Supplementing with 5v power supply

I have a Mega powered by a 12V power supply. I am needed this Arduino to power 3 load cells, 3 LCD screens, and 3 LED's. I realize I need an external power supply to carry some of the weight. If I connect the 5v+ of my external directly to my 5v pin (which is already connected to each of the LCD screens and LED's) and ground to ground, would that be fine, or would there be any weird consequences of having essentially two 5v supplies that merge?

Isn’t it possible to power your entire project from the external power supply? In that case I would recommend to connect the +5v directly to the VIN pin of your Aduino. Beware, what ever option you choose, you can never draw more then 40mA from each output pin from your Arduino, maybe even less on the Mega, look it up in the datasheets. I can’t remember the amount from the top of my head.

I hope this helps you,

Mike

If you're right, that would be great, but I feel like I remember reading that the VIN required a minimum input voltage of 7 volts. Maybe even a minimum of 9? So I'll need two separate power supplies anyway, unless I connect my 5V power supply to a USB jack...which may actually be what I end up doing, come to think of it.

Mikeb1970:
I would recommend to connect the +5v directly to the VIN pin of your Aduino.

NO.

V-in is the input of the onboard 5volt regulator. It needs >=6.6volt to make a stable 5volt for the board.

If you have a reliable 5volt source, then you can connect that directly to the 5volt pin.

The USB socket can also be used to power the Mega (with 5volt).
A bit safer, because of an onboard 500mA polyfuse.
Leo..

I have done some research, and your right. The best option for you to go is to power it using the USB port, the Vin needs between 7 to 12 volts. I also found that instead of the 40mA per digital I/O pin I mentioned this is restricted to 20mA. Good thing I researched this now, because I ordered a Mega and it should arrive soon.
Here is the source of this info.

Arduino Mega 2560

I hope this helps, sorry if I confused you earlier,

Mike

So my 5V power supply is reading at a steady 5.21 volts. Is this too much variance for me to connect this to the 5V pin? Or, say I cut off the male end of a USB cable and connect my power supply to that. Would this work or could it introduce problems?

The USB socket can also be used to power the Mega (with 5volt).
A bit safer, because of an onboard 500mA polyfuse.

Leo, This would limit the current available even further? I'm just considering the digital I/O pins now, and there are 54 of them, if they would only have 500mA to work with then your limited pretty fast.

Is it possible to have a 7-12volt supply available? I think it would be more efficient if you used that on your Vin. Otherwise you will "trip" this polyfuse all the time.

Mike

Bapstack:
So my 5V power supply is reading at a steady 5.21 volts. Is this too much variance for me to connect this to the 5V pin?

Or, say I cut off the male end of a USB cable and connect my power supply to that. Would this work or could it introduce problems?

5.21volt is within the +/- 5% tolerance of a 5volt regulator (e.g. the one on the Mega).
5.5volt would still be fine.

Only problem I see is that you connect it with the wrong polarity. Triple-check.
Leo..

Mike, regardless of whether I connected the power supply to the 5v pin or run it through the USB socket, I will have branched the 5v off before the Arduino in order to power my various external lights/modules. The Arduino itself won't be powering these.

The whole reason I'm doing this is that space is at a minimum, and if I could avoid running a second power supply and just use a single 5v supply, that's what I want to do.

And sounds good, Leo. But looking at whether I connect my supply via the USB cable or 5V cable, is one particularly better than the other?

Mikeb1970:
This would limit the current available even further?...

A Mega can't supply more than 150mA when on 12volt power (DC socket/V-in) before overheating and shutting down.
That is increased to 500mA when powered directly with 5volt.
That might be enough for the sensors/LEDs OP is using (not posted).
Leo..

Bapstack:
But looking at whether I connect my supply via the USB cable or 5V cable, is one particularly better than the other?

Not really. Just the extra 500mA fuse (and a mosfet switch) in the USB power line.
Leo…

Wawa:
Not really. Just the extra 500mA fuse (and a mosfet switch) in the USB power line.

Okay, thanks.

Wawa:
That might be enough for the sensors/LEDs OP is using (not posted).

Possibly, but I'd rather play it safe and power those directly. 3 LCD screens each with a back light can apparently pull up to 160mA apiece, plus 3 LED's at ~20mA, and whatever amount my 3 load cells take (I'm assuming not much) is taking me right up to the 500mA max.

Wawa:
A Mega can't supply more than 150mA when on 12volt power (DC socket/V-in) before overheating and shutting down.

I must have really been playing with fire before, because I was (stupidly) powering all the aforementioned lights/modules with the 5v pin while using just the 12v power supply. No shutting down, but the back lights of my LCD screens were not as bright as I'd like and my load cells were giving me funny, inconsistent readings over time.

On an unrelated note, when does the "1 post per 5 minutes" rule go away?

To be honest I kind of regret now that I bought that Mega, if I wanna use it for what I had in mind I'm gonna need a shitload of transistors and/or mosfets. :cry:

Leo, when I quoted you, I never wanted to criticize you, I just wanted to mention the limitation of the 500mA and learn something in this forum.

And about that 5 min posting rule, I would like to see that go too. Especially when you want to edit your own post, for example because you made a very stupid typo, I hate waiting for 5 mins.

Yours,

Mike

Mikeb1970:
To be honest I kind of regret now that I bought that Mega, if I wanna use it for what I had in mind I'm gonna need a shitload of transistors and/or mosfets. :cry:

Leo, when I quoted you, I never wanted to criticize you, I just wanted to mention the limitation of the 500mA and learn something in this forum.

And about that 5 min posting rule, I would like to see that go too. Especially when you want to edit your own post, for example because you made a very stupid typo, I hate waiting for 5 mins.

A Mega is great to have in the parts box. What project needs a shipload of transistors.

Haha, not a problem.

You have just passed the 100-post limitation.
leo..

Wawa:
You have just passed the 100-post limitation.

Wait, really? The way to prove that I'm not posting too much is to post more?

There are some posting restrictions in place to slow down spammers, and newbies who haven't read the forum rules yet. A small price to pay for a better/cleaner forum.
Leo..

If you're gonna need a load of transistors or MOSFETs, I have boards with 32 MOSFETs for sinking an amp of current, and another with 12 TPIC6B595 for 96 outputs that can sink 150mA each or 250mA each if TPIC6595 are used.
http://www.crossroadsfencing.com/BobuinoRev17/

Tyvm, I will def look at that link, and most probably order some parts. It will be tomorrow though, since I am installing an SSD in my laptop now.

Yours,

Mike