Arduino mega barrel jack has power coming out when USB is connected

Official Hardware Mega Mega 2560
1

Hi

I got a Mega ADK and as I plugged the programming USB cable from the computer, the 5V jack started to power the rest of my circuitry including a fan

I am surprised there is no Schottky diode or what not, to prevent current going out of that jack

any thoughts on this ?
I don't even understand how this could be possible, looking at the schematic

thanks

image
image1242×596 45.8 KB

2

There is no 5V jack on that board. What do you mean?
Do you mean the big barrel jack? If yes, that is NOT a 5V output or input. Using it as such will damage the board.

The barel jack is for power input only and the voltage must be between 7V and 12V

3

You are seeing DC power coming out of the Vin jack because:

USBVcc goes past T1’s internal diode to the 5V line. 5V line is connected to IC2’s output. IC2’s output has an internal Schottky diode back to the Vin pin which is attached to the Vin jack. You will not get 5V out of the jack but 4V is very likely, maybe even 4.5V at lowish current. Hope that explains it?

4

that jack/barrel whatever is connected to my power supply PCB (that was not plugged in at the time)
as the USB cable is in, that jask not only shows 5V but started to power other devices plugged on my power supply PCB

which should not be the case, I'll have to add a diode to avoid that sort of dangerous behaviour

5

See post #2

You should not have anything connected in the first place.
It is NOT 5V

6

I measure 4.53V

and it is used to power the Arduino in normal (no usb cable) running conditions

I don't want any current to feed back to the power board. as mentioned it powered a fan connected to the power rails of my power board

I don't want to forget to unplug that cable when I program my arduino

7

Is the voltage between 7V and 12V?

8

it is a simple pcb power concentrator

I measured 5.07V

image
image856×475 5.64 KB

9

The voltage on the barrel jack or Vin must be between 7V and 12V, otherwise the Mega may not operate properly.

See the documentation for your board.

10

ok, good to know, thanks

11

Adding a diode will drop the voltage even further and nothing will work.

12

I can supply more with a regulator and shoktey have low drops

gf1a_gf1b_gf1d_gf1g_gf1j_gf1k_gf1m.pdf

13

You mean a boost converter and the Vf of a diode will vary with current.

14

I am trying with a node MCU
dont like the fact that barrel jack is leaking for some reason
if it does not work I'll try with a regular MEGA

15

It has the diode on the board but still requires 7 to 12V.

The node MCU might work if what you have connected will work with 3.3V logic.