[Resolved]Problem with 5V connection on Arduino Uno

I measured the resistance between the gnd and 5V pins on the Arduino Uno to be 800 Ohms, without anything attached. I actually had 2 arduino unos and they both had the same value, so I don’t think that was just a mistake on one board.

I made a shield (pdf of the layout attached), and the resistance between gnd and 5V pins (on the shield) can’t be read using a max of 2MOhms (so it’s larger than that).

When I connect the shield to the arduino, the resistance between 5V and gnd is very low that its as if they’re shorted (the multimeter beeps). This halts the uploading process; I’m guessing because the program doesn’t receive the data its expecting from the arduino.

When I tried the circuit on a breadboard, it actually works and I can upload programs. I think the resistance between pins on the breadboard is much higher (open circuit) than that on the shield that I made (a finite resistance, albeit higher than 2MOhms), so it’s acting like 2 resistors in parallel, with the resulting resistance lower than the 800 Ohms.


I am not doing this as a hobby or playing around; I have a project with very tight deadlines, and I need to solve this asap (1 week max). Unfortunately, I don’t have time to change the pcb to have wider wires or more space between wires, if that would solve the issue.
BUT I do think that the main issue is with the Arduino - 800 Ohms between pwr and gnd? Seriously?

projectbak.pdf (35.3 KB)

Thanks for quick reply!

1) How are you powering the Arduino?

I tried different methods: USB, 9V battery, 4x1.5V (AA) batteries (6.5V).

2) Measuring the resistance between the Arduino GND and the +5V pin is NOT a valid measurement or a debugging technique. The fact that two different Arduino boards measure the same would appear to confirm that they are both good.

OK. Thanks.

3) You should be measuring the VOLTAGE not the current between the GND and the +5V pins. When the Arduino is powered (by the undisclosed method) do you see the expected 5V on the +5V pin?

I did try to measure the voltage from ground to 5V pin, it was always less than 5V, with any of the sources. Without the shield, the 5V pin was at 4V.

4) When you connect your shield board, does the voltage remain at 5V?

With the shield, the voltage was less, closer to 3V.

The tests I did were casual, and I'm getting these numbers from memory (I'm at home now and the stuff is on campus). I'll re-do the tests in a more organized fashion tomorrow and I'll try removing components like you said and I'll get back to you.

The schematic is a little awkward because I was rushing it to do the board to get the pcb manufactured in time. I'll reorganize it so its more readable and post it tomorrow.


When I connect the shield to the arduino, the resistance between 5V and gnd is very low

Remove the shield from the arduino and check for a short on the shield.

I had the power connector inverted in the pcb layout :S

So Vin on my shield actually connects to reset, 3.3V = gnd, 5V = gnd, etc

Re-wired the connectors and some parts of the circuit is working. I suspect the others that aren't working have some other issue.

Thanks for your suggestions everyone.