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Topic: Arduino Mega: Issue with Heat Dissipation of Onboard 5V Linear Voltage Regulator (Read 593 times) previous topic - next topic

ArianKS

Hello all,

Thank you for reading this.

Background
To keep things short and relevant, my current project (with Arduino Mega 2560 Rev3) involves a number of periphery (most use 5V input) that require a total of about 900 mA to function. Separate of those periphery, I also have two 12v solenoids that are also part of the project (controlled via relays).


Goal
To use the output of a 12v power adapter to power the Arduino board, the 5V periphery, and the solenoids.


Issue
I originally wanted to use the 12v DC jack of the Mega to power it up. I've since learned that doing so may not be wise since the onboard 5v linear voltage regulator has very poor heat dissipation.


Potential Solution
I was thinking about using a DC-DC 12-5v converter module that outputs a USB connection (like one of these). This way I could still power the Mega via its USB port, and all components of the project share a common ground.

What do you guys think? Do you happen to have a better solution than this?

Please let me know if I need to provide anything else. I've attached PDFs of my schematic (EAGLE) in case it would be useful.

Paul__B

A very real danger is that the obsolete tutorials on the Arduino site and others misleadingly imply that the largely ornamental "barrel jack" and "Vin" connections to the on-board regulator allow a usable source of 5 V power.  This is absolutely not the case.  It is essentially only for demonstration use of the bare board back in the very beginning of the Arduino project when "9V" transformer-rectifier-capacitor power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes.  And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.

If you are asking this question, it is highly likely that you will wish to connect something else.  In which case, the answer is regulated 5 V.

This is because the on-board regulator is essentially capable of powering only the microcontroller itself and no more than a couple of indicator LEDs.  The on-board regulator might be able to power a few other things if it had a heatsink, but on the (older) Arduinos, it does not.

Powering via the "barrel jack" or "Vin" connections is asking for trouble.  The "5V" pin is not by any means an output pin, if anything a "reference" pin but most certainly the preferred pin to which to supply a regulated 5 V.

ArianKS

A very real danger is that the obsolete tutorials on the Arduino site and others misleadingly imply that the largely ornamental "barrel jack" and "Vin" connections to the on-board regulator allow a usable source of 5 V power.  This is absolutely not the case.  It is essentially only for demonstration use of the bare board back in the very beginning of the Arduino project when "9V" transformer-rectifier-capacitor power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes.  And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.

If you are asking this question, it is highly likely that you will wish to connect something else.  In which case, the answer is regulated 5 V.

This is because the on-board regulator is essentially capable of powering only the microcontroller itself and no more than a couple of indicator LEDs.  The on-board regulator might be able to power a few other things if it had a heatsink, but on the (older) Arduinos, it does not.

Powering via the "barrel jack" or "Vin" connections is asking for trouble.  The "5V" pin is not by any means an output pin, if anything a "reference" pin but most certainly the preferred pin to which to supply a regulated 5 V.

Thank you for your reply; I apologize if I did not clarify in the original post: I intend to use a DC-DC 12v to 5v converter that outputs USB to connect a 12v adapter to the USB input of the mega.

Paul__B

Thank you for your reply; I apologise if I did not clarify in the original post: I intend to use a DC-DC 12v to 5v converter that outputs USB to connect a 12v adapter to the USB input of the mega.
I saw that.  But still a very clumsy way to do it.  :smiley-roll:

The UNO and Mega use a 500 mA "polyswitch" to protect the USB input.  If you draw 900 mA, it will - after some time - shut the system down.  I suspect that is not what you want.  :smiley-eek:

The "5V" pin is the proper point to feed regulated 5 V.  The Arduino itself will not draw much power (it can't really - there are limits to how much each port can drive and the supply pins can take) so there will not be too much going into that pin and your 5 V supply will connect to that pin and all the other 5 V devices.

ArianKS

I saw that.  But still a very clumsy way to do it.  :smiley-roll:

The UNO and Mega use a 500 mA "polyswitch" to protect the USB input.  If you draw 900 mA, it will - after some time - shut the system down.  I suspect that is not what you want.  :smiley-eek:

The "5V" pin is the proper point to feed regulated 5 V.  The Arduino itself will not draw much power (it can't really - there are limits to how much each port can drive and the supply pins can take) so there will not be too much going into that pin and your 5 V supply will connect to that pin and all the other 5 V devices.
Thank you for your response, and again sorry I did not clarify; the situation is that I already have a 12v-5v linear voltage regulator (with a massive heatsink) that will feed most of the 5v peripherals. So my setup would have the following: 12v power adapter powering the solenoids, the 12v power adapter feeding the 5v linear voltage regulator powering the 5v periphery, and the 12v power adapter feeding the DC-DC 12v-5v step down with USB out that would power the actual Arduino (via USB).

On a different note, if I feed the Mega regulate 5V at the '5V' pin, how much current can I pull from each of the available 5V pins?

Paul__B

On a different note, if I feed the Mega regulate 5V at the '5V' pin, how much current can I pull from each of the available 5V pins?
Curious question!

Do you refer to distributing power from one 5 V pin on the Mega to another?  Presumably in order to group wiring together.  Not critical, a couple of hundred mA would be no great problem.  You may need decoupling capacitors across 5 V and ground of sensors.

ArianKS

Curious question!

Do you refer to distributing power from one 5 V pin on the Mega to another?  Presumably in order to group wiring together.  Not critical, a couple of hundred mA would be no great problem.  You may need decoupling capacitors across 5 V and ground of sensors.
Got it, thank you.

Do you happen to think that, if I provide my schematics and board design here, you'd by chance be able to take a look at them?

Thank you again.

Wawa

How do you power the EMR1/2 relay coils.
Not straight from Mega pins I hope.
Leo..

ArianKS

How do you power the EMR1/2 relay coils.
Not straight from Mega pins I hope.
Leo..
The relay coils are rated for 3V. I've attached a flyback diode in the correct orientation between the GND and Signal of the relays. The relays are then directly powered up (though in very short bursts) by the Arduino.

Wawa

The relay coils are rated for 3V.
Yes, but what is the current draw of the coils.
Absolute max pin current of a Mega is 40mA, and should normally not be higher than 20mA.
Common 3volt relays draw 100-200mA.

If you have 12volt available, then use 12volt relays.
With a 2N2222 or similar transistor (with 1k base resistor) driving the coils.
Leo..

ArianKS

Yes, but what is the current draw of the coils.
Absolute max pin current of a Mega is 40mA, and should normally not be higher than 20mA.
Common 3volt relays draw 100-200mA.

If you have 12volt available, then use 12volt relays.
With a 2N2222 or similar transistor (with 1k base resistor) driving the coils.
Leo..
Thank you so much for that idea; I will implement it and get back to you.

Paul__B

The point is that by definition, 5 V relays (of the same size) use 2½ times as much current as a 12 V relay, so you need the transistor even more to switch 5 V relays from the Arduino.  :smiley-roll:

TomGeorge

Please let me know if I need to provide anything else. I've attached PDFs of my schematic (EAGLE) in case it would be useful.
Please attach, jpg export images of your schematics, it makes them easy to insert into your post, pdf's are difficult.
Please read the post at the start of any forum , entitled "How to use this Forum".
OR
http://forum.arduino.cc/index.php/topic,148850.0.html.
Thanks..Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

CrossRoads

The 12V to 5VUSB is a great solution. The particular one you linked to is rated for 3A, plenty for a Mega with 2.5A to spare for other loads.

Use a ULN2803 or similar to sink current for the 12V relay coils.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

ArianKS

The 12V to 5VUSB is a great solution. The particular one you linked to is rated for 3A, plenty for a Mega with 2.5A to spare for other loads.

Use a ULN2803 or similar to sink current for the 12V relay coils.
Got it, thank you! Since I only have 2 lower power (140mW) 12V relays, at least based on my calculations it seems like I could get away with using two 2N2222As.  What do you think?

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