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Topic: Overpowering Mega1260? (Read 759 times) previous topic - next topic

_Roms_

Hello,

It's smelling like burnt plastic today...  :smiley-red:

I've connected a TFT + SD card + RTC clock to the Arduino mega, which is clearly over the top. It worked fine with TFT+SD or TFT+RTC but here's what I saw when everything was plugged:

- The USB controller wouldn't recognize the Arduino anymore
- When added the external 12V power in addition to the USB cable, I think I must have damaged something.

Yes, I should have used an external power supply to power my devices, but what just happened?


PaulS

Quote
It's smelling like burnt plastic today...  :smiley-red:
And this is a Programming issue how?
The art of getting good answers lies in asking good questions.

_Roms_

Right, let me move this out of here. Thanks,

Wawa

The DC socket on a Mega/Uno needs a minimum of 7.5volt for the regulator to make a stable 5volt and to switch off USB supply. Anything more than 7.5volt is burned off as heat.
12volt on the DC socket with the extra current draw of TFT and SD would have cooked the regulator.
That could have taken out other parts.
Time to replace the Mega.
Leo..





_Roms_

Hi Leo,
Argghhh. Thanks for the reply.

Nick_Pyner

#5
Sep 28, 2017, 03:36 am Last Edit: Sep 28, 2017, 03:43 am by Nick_Pyner
It's smelling like burnt plastic today...  :smiley-red:

I've connected a TFT + SD card + RTC clock to the Arduino mega, which is clearly over the top. It worked fine with TFT+SD or TFT+RTC
I submit that there is nothing clearly over the top, indeed your conclusion is utter nonsense. What you imply is that the RTC is the straw that broke the camel's back, and I reckon this has to be one hell of an RTC to do that. You indeed should use an external supply for motors etc., but the idea of using one for an TFT RTC etc. is absurd.

I suggest you should carefully check that the Mega is really dead, rather than smelling like it might be, before you go get a replacement. If it is dead, I think it is more likely that the "just what happened" is from some other abuse you haven't mentioned and, if you don't address that, you will probably cook the new one too.  A typical supply for Arduino is 9v, but 12v is within recommended specification. The upper limit is 20v.

I have used Mega+RTC+TFT+SD+Bluetooth+sensors for some years, without any suggestion that anything is "over the top". OK that is with a 9v supply, and yes, the regulators do get hot, but they always do, and I find it hard to believe that things are so marginal that a 12v supply would be fatal. If it was, I'm sure we would all know about it.

Adding external power as well as using the USB cable is normal practice, and you should not have damaged anything.  Something else is going on if your Arduino can't be recognised by USB - possibly an inadvertent disconnection.

You might find that Mega is OK, and your money is better spent on a 9v supply. A 12v supply should not be fatal, but is simply not for the faint-hearted.



Wawa

Voltage limit on the DC socket of some Mega clones is 16.7volt (16volt electrolytic caps + 0.7volt reverse protection diode). Current limit could be ~800mA, depending on which regulator is used.
Problem is that those two limits won't go together.

If we assume absolute max power (heat limit) of the Mega's regulator is 1.5watt (educated guess), then with a 12volt supply, max current draw is: 1.5watt / (12 - 0.7 - 5) = ~240mA.
The Mega itself uses ~70mA, so 170mA left for TFT and SD.

So a 170mA draw from the 5volt and/or 3.3volt pin on a 12volt supply will make the regulator stinking hot.
It should shut down, but did it.
Leo..
 

_Roms_

Hi guys - thanks Nick, I like to hear what you're saying.

I pulled everything apart and tested the components. It seems that only the Mega suffered from the event. I ordered another one (cheap $12 Elegoo).

I also got a more reasonable 9V DC supply. I realized I could power the SD card reader with 3V and this seems to be working well.

I think (tell me if you think it's stupid) my plan is to try to flash the Mega with the code first, then add the TFT and then run it with the DC power only. Comments are welcome! :)

Wawa

I realized I could power the SD card reader with 3V and this seems to be working well.
From the Mega's 3.3volt pin?
That pin has a current limit of 150mA (on a Mega/Uno).
Don't know what your card draws, but I have seen (peak) values of 200mA (never measured this myself).
It doesn't make any difference for the dissipation of the 5volt regulator, since the internal or an external 3.3volt regulator is powered from the same 5volt regulator of the Mega.
Leo..

_Roms_

I see. It looks like the SD card reader is taking up to roughly 100mA so it should be good.

Nick's comment seems to be pointing to the direction that something else is going on and I should be good having the TFT+RTC+SD card reader together. Or do you think I should search for additional power supply options?

Thanks!!!!

Wawa

Without posting links to the specs of the parts you're using, we can only guess.
Forget about the RTC. That one runs on fumes.
The TFT might be using a bit for the backlight.
There is a lot more headroom with a 9volt suppy though.
1.5 / (9 - 0.7 - 5) = ~450mA
I think you should be ok with a 9volt supply.
The 5volt regulator is the black block next to the DC socket.
Put your finger on it after 10 minutes.
It should be warm/hot, but not burn your finger.
Leo..

Nick_Pyner

#11
Sep 29, 2017, 02:15 am Last Edit: Sep 29, 2017, 02:18 am by Nick_Pyner
It looks like the SD card reader is taking up to roughly 100mA so it should be good.

Nick's comment seems to be pointing to the direction that something else is going on and I should be good having the TFT+RTC+SD card reader together.
My comment is indeed exactly that. Such an arrangement really is normal, and the idea of using external power just for them is absurd. I am still surprised that using 12v would be fatal. Even if there is nothing going on that you aren't talking about, maybe you were just unlucky.

However, I would be a bit suss about using a 3.3v SD card on a Mega, and I can't see why you would not use 5v like everybody else does. You have no good reason for using 3.3v, at best you are merely moving a problem rather than fixing it, it appears that you do have a choice of supply, and using 3.3v is clearly and unnecessarily asking for trouble. I think it is time to read the datasheet, which clearly states, twice, that the 3.3v pin is good for 50mA.

Wawa

which clearly states, twice, that the 3.3v pin is good for 50mA.
Which has been wrong for a long time.

This has been taken from Arduino's where the 3.3volt supply comes (came) from the FTDI chip.
And copied without thinking.

The Uno R3 and the Mega have a dedicated 3.3volt chip that can deliver 150mA.
Leo..


Nick_Pyner

#13
Sep 29, 2017, 04:54 am Last Edit: Sep 29, 2017, 04:59 am by Nick_Pyner
OK
I still don't think it fixes anything for OP, but I now feel a bit more comfortable myself with a little LCD on the 3.3v.

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