Mega 2560 + 78S05

Can I power the Mega board using the external power source from 78S05 (2A) directly to 5v pin with diode (1N4001) from 5V (a) to Vin (c)? Input voltage of 78S05 is 12V.

Thanks in advance.

You better use a DC-DC converter, and preferably a 5V power supply that is limited to about 500mA and connect that to the USB connector.

When you apply 5V to the 5V pin, current might flow into the computer.
When the computer is connected with the usb cable, and the computer is turned off, then current from the 5V pin flows into the computer. That might damage things. It is therefor dangerous, and you should not do it.

However, I apply 5.0V to my Mega 5V pin all the time, and yes, there is current flowing into my computer. My DC-DC converter gets hotter than normal when that happens, but so far that is the only thing and so far nothing is damaged.

I don't want to connect Mega with the computer via USB. If I want to upload code, I will not use that setup.

The question is about 78S05. In the datasheet, max. output voltage could be 5.2V, so I'm not sure how to provide exactly 5V.

Peter_n:
When you apply 5V to the 5V pin, current might flow into the computer.
When the computer is connected with the usb cable, and the computer is turned off, then current from the 5V pin flows into the computer. That might damage things. It is therefor dangerous, and you should not do it.

I think it does damage your pc... I had it hooked up like this. And with my computer shut off an led fan monitor and some status lights were still on...

A few days later my hard drive failed, and it was the logic board... Can't say for sure, but the hdd was a little over a year old... Lost some eagle files I neglated to back up....

Luckly posts my code often enough on the forums saved me from loosing too much progress...

Ouch! That is not good at all.
Thanks for sharing. I have to put in a Schottky diode.

What's wrong with feeding 12volt directly into the DC socket.
The only problem that could arrise is overheating of the onboard 5volt regulator is you draw a lot of current from the 5volt line.
Do you?
Leo..

Yes, I do. :slight_smile: That's the problem.

Can't you feed the Mega from 12volt.
And the OTHER things from the 7805.
How much extra current are we talking about.
Leo..

Ethernet shield, HC05, DS1302, DHT11, LCD, LEDs… The 5V voltage regulator is very hot (~70 °C) when I use 9V on dc jack.

Why I should not use 78S05 with 1n4001(from 5v pin to Vin) to power all things together?

Not a big problem if the regulator runs hot, as long as it does not shut down.
I would try two 1N4001 in series with the 9volt source, so you feed 7.6volt into the Mega.

Using an external 5volt source could introduce other problems.
Avoid if you can.
Leo..

Can you tell me what kind of problems?

Here's one.
The Mega has an external power source detector.
It switches off USB supply when external power is above 6.6volt on the Vin pin, or 7.3volt on the DC socket.
That circuit does not work anymore if you feed 5volt directly into the 5volt pin.
A computer, connected to the USB socket, and off, might or might not like backfeeding 5volt into it's USB socket.
Leo..

That's ok, but I don't want to connect Mega to the USB socket.

So you never want to upload any code.

Remember what Peter_n told you about using a DC/DC converter (less heat, no heatsink).
Have a look here.
Leo

insense, as I wrote, I do power the Mega with 5V to 5V pin. Because then I can use a single DC-DC converter for my whole project and use any DC wall wart as power source (7...24V).
I always knew that it was dangerous, and Qdeathstar wrote that his computer got damaged.
Wawa is right of course, you certainly are going to connect that USB cable some day, because the sketch needs to be altered and updated.

On the Arduino board is a polyfuse of 500mA in the USB 5V power line. That should prevent most trouble, but it is not known how any random computer will behave when 5V is powered to the USB.

The Ethernet Shield will be powered by the 5V of the Mega board.
If you use 7.5V...9V for the Mega power jack, and use the 78S05 for the LCD background and the LEDs, then you should be okay. The heat created by the 78S05 will be limited if you use the same 7.5V...9V as input.

The W5100 gets hot and requires a lot current, the newer W5500 might be better, but not yet supported here at Arduino.cc

I am going to add a Schottky diode to the Arduino board (scratch a copper trace, add a Schottky diode).

insense:
Ethernet shield, HC05, DS1302, DHT11, LCD, LEDs... The 5V voltage regulator is very hot (~70 °C) when I use 9V on dc jack.

Why I should not use 78S05 with 1n4001(from 5v pin to Vin) to power all things together?

in addition to what other people have already told you, the answer is also in the 1st part of your post !
If the 5V reg gets hot with 9V input, you can imagine that an external 7805 will get very hot with 12V input.
That means 7V through the 7805 !
Do the math using the current all these components will draw, and you'll know how much power the 7805 will dissipate! 1A gives 7W...... with 0.5A , it will be 3,5W . Fortunately, 7805s have an overheat protection, which put them off in case of overheating, but I guess you'd prefer your design to work flawlessly, wouldn't you ?