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### Topic: 12V but what Ampere? (Read 990 times)previous topic - next topic

#### tsunaliew

##### Nov 02, 2019, 08:36 pm
Hi guys, i am not sure if my plan will work so i need people to correct me if i am wrong. My plan is to get a 12V 3 pin adapter plug for the whole circuit, everything here is powered by Arduino. 16x2 LCD with backlight on (120-160mA), 3-24V piezo buzzer (20mA), 5V relay module (7-10mA), 4x4 Matrix Membrane Keypad (30mA), total 220mA.

From what I researched, Arduino can hold up to 7-12V (Total maximum current draw when powered via external power supply: 1A) and I have a 12V solenoid lock around (1.2A). I am planning to connect it in parallel circuit so the voltage will be the same and sum of the currents through each path is equal to the total current that flows from the source.

So here comes the question, if I were to get a adapter of 12V what should my current supply be? If I calculate Arduino max current 1A + Solenoid lock 1.2A, i will have to get around 2A or 2.5A supply. But if I only calculate the Ampere I need, I only need to get 1.5A supply.

#### groundFungus

#1
##### Nov 02, 2019, 09:13 pmLast Edit: Nov 02, 2019, 09:13 pm by groundFungus
It is always good to have more current capacity.  The circuit will use what it needs.  You could use a 100000A supply and as long at the output is regulated to 5V (or whatever your mystery Arduino is) it will work fine.
You will save everyone's time if you read and follow the forum guidelines.
https://forum.arduino.cc/index.php/topic,148850.0.html
and
https://forum.arduino.cc/index.php?topic=97455.0

#### tsunaliew

#2
##### Nov 02, 2019, 10:00 pm
It is always good to have more current capacity.  The circuit will use what it needs.  You could use a 100000A supply and as long at the output is regulated to 5V (or whatever your mystery Arduino is) it will work fine.
I am sorry if i didn't make it clear on my first post, the board is arduino uno.

And by 5V, do you mean the output pins of the arduino?

#### raschemmel

#3
##### Nov 02, 2019, 10:09 pmLast Edit: Nov 02, 2019, 10:11 pm by raschemmel
Probably not a good idea to drive a solenoid
off the same unregulated supply that feeds
the arduino 5V regulator but you can try it.
And the Arduino current max is 800mA @ 5V,
which, if you know your Ohms Law is 4W, which
you should know is only 333mA @ 12V.
(do the math: P= I*V)
NOT 2.5A@12V.

Groundfungas knows that but he was probably distracted or had something else on his mind.

#### tsunaliew

#4
##### Nov 02, 2019, 10:35 pm
Probably not a good idea to drive a solenoid
off the same unregulated supply that feeds
the arduino 5V regulator but you can try it.
And the Arduino current max is 800mA @ 5V,
which, if you know your Ohms Law is 4W, which
you should know is only 333mA @ 12V.
(do the math: P= I*V)
NOT 2.5A@12V.

Groundfungas knows that but he was probably distracted or had something else on his mind.

The solenoid is connected to the common and 12V is connected to normally open on the relay module so I do not think any reverse voltage will be produced.

If the current is around 1.533A, I assume that 1.5A is good enough?

And what will happen to my arduino if my solenoid lock is not active or in use? Will the 1.5A damage the arduino board and the rest of the components?

#### raschemmel

#5
##### Nov 02, 2019, 10:39 pmLast Edit: Nov 02, 2019, 10:50 pm by raschemmel
You really need to stop and learn Ohm's Law.
Then you wouldn't ask such questions.

Even with all you extras I doubt you are anywhere close to your 800mA @5V maximum so yes,
1.5A would be fine.
And as Groundfungas ALREADY told you, the
arduino uses shat it needs so why would you
ask if the 1.2A solenoid would damage the arduino
if the solenoid was not in use.
Seriously, if your going to use electronics, at
least learn the basics of Source and Load and Ohms Law (P=I*V/V=I*R).

#### Wawa

#6
##### Nov 02, 2019, 11:10 pm
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..

#### raschemmel

#7
##### Nov 02, 2019, 11:47 pmLast Edit: Nov 03, 2019, 01:01 am by raschemmel
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 .

#### tsunaliew

#8
##### Nov 03, 2019, 12:09 am
You really need to stop and learn Ohm's Law.
Then you wouldn't ask such questions.

Even with all you extras I doubt you are anywhere close to your 800mA @5V maximum so yes,
1.5A would be fine.
And as Groundfungas ALREADY told you, the
arduino uses shat it needs so why would you
ask if the 1.2A solenoid would damage the arduino
if the solenoid was not in use.
Seriously, if your going to use electronics, at
least learn the basics of Source and Load and Ohms Law (P=I*V/V=I*R).

I am sorry if my questions sound stupid to you but i do not want to take any chances that will over current my circuit. But still I am very thankful that you answer my questions.

The 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 .
And yes, that's what I want to do.

#### raschemmel

#9
##### Nov 03, 2019, 01:00 amLast Edit: Nov 03, 2019, 01:04 am by raschemmel
You said:
"If the current is around 1.533A, I assume that 1.5A is good enough?"

That's a BIG 'if' because for that to happen,
the arduino 5V regulator would have to be
drawing 800mA. In the 7 or 8 years since I
started using arduinos, I can't recall ever
reading a post where someone claimed to
be drawing 800mA @5V (333.333mA @12V).
In all likelyhood, your arduino would probably
be under 100mA @12V (1.2W), which is 240 mA @5V. With nothing connected to the 5 Vcc pins,
185mA for things like LCDs and whatnot.
Can you list enough 5V addons to draw 185mA ?
As much as it sounds like it's still only a 1/3rd
of what it could support.

#### Wawa

#10
##### Nov 03, 2019, 04:04 am
Can you list enough 5V addons to draw 185mA ?
As much as it sounds like it's still only a 1/3rd of what it could support.
But it can't.

An Uno with 12volt on the DC socket (or V-in) will eventually go into thermal shutdown with more than ~150mA total drawn from any pin.

A 7volt drop, with 0.8Amp, is 5.6watt dissipation inside the tiny 5volt regulator.
Leo..

#### Wawa

#11
##### Nov 03, 2019, 04:14 amLast Edit: Nov 03, 2019, 04:17 am by Wawa
You should be just fine drawing that extra current for a relay for a second or so to activate that lock.
Total constant current draw for the Uno/LCD/keypad seems within the limit.
Just use a regulated 12volt supply that can provide a bit more than the solenoid current.
Leo..

#### raschemmel

#12
##### Nov 03, 2019, 04:21 amLast Edit: Nov 03, 2019, 05:14 am by raschemmel
Nobody's talking about exeeding pin limits.
Nowhere in this thread is that even remotely suggested. Everything in this post is about

You're 100% right about the 5.6W heat dissipation
from the 7V drop though (7V * 0.8A).

@OP,
If you insert a (12V-7V)/I_load= 5V/0.5A=10 ohms
dropping resistor rated for I*V= 0.5A*5V= 2.5W , (round up to 3W) , between the 12V p.s. and the external dc barreljack input , thus reducing it to 7V ,then you can use the the 12V p.s. that you plan to use for the solenoid. Without the dropping resistor
the 5V regulator will go into thermal shutdown
if you draw more than 266mA ? (1/3 of 0.8A).
With the given values , the resistor would increase
Without it, whether or not the regulator shuts down
will depend on how much you load it down.

@Wawa,
The isn't powering a 12V solenoid lock , which has no impact on the arduino load.

#### Wawa

#13
##### Nov 03, 2019, 04:37 am
Nobody's talking about exeeding pin limits.
Correct.

But don't forget that all current on external power flows through the 5volt regulator.
Leo..

#### wvmarle

#14
##### Nov 03, 2019, 05:09 am
From what I researched, Arduino can hold up to 7-12V (Total maximum current draw when powered via external power supply: 1A) and I have a 12V solenoid lock around (1.2A). I am planning to connect it in parallel circuit so the voltage will be the same and sum of the currents through each path is equal to the total current that flows from the source.
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.

Quote
So here comes the question, if I were to get a adapter of 12V what should my current supply be? If I calculate Arduino max current 1A + Solenoid lock 1.2A, i will have to get around 2A or 2.5A supply. But if I only calculate the Ampere I need, I only need to get 1.5A supply.
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.
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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