9V Vin powered Mega - 3V instead of 5V

I'm experiencing an issue with an Arduino Mega 2560 : I powered the board through the Vin using a +24 --> 9V voltage regulator. When I power the board and measure the 5V output from the board, the voltage quickly drops from 5 to ~3V within seconds.

However, while powering with USB, the 5V output from the board gives us a clear 4.8V.

I have checked the 9V Voltage regulator (connected to the Vin), and it has a +24V input which remains stable (as should the 9V).

Any thoughts ?

• How much current are you drawing from the regulator? In other words, what is connected to the regulator (if any) other than the Mega? Also, what is powered by the Mega? Sep 12, 2016 at 11:54
• Much more essential question is what `24V -> 9V` voltage regulator. Even 100mA means 1.5Watts turned into the heat on it if it's simple 7809 linear voltage regulator.
– KIIV
Sep 12, 2016 at 12:56
• And what are the specs, esp. max output current, of the 24v - 9v regulator? Sep 12, 2016 at 12:57
• Roberto Lo Giacco > The Arduino board is the only component connected to the 9V regulator. I measured only 2mA from the +9V to the Vin. The mega powers two Leds, 1 TFT (1,8"), and 1 SD card reader. Sep 12, 2016 at 13:05
• Power consumption about 2mA is highly unlikely as ATMega2560's consumption should be about 1mA per 1MHz so at least 16mA and add another for voltage regulator, LCD, LEDs...
– KIIV
Sep 12, 2016 at 14:24

So, according to the comments the main problem should be a thermal protection in `LM7809C` regulator.

The voltage drop between regulator's input and output is about `15 Volts` and even with current consumption about `150mA` it means `2.25W` of power dissipation on it.

For such a big difference between input and output voltage switching regulator is recommended. (Or 9V power supply instead of 24V)

And it is not possible to have `2mA` current consumption. I've just measured power consumption of bare `Arduino Mega 2560` and it is `81mA` through barrel jack input (`9V`) without anything on it. Not even blinking Led. So if you've measured for example `0.2A` it's not `2mA` but `200mA`

• I cannot change the power supply as it is powering another device (24V). I will have a look at the switching regulators, ty. However, how can the device not work with ~1% of its maximal current ? Could this be an issue related to the capacitors ? The manual I used gives a circuit at the page 18 : docs-europe.electrocomponents.com/webdocs/0e1f/… Sep 12, 2016 at 14:52
• Use DC-DC converter then. There are loads of them at ebay or so. And it's not possible to have only 2mA consuption not even with bare arduino board.
– KIIV
Sep 12, 2016 at 14:56
• Thank you KIIV. It seems like the voltage regulator was the cause of this issue, since providing it a +12V (instead of +24V) has given us a good and steady +9V output (connected to the Arduino Vin). I think this was an overheating problem. We've bought another DC-DC regulator as adviced to prevent this issue. Sep 22, 2016 at 14:15
• @EtienneP. Well, then you can mark it as a solution.
– KIIV
Sep 22, 2016 at 14:22
• "how can the device not work with ~1% of its maximal current" because your measurement is wrong, and because even with a correct current measurement, the limit being hit is the power one, where the huge voltage drop you are asking it to regulate across is a multiplier. Oct 12, 2016 at 19:28