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### Topic: 12V but what Ampere? (Read 987 times)previous topic - next topic

#### raschemmel

#15
##### Nov 03, 2019, 05:24 amLast Edit: Nov 03, 2019, 05:32 am by raschemmel
I dissagree because the OP's 5V load is only 100 mA @12V. (P=I*V)(The devices he plans to power wth the 5V Vcc pin do not exceed 266mA@5V (100mA@12V)
I also disagree with your statement about using the dc barreljack unless as Wawa pointed out, he
overloads the 5V, (which he isn't , with the devices he has listed.

#### wvmarle

#16
##### Nov 03, 2019, 05:54 am
I dissagree because the OP's 5V load is only 100 mA @12V. (P=I*V)(The devices he plans to power wth the 5V Vcc pin do not exceed 266mA@5V (100mA@12V)
Those numbers imply a buck converter...

266 mA at 5A is 266 mA at 12V if using a linear regulator, and it'd mean a heat dissipation of   7 V * 266 mA = almost 2W. That's way too much for that regulator, which for lack of any external heatsink would struggle with dissipating a sustained 400-500 mW (that's what it gets to endure just powering the Arduino itself at 12V, without any external loads).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

#### tsunaliew

#17
##### Nov 03, 2019, 06:43 am
Don't power the Arduino with anything >5V for a long term project. That RAW input is for prototyping/testing only, and even then not the greatest of ideas.

So what you need is a 2-3A 12V supply (some derating here never hurts, a 4-5A supply will do just fine as well), plus a 12V-5V converter for your 5V parts including the Arduino, a buck converter will do great here. Of course you power the Arduino over its 5V pin (that's what it really is for), and the rest of the 5V stuff directly to your 5V supply.

Remember to connect grounds. Consider using a MOSFET instead of a relay to switch that solenoid.
Yes, thank you for your suggestion, I will use DC-DC Step Down Buck Converter just for Arduino.

If I choose not to use mosfet but stay with the relay, will this work?

#### raschemmel

#18
##### Nov 03, 2019, 06:59 am
That depends on the relay current being < the transistor continuous max current rating.

#### krupski

#19
##### Nov 03, 2019, 08:42 amLast Edit: Nov 03, 2019, 08:48 am by krupski
The OP isn't planning to overload the 5V.
All he wants to do is power his solenoid
and the uno and few other things from a single
12V power supply .
Yes, but by using the tiny 5V regulator on the UNO which will simply cause it to get hot enough to unsolder jtself.

The OP should just use a 5V (well regulated) supply and power all of it directly (and use a relay with a 5V coil).
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### krupski

#20
##### Nov 03, 2019, 08:45 amLast Edit: Nov 03, 2019, 08:45 am by krupski
An Uno will eventually overheat/reboot if you draw more than 150mA total from any pin.
Make sure your supply is regulated. Unregulated ones could have >17volt unloaded, which will fry the Uno.
12volt is already borderline for an Uno/Mega, and smaller boards will eventually release the magic smoke at that volatge.

Common 5volt relays draw about 75mA coil current, plus 2mA I/O pin current.
A membrane keypad runs on fumes, unless it has got some LED illumination inside.
160mA sounds a lot for an LCD (backlight). The LCD itself (no backlight) could draw 20mA (educated guess).

Don't forget the back-emf diode across the solenoid, even if you use a relay to switch the coil.
Otherwise you will be rewarded with random resets of the Uno.
Leo..
Haha!!! That made me LOL!
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### Willem43

#21
##### Nov 03, 2019, 11:45 amLast Edit: Nov 03, 2019, 11:46 am by Willem43
Just did a quick check.  I have an ATtiny85 and some photo transistors on a breadboard, plus a LCD 20x4 (connected via I2C), with backlight, all connected to my ISP programmer (using a 32u4 MCU - also supplying 5V to the rest).

I have one of these USB inline units measuring USB voltage and current.  Current is displayed in amps (for some strange reason) with only 2 decimal places - so low current is a bit of a guess.

All connected reads 0.07A.  If I unplug the LCD it goes to 0.02A, so the LCD draws less than 60mA total.

Hope that helps.

Willem

#### wvmarle

#22
##### Nov 03, 2019, 12:54 pm
Try with just switching on and off the LCD backlight. You probably also see that 0.05A difference. Or at least it should be about 0.04A. The display itself needs only very little current.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

#### tsunaliew

#23
##### Nov 03, 2019, 06:14 pm
Thank you so much guys, for your endless advises, time and patience for guiding me, this is my first time creating a project on my own so I did not know how to use power rating as the main calculation, but now I know.

So I have decided to choose the 12V 1.5A (18W) supply connected to 12V 1.2A solenoid and a DC-DC Step Down Buck Converter, stepping down from 12V to 7V, that will cause the current to increase from 1.5A to 2.5A for the Arduino but since there is no maximum current rating for the power supply connected to Arduino, the Arduino will draw as much current as it needs (and the voltage regulator can cope with). So there should not be any problem with the current.

And the reason why I choose 7V is because the recommended range is 7 to 12 volts. The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. Additional reason is for preserving the Arduino board as a long term project.

Additionally, I dropped the idea of adding the transistor on the solenoid, even without the transistor, the flyback diode will protect the relay. From what I learnt, diode does not prevent a voltage spike from travelling to some downstream load but, it provides a path with low resistance that reroutes the current, thus the voltage spike at the downstream load to be much lower. Using a simple 1N4007 diode is sufficient to suppress large voltage spikes in a 12VDC relay with a diode protection circuit.

#### wvmarle

#24
##### Nov 03, 2019, 06:45 pm
That buck converter will not create 2.5A just like that. Many are not even capable of such output currents. That you even write it like this tells me you still don't understand how current works.

Also you could of course simply have the buck converter produce 5V and not need the on board regulator at all...

A 1.5A supply is anyway pretty minimal for a 1.2A solenoid plus the rest.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

#### tsunaliew

#25
##### Nov 03, 2019, 07:06 pm
That buck converter will not create 2.5A just like that. Many are not even capable of such output currents. That you even write it like this tells me you still don't understand how current works.

Also you could of course simply have the buck converter produce 5V and not need the on board regulator at all...
True, DC-DC Step Down Buck Converter Output Current: Rated current 2A, maximum 3A (Additional heat sink is required)

I calculated it using https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.html I am sorry that I left out such a important note. Thank you for pointing it out.

I am following the recommended voltage just in case since its a external power supply for AMS1117-5.0.

A 1.5A supply is anyway pretty minimal for a 1.2A solenoid plus the rest.
Alright, thanks.

#### Willem43

#26
##### Nov 03, 2019, 10:05 pm
Try with just switching on and off the LCD backlight. You probably also see that 0.05A difference. Or at least it should be about 0.04A. The display itself needs only very little current.
Spot on.  Tried it without backlight - back to 0.02A.  The resolution of that inline unit is not even seeing the difference with or without the LCD with the backlight off.

Problem is, with the backlight off, it is not really possible to read the LCD in normal room light.

Willem

#### SteveMann

#27
##### Nov 03, 2019, 10:11 pm
Don't power the Arduino with anything >5V for a long term project
Define "long term".  I have a UNO project with 12V in to the barrel connector that's been running for the better part of a year.
I am usually so far out of the box that most people don't know what I am talking about.

Please do not ask for help by PM. I will not respond.
If you need help, post a question on the appropriate forum.
Click on Add Karma if I helped you.

#### raschemmel

#28
##### Nov 03, 2019, 11:58 pm
As already explained,the buck converter output @5V will draw < 266mA @5V , which is only 100mA @12V.

Either you are not paying attention or you have not
done  the math:
P= I*V
(Ohms Law)
Continued ignorance of this is going to get you
into trouble later.
All this talk about the buckconverter supprting 2A is
pointless because it is only going to draw 100mA @12V. (266mA @5V).

How many times do we have to tell you this ?

#### wvmarle

#29
##### Nov 04, 2019, 04:05 am
Define "long term".  I have a UNO project with 12V in to the barrel connector that's been running for the better part of a year.
That something works doesn't necessarily make it the best of ideas. As is using an Uno for anything long term; those DuPont connectors are prone to working their way out or losing contact due to corrosion.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

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