode supply.
The first 24volt supply also feeds the Mega and Ethernet shield with that Pololu DC/DC converter.

Very cool. Thanks for the help!

Update. Measured things. Hope I have done it right. My Mega (other ones could be different) switches to DC socket power when that is more than 7volt. The onboard mosfet stops any current TO the USB socket. But does not stop current FROM the USB. When USB voltage is slightly higher (>0.1v) than the onboard's 5volt, it STILL USES the USB power. Leo..