Problems with various power supplies / ground loops?

Hi everyone,

I’m building a start box for a small turbine engine around an Arduino MEGA.
It comprises :

  • Arduino MEGA
  • H-Bridge
  • Relay boards (with optocouplers)
  • LCD 4*20

and the different voltages / needs are supplied by :

  • 12V Battery
  • 300W DC-DC (to lower to +6,5V)
  • 7805 module for +5V

Now for the problem :

When I’m using the H-Bridge to fade a LED Strip I have at the output (maybe 1-2W, nothing serious), if the arduino is fed by the 6,5V, the H-bridge will barely light up the LED.

BUT If I plug the arduino to the USB, it will work fine and the LED strip will light up!
I measured the voltage on the power terminal screws of the MEGA (I have a sensor board) and it shows around 4,5V while connected only to the battery (which delivers perfect 12V and enough Amps also) and goes back to a nice +5V when I connect it to the arduino.

I also noticed that when one (or more) of the relay is working, the LCD backlight will lower.

I can’t be putting my 7805 through it’s paces? Could I? If I’m correct I’m only asking for 2-300mA in the worst case and it’s rated for 1,5A…

Enclosed is the schematic of my build.

Thanks for your help!


Probably not a ground "loop", but it does sound like a grounding problem. It seems like there may be a missing ground between the 6.5V supply and the H-Bridge. The USB supply may be getting grounded through the computer and "back around" to the H-bridge.... somehow??? ...Then, you disconnect the USB, and you loose the ground connection???

Do you have a multimeter? Is Arduino running OK with the 6.5V supply? Is there a common ground between the H-bridge and the Arduino? If you've got a multimeter, check the resistance between all of the grounds (with the "fail' condition, with the 6..5V connected, the USB disconnected, and the power off.)

FYI - A ground loop can cause noise in audio or data-communications applications and it might even cause enough noise to "crash" your Arduino. But, it won't cause you to loose power.

Grab a multimeter and measure voltages everywhere, between every point of the circuit…

ie. Don’t just measure Arduino voltage relative to the Arduino GND pin, measure it relative to everything else as well.

6.5 V supplying a 7805? Doesn't the 7805 require input at least 2 V over the nominal output?

Ok guys, I will measure everything!

Just for the record, the 7805 is fed by the +12V (I had made a mistake but corrected it straight away).

How are you powering the arduino ? If its th jack socket you really need 7 v or more.

Is your psu adjustable ?

If 6.5 v is needed for the motor there is some voltage drop through the bridge. Increasing to 7.5 to 8 v should take care of both problems.

Edit Im only looking for 200 to 300 ma.

Check the current on the 5 V regulator. Backlights can have a substantial current consumption depending on type. If chinese i would not entirley trust the spec sheet.

Simple test , does the 5 V reg get hot.

Hi, we need a proper full circuit diagram, you must have one to get this far, if not then STOP and draw one. A a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf? The block diagram you have posted tells us nothing about your project is connected.

I agree with Boardburner2, check the voltages around the 7805 and if it is getting to warm, it sounds like current limit due to thermal shutdown.

Tom....... :)

And measure the current drawn by the arduino.

It should really have its own supply seperate from the motor.
If the current is low enough you may be able to power direct from the battery.

Is it a 12 v battery ?
If its lead acid the terminal voltage could be 14.6.