Hi, I am powering my arduino using USB cable connected to USB jack of my arduino nano. I want to know how much will be the voltage at VIN?
0 Volt
Really 0v? I thought that at least 5V should be sent there...
I just powered an LED with a 100 ohm resistor from VIN using only USB connected to my arduino nano... so it is not 0..
Checked an Uno and it's 4.5V. So I'm wrong. I find it strange that 5V can 'travel' in reverse direction through the voltage regulator.
I also suspect a 0v but the meter shows a voltage close to 5v.
yeap, really strange, I have no idea why this happens. Any body knows the current limit I can draw from VIN powering my arduino from USB?
sterretje:
Checked an Uno and it's 4.5V. So I'm wrong. I find it strange that 5V can 'travel' in reverse direction through the voltage regulator.
I faced a similar issue before, I was using a LM7809 and current could travel reverse. Later I used a diode to stop reverse flow.
check this post, might help you max ampere for VIN pin - Motors, Mechanics, Power and CNC - Arduino Forum
thanks man 
Look at the schematic on page 10 of data sheet, there's a path from output to input terminals through transistor junction diodes.
I thought that the vin was good for 6v and up (to a limit), so if you use a usb pushing a constant 5v, nothing goes to the VIN. I personally don't know if reverse voltage plays into this at all, but it's definitely worth checking out.
V-in is the input of the onboard 5volt regulator.
If you want to supply the Arduino externally, you can connect >=6volt to V-in.
Better to use >=6.6volt. This also disconnects USB supply. Then the Arduino runs fully on external supply.
Safer still is to use the DC jack.
That input has a reverse polarity protection diode before it reaches V-in.
The diode drops 0.7volt, so add 0.7volt to the above voltages.
When supplying from USB only, you should connect nothing to V-in.
Doing so draws current through the onboard regulator the wrong way. Out to in.
The regulator could fail.
Leo..
sterretje:
Checked an Uno and it's 4.5V. So I'm wrong. I find it strange that 5V can 'travel' in reverse direction through the voltage regulator.
The UNO regulator has an inherent diode form output to input. As Wawa says though, current should not be drawn through it under normal operating conditions. Vin should not be loaded when 5V is supplied to the board via USB or direct connection.
From the NCP1117 datasheet:-
The NCP1117 family has two internal low impedance diode paths that normally do not require protection when used in the typical regulator applications. The first path connects between Vout and Vin,
I think so..... better not use Vin for supplying power when the USB is plugged in. However, if you have a bunch of UNOs etc, then I guess you could see what happens if you do it. If everything keeps operating over a long period of time....and if the application isn't a critical or life-threatening or house-burning one.... then that would be nice. Will be interesting to see the results. I might try it for myself sometime heheheh. Probably like everybody here.... I have a stack of UNOs and 2560 waiting in the wings to use. Will report on results when I try it. Will see what happens.
Southpark:
I think so..... better not use Vin for supplying power when the USB is plugged in. However, if you have a bunch of UNOs etc, then I guess you could see what happens if you do it. If everything keeps operating over a long period of time....and if the application isn't a critical or life-threatening or house-burning one.... then that would be nice. Will be interesting to see the results. I might try it for myself sometime heheheh.
Why? The voltage will only be about 4.4V to 4.5V, lower under higher load, and 5V can already be drawn directly from the 5V header when USB or an external 5V is connected. With an external 5V supply, the supply can provide 5V to the Arduino and 5V to other loads.
Seems like a pointless exercise to me.
OldSteve:
Why? The voltage will only be about 4.4V to 4.5V, lower under higher load, and 5V can already be drawn directly from the 5V header when USB or an external 5V is connected. With an external 5V supply, the supply can provide 5V to the Arduino and 5V to other loads.
Seems like a pointless exercise to me.
If everyone reckons it's not a good idea...... then it's not pointless to me if I try it to see what happens. I don't mind if it's pointless to you though. Only half a volt under. Still got 4.5 V to try. The exercise is to see if that internal diode can handle significant current, and to see if that Vin pin is useable at all.
Southpark:
If everyone reckons it's not a good idea...... then it's not pointless to me if I try it to see what happens. I don't mind if it's pointless to you though. Only half a volt under. Still got 4.5 V to try. The exercise is to see if that internal diode can handle significant current, and to see if that Vin pin is useable at all.
You'll get away with it - the regulator's internal diode is rated at 15A peak, but it's not good practice and there's no real purpose.
OldSteve:
You'll get away with it - the regulator's internal diode is rated at 15A peak, but it's not good practice and there's no real purpose.
Thanks OldSteve. Thanks for that info about the current handling. Ok.... I won't need to try it then.
OldSteve:
You'll get away with it - the regulator's internal diode is rated at 15A peak, but it's not good practice and there's no real purpose.
Well that might be true for 78XX, LM3XX regulators.
But can you find those specs in a modern LDO regulator, like the xx1117.
Leo..
Edit.
Found it on the TI LM1117 datasheet.
Bottom of page 12.
10-20A for microseconds.
No continious current is mentioned.