Current Draw with two seperate power supplies

Projects General Guidance
1

So in this project I'm building, I plan to have around 30 LED's which is obviously too much to power off arduino's 500ma output. I also have many potentiometers, buttons and encoders connected to arduino's inputs
My idea is to have one 5V/3A power supply connected to all these LED's and also powering the pots and buttons.
I would then power arduino off the USB socket on the board.
In theory, the pots, buttons and LED's are all receiving power from the 5v/3a power supply, while arduino recives it own 5v to power itself, and hopefully the pins should not exceed the current draw from the USB power supply because they are only receiving voltage from the pots and buttons.

Can somebody give their opinion on this? It would be greatly appreciated.

2

i cant figure out how exactly you plan to connect the arduino and the leds on your project. but i shold work in general. i did so powering a motor with an extra battery and one battery for the arduino-board.
you have to pay attention, that you dont connect the two +5V together!

3

operators123:
hopefully the pins should not exceed the current draw from the USB power supply because they are only receiving voltage from the pots and buttons.

I don't understand what that means.

4

JimboZA:

operators123:
hopefully the pins should not exceed the current draw from the USB power supply because they are only receiving voltage from the pots and buttons.

I don't understand what that means.

I'll show a quick diagram which should help explain, (imagine that LED is also conected to ground)

jn-wp:
i cant figure out how exactly you plan to connect the arduino and the leds on your project. but i shold work in general. i did so powering a motor with an extra battery and one battery for the arduino-board.
you have to pay attention, that you dont connect the two +5V together!

Of course, as that would be detrimental!

5

Couple of observations, then:

  • You need to hook the ground shown to the Arduino ground, else (for example) the output from the pots' wipers will have no meaning. A bit like imagining the sound of one hand clapping
  • Do you mean the LEDs to be on all the time?- if you intend to control them from Arduino there will need to be some switching mechanism like a transistor whose base is connected to an Arduino digital pin
  • You'll need a resistor on one side of the button as a pull-up or pull-down (or use the internal pull-up via code) else when it's closed you'll short 5v to ground. Have a look at the circuit and sketch here
6

JimboZA:
Couple of observations, then:

  • You need to hook the ground shown to the Arduino ground, else (for example) the output from the pots' wipers will have no meaning. A bit like imagining the sound of one hand clapping
  • Do you mean the LEDs to be on all the time?- if you intend to control them from Arduino there will need to be some switching mechanism like a transistor whose base is connected to an Arduino digital pin
  • You'll need a resistor on one side of the button as a pull-up or pull-down (or use the internal pull-up via code) else when it's closed you'll short 5v to ground. Have a look at the circuit and sketch here

-Interesting, I never knew you'd have to connect both grounds, I thought if the pots were connected to any 0v it would work.

  • The LED's are simply on, not controlled by arduino for now. (I'd like to have them flash from recieving MIDI data, but that's another story for another day)
    -And yeah I left the resistors out the diagram because I was too lazy to add them, I usually just put 10k between each button and GND.
7

Ok well, if....

I left the resistors out the diagram because I was too lazy to add them

.... don't be surprised when other members are too lazy to respond.

8

Now, the potentiometers are presumably at least 10k each, so even ten of them in parallel would come only to 1k and draw 5 only mA.

The potentiometers are supplied from the 5V Vcc line on the Arduino. This is not only for safety - ensuring that the voltage from the wiper cannot exceed that 5v supply voltage, but also so that the readings are always relative to that voltage and read the same for any given position.

Pushbuttons should be wired to ground, not Vcc, to ensure that wiring to them is safe in the event of accidental grounding. You then use the internal pull-ups of the Arduino and need no other connection.

Encoders will probably need to be powered from the Arduino Vcc as well, unless they draw significant current (more than 10 or 20 mA).

Now all that said, if your 5V 3A supply is a switchmode design and fully regulated, preferably with a "crowbar" circuit to shut it down should it fault and exceed 5.6V, then you can use it to feed Vcc on the Arduino. Otherwise, it sounds as if it is primarily for powering the LEDs. If you intend to use the LED strips which do not employ integrated IC drivers, you probably want to use 12V instead. You will require some sort of driver circuit to switch the LEDs from the Arduino.

9

Now, the potentiometers are presumably at least 10k each, so even ten of them in parallel would come only to 1k and draw 5 only mA.

That's right, they're 10k each

The potentiometers are supplied from the 5V Vcc line on the Arduino. This is not only for safety - ensuring that the voltage from the wiper cannot exceed that 5v supply voltage, but also so that the readings are always relative to that voltage and read the same for any given position.

Makes sense, I'll think about it when designing the pcb's for my pots.

Encoders will probably need to be powered from the Arduino Vcc as well, unless they draw significant current (more than 10 or 20 mA).

I thought encoders were not connected to vcc, only GND and two digital inputs (interrupt pins for best performance right?)

Now all that said, if your 5V 3A supply is a switchmode design and fully regulated, preferably with a "crowbar" circuit to shut it down should it fault and exceed 5.6V, then you can use it to feed Vcc on the Arduino. Otherwise, it sounds as if it is primarily for powering the LEDs. If you intend to use the LED strips which do not employ integrated IC drivers, you probably want to use 12V instead. You will require some sort of driver circuit to switch the LEDs from the Arduino.

My 5v/3a supply is a power cable that's used to power a cisco device or something like that, it looks like a laptop charger cable I guess, just says 5v/3a on the box.
Feeding it to arduino is something I considered, (through VIN and GND pins) I was just unsure as to whether this would damage the atmega2560's pins if I'm drawing about 1A from the power supply, or whether arduino would regulate this down to 500ma.

I must point out the fact that because the LED's are on the same PCBs as the pots and buttons, this means that I want to only have 1 vcc and 1 gnd rail for each pcb. (having two would get too complicated)

Thank you very much for the information though :slight_smile:

10

operators123:
I thought encoders were not connected to vcc, only GND and two digital inputs (interrupt pins for best performance right?)

There are encoders and there are encoders. Optical and Hall Effect encoders require a power source, and any with open-collector outputs or switch contacts require a pull-up - the internal pull-up of the Atmel chip.

You only ever require interrupts for encoders driven by motors which generate more than a few hundred signals per second and are fully de-bounced (hall/ optical). Interrupts are a problem as contact bounce will generate barrages of interrupts which if fast enough, will cause the code to crash - I have given up trying to help people who are using interrupts to service encoders and pushbuttons. Pushbuttons and manual encoders require debounce to prevent spurious events faster than 100 per second (if even that),

operators123:
My 5v/3a supply is a power cable that's used to power a Cisco device or something like that, it looks like a laptop charger cable I guess, just says 5v/3a on the box.
Feeding it to Arduino is something I considered, (through VIN and GND pins) I was just unsure as to whether this would damage the atmega2560's pins if I'm drawing about 1A from the power supply, or whether Arduino would regulate this down to 500ma.

Given my previous comments about how much you trust the power supply, if you feed the Arduino with regulated 5V via the Vcc connection (you cannot use Vin as you need 7V minimum for the regulator), it only draws what it needs, the current drawn by the other things connected to the same supply never passes through the Arduino, so is irrelevant.

operators123:
I must point out the fact that because the LED's are on the same PCBs as the pots and buttons, this means that I want to only have 1 vcc and 1 gnd rail for each pcb. (having two would get too complicated)

You only ever have one ground rail, except for very special considerations of audio, video or analog sensing. Everything connects to that ground rail with as little resistance (lead length) as possible.

You have as many Vcc rails as different devices have different requirements. If everything will operate from 5V and you have a well regulated supply, then that is "the" Vcc.

The supply for the potentiometers however, should come back separately from the Vcc terminal on the Arduino (and also separately from the ground terminal - that's the bit about "analog sensing").