VMA412 turns off after app. 10 seconds

Hi Guys!
I am not that new to arduino, but it keeps up the challenge :slight_smile:

I have a setup with 4 Unos (three with motorshield as slaves, one with a display as the master) connected with i2c. So far everything works fine. Exept one - as i hope - minor thing:
As the i2c uses the pins A4/5 for communication, these pins (especially A4) cannot be used b the display. Usually the A4 ist used for the reset of the display. On the other hand, the pin connecting to the Uno's Reset-slot ist not used by the display. So i bridged the two pins on the display and it starts up as it should.
BUT...as long as i am connected to USB everything is fine. When i disconnect USB and use external 12V-Power it starts up as it should, but after about 10 seconds it starts kind of flickering just slightly and the display turns white. When i hit the restart-button on the uno the procedure repeats.
Any ideas?
Kind regards,

my setup:


I localized the issue. The voltageregulator is oveheating, when i run it on external power. Fixed a coolingunit on the regulator, but this just delays the problem. Looks like i need to add a powerful ventilation...
Or is there any chance to reduce powercomsumption of the display (VMA412)?

Oh, you thought the on-board regulator on the Arduino could be used to power it in addition to other modules? Oops! :astonished:

The "Vin" or "RAW" terminal is essentially a legacy part, some "clones" such as the "RoboRed" and more sophisticated Arduinos do incorporate an actually functional switchmode regulator but it should simply be ignored on the older designs. :grinning:

The obsolete tutorials on the Arduino site and others imply that the largely ornamental "barrel jack" and "Vin" connections to the on-board regulator imply that this is a usable source of 5 V power. This is absolutely not the case. It is essentially for demonstration use of the bare board back in the very beginning of the Arduino project when "9V" power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes. And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.

Nowadays, 5 V regulated switchmode packs are arguably the most readily available in the form of "Phone chargers" and switchmode "buck" regulators are cheap on eBay so these can be fed into the USB connector or 5 V pin to provide adequate power for all applications. Unfortunately, many tutorials or "instructables" are seriously outdated or misleading and have not been updated to reflect the contemporary situation.

Thanks for the input.
I was assuming, the external powersupply would be ok to power at least one "stacked" board, like my screen. So i have to establish a power-supply (5V) on the USB-Port of my Uno, correct?

Just use some common sense.

Add up your external currents and calculate power dissipation in the Arduino's 7805 regulator.
e.g. 9V battery will mean 9V - 5V = 4V.
200mA * 4V = 800mW will be very warm.

e.g. 12V battery will mean 12V - 5V = 7V
100mA * 7V = 700mW will be very warm.

Higher currents or higher input voltages will mean overheating or failure.
Wall-warts provide their rated voltage at their rated current. At lower currents the output voltage rises.

7805 style regulators dissipate all the power as heat.
AMS1117 style LDO regulators operate more efficiently by switching. Much less heat is involved.


AMS1117 style LDO regulators operate more efficiently by switching. Much less heat is involved.


I was assuming, the external power supply would be OK to power at least one "stacked" board, like my screen.

And in fact, it is simply not. :astonished:

So I have to establish a power-supply (5V) on the USB-Port of my UNO, correct?

That would be the easiest way. Almost all "phone chargers" are rated at least 1 A, but the polyswitch on the UNO is rated at 500 mA so that is the most you can draw for the Arduino itself plus from the "5V" pin. If something else required more, you would need to connect it separately to the 5 V supply.

Quote from: david_prentice on Jan 11, 2019, 10:03 am

AMS1117 style LDO regulators operate more efficiently by switching. Much less heat is involved.


Oops. I had never checked the AMS1117 datasheet. I had (erroneously) assumed that this was an integrated "Buck Converter". It seems to be a Linear Regulator but with "Low Drop out" compared to the 7805 style Linear Regulators.
From the AMS1117 datasheet:

PD = ( VIN - VOUT )( IOUT )

A LDO regulator has a smaller power dissipation due to the lower voltage drop. e.g. to drop from 5.0V down to 3.3V is only 1.7V. The Uno 3.3V pin is rated for 40mA. This means 68mW in the SMD package.
Other Arduino boards provide a 3.3V regulator in a bigger SMD package with adequate heatsinking for higher currents.


The AMS1117 - or some similar devices - is the "generic" popular regulator used on Arduino clones if not the originals, and all the little eBay modules. Generally speaking, the modern version of the 78xx.

Yes, the heat dissipated is directly proportional to the voltage drop across the regulator, so if it is dropping a small voltage, it will not dissipate much, but there is clearly no benefit to a "LDO" regulator at any given voltage drop and given the negligible heatsinking on the UNO, Nano and Pro Mini boards, they are essentially useless when used with a 12 V supply and barely useful for the processor itself and a few trivial devices such as indicator LEDs at 9 V. The AMS1117 would be capable according to the datasheet of dissipating as much as 2 W with a generous heatsink. That would be all of 500 mA with a 9 V supply.

I have no real idea what a VMA412 is and no reason to research it, but just popped in to explain that in any such situation, you need to find a proper power supply for the project.