Power Question

Hi,

I made a circuit that needs 5v and about 400mA of current.

The issue is that when the circuit is powered by the Arduino Nano via a USB cable (USB plugged into Nano), the circuit does not power on.
The circuit Vcc connected to the +5V pin of the Nano
and the GND of the circuit is connected to one of the GND pin on the Nano.
I gues the +5v Pin is not able to give the 400mA required for the circuit.

If I connect the USB directly to my circuit, both the Nano and circuit works fine.

The problem now Is that I need to transmit data from the Nano to my computer via the USB…

Now Can I have both USB plugged at the same time…
One to my circuit (just for the power), and the other to the Nano
for the USB data transfer ?

How would the Nano react seeing Power on the USB connector and and other source on the +5v pin?

What ever you do don’t connect the +ve of two supplies together and always connect the -ve lines together.
For further discussion please see:-
http://www.thebox.myzen.co.uk/Tutorial/Power.html

Thanks… but does that also apply if I’m using 2 USB ports from the same computer?

Also, if I understand correctly the Vin pin on the Arduino Nano is actualy the “input” of a voltage regulator, and the +5v pin is its “outpu”, which is then used to power the chips on the Arduino.

But when the Arduino is powered useing the USB cable… is the Voltage Regulator also in play ? I mean is it also regulating the 5v from the USB?

If it is regulating the 5v from the USB… what would happen if I plug an other USB cable to my circuit… thus putting a 5v on the “output” of the voltage regulator.

Or would it be better if I just remove the wire that goes from the Arduino +5v pin to my circuit Vcc

I mean is it also regulating the 5v from the USB?

No, when powered from the USB the +5vdc from the PC is wired to the output side of the on board regulator. If you are using two USB connections, one for the Arduino and one for external circuitry then you really should not have to connect the grounds together. That is a rather unusual configuration but should work OK.

Lefty

I think that if you have to ask the question you don’t know enough to do it.

Don’t join +5v together if you have the grounds together, you are asking for problems.

Even if they start off as the same voltage it goes through a poly fuse before it reaches the +5v line and so they are going to be slightly different when they arrive.

I know that we can’t plug the “+” together,.

I thinking of powering the whole thing using on the USB Connected to the Arduino… That is why I connected the circuit power to the Arduino’s +5v and GND

But that way… the circuit does not power on… the 5v is there but I beleive there’s not enough current…

So i guess the best think I could do is completly separate the power from the circuit and the Arduino. and power them both independly…
Which is a bummer because I will 2 USB cables instead of only 1… which was my goal…

Thanks everyone for your input

the circuit does not power on… the 5v is there but I beleive there’s not enough current.

This is odd because if there was not enough current there would not be 5V showing, it would be much lower than this. The excess current would cause the voltage to sag.

As an arduino only takes about 50mA or so. I don’t think powering the rest of the circuit from another USB port will be the answer. You could use a separate wall supply if you think it is a current issue.
However because you report:-

If I connect the USB directly to my circuit, both the Nano and circuit works fine.

makes me think there is probably something else going on here. There might be an issue with he amount of current your USB port can supply, it should supply 500mA

Thanks Grumpy_Mike…

I’ll be taking an other look at my circuit and double/triple check there’s nothing wrong or shorting…

And are you sure about that 50mA?
Doesn’t the spec says that each Digital output is able to source 40mA…
What happens if I use 40mA from all Ditigal outputs at the same time ?
Or is that 40mA shared between all outputs ?

What happens if I use 40mA from all Ditigal outputs at the same time ?

Yes they all add up, there is an overall current limit of 200mA on the chip as a whole. So the 50mA reefers to a board driving only logic loads or acting as inputs. If you have say lots of LEDs being driven then you will add that current to the 50mA or so for the processor as a whole.
Hope that makes sense.