Why doesnt anybody use the DC jack on Arduino?

Hi,

I am sorry if this is a dumb question but I want to built two projects with Arduino - this one and this one. My question is: why do they always use sort of external power (eg batteries) and just the Arduinos main DC plug?

Thank you for any answers on the matter.

Because the DC jack goes to a linear regulator, and linear regulators are inefficient?

The first link is to a youtube video; I don't watch video, so I can't comment on that.

Second one is meant to run with the Arduino powered from USB, as far as I can tell. They use a battery - but the battery is being used to provide a positive and negative supply for the opamp, and isn't powering the Arduino. Also, wtf, their "clipping indicator" shows an LED stuffed between two pins, with no resistor in series...

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I recently ran across a situation. My ethernet shield won't work when the power jack on the Mega is used. It only works with USB power. It probably draws too much current for the regulator. Also, it's often easier to find a USB adapter and cord in the stores, than a 9-12V adapter and barrel plug.

Because the DC jack is the tallest component on the board and I usually desolder it on every Arduino I have, so that the stack is smaller.

It also has a reverse-voltage protection diode, which wastes a little more power. (Only significant in battery-operated systems.)

Then I'm usually powering other 5V or 3.3V equipment which draws more current than the Arduino's regulator can provide. So I have a "good" regulated supply and run that to the 5V or 3.3V pin on the Arduino.

As mentioned, the onboard regulator is linear... and a bit weak. The onboard regulator needs to be de-rated due to the fact that it has a very insufficient heatsink so it can power the board and not a lot else. The USB port relies on the attached PC regulator and the 5 Volts fed from the USB plug... usually up to 500mA... which is actually a bit more than the onboard can provide unless you feed it from a 7.5V supply. If you feed the DC jack with 9-12 volts, it generates more heat due to the voltage drop that must happen. The heat generated means even less current can be provided at the 5V side of the onboard regulator.

This is why.

Often, if people have a good external supply of 5V... and more than just the main board needs to be powered, it is the preferred option.

Besides what's already been said about powering the 5V rail directly with a regulated 5V supply, (which I also do), many people do use the DC jack or Vin connection. It just depends on your application, and how much current it draws.
If the Arduino isn't drawing much current, there's nothing wrong with a 9VDC supply connected to the DC jack, or even 12VDC if the current consumption is really low. It's just not as efficient as using an external switching DC-DC converter.

So in summary, because the "barrel jack" feeds the on-board 5 V regulator which cannot supply much current, you cannot use it for much - you cannot drive motors, you cannot drive relays, you cannot drive more than a few indicator LEDs and you cannot even use it to power many of the "shields", so to do any of those things, you have to use another power supply for those things which then makes it inconvenient to use the jack.

Because a USB 5V supply is easier to find than one with a DC jack I suppose... With the motor example
the motor driver needs a high current supply and probably more than 5V anyway. You'd normally no
risk feeding a motor supply into the Arduino (even on dc jack or Vin) due to the drop-outs and noise
spikes that occur on motor supply rails.