EXT vs USB-supply

Hi!

I have created a servo project. Due to great tips from this forum I have used a separate battery pack (6V) for the servo and then supplied the UNO by the USB.

Now I however wish to use my 9VDC/800mA (+@pin) adaptor instead of the USB (which it is capable of 100mA only?).

I have googled on the diode between the adaptor (EXT) and Vin and it can withstand 1A but this is probably mainly for protection if you should have the crazy idea to supply voltage at Vin while the adaptor is connected.

I think I get this.

I do however not get how the UNO is supplied if I should use both EXT and USB. I need EXT for the servo but how does the board know what source to use if the voltage is the same? I.e regulated 5V from EXT and USB-5V at the same time.

While both are 5V the source cannot be selected by a couple of diodes.

So, can I supply my UNO externally (EXT) at the same time as it is connected by USB to drive my servo or am I missing something important?

Best regards, Roger

Now I however wish to use my 9VDC/800mA (+@pin) adaptor instead of the USB (which it is capable of 100mA only?).

USB 1.1 and 2.0 are 500mA if I remember correctly and 3.0 is 900mA or more.

I need EXT for the servo but how does the board know what source to use if the voltage is the same? I.e regulated 5V from EXT and USB-5V at the same time.

The trick is common ground between the two supplies.

So, can I supply my UNO externally (EXT) at the same time as it is connected by USB to drive my servo or am I missing something important?

That's the purpose of the diode, it is for protection.
I always recommend unplugging the Ext AC/DC supply when using USB, but an authentic UNO should work correctly if you do not. However, if you have another supply w/common ground connected to LEDs, servos, steppers, etc. Then that supply should be powered off prior to reprogramming the UNO.

Ray

rogerk8:
I have used a separate battery pack (6V) for the servo and then supplied the UNO by the USB.

Now I however wish to use my 9VDC/800mA (+@pin) adaptor instead of the USB (which it is capable of 100mA only?).

I do however not get how the UNO is supplied if I should use both EXT and USB. I need EXT for the servo but how does the board know what source to use if the voltage is the same? I.e regulated 5V from EXT and USB-5V at the same time.

So, can I supply my UNO externally (EXT) at the same time as it is connected by USB to drive my servo

I’m confused by the excerpts I have copied here.

It sounds like you are thinking of using a 9v/800mA adaptor to drive your Arduino through the EXT input and to power your servo from the Arduino 5v pin.

You already have a separate power supply for your servo. Continue to use that. Using the Arduino 5v to power a servo (or any motor) is a bad idea no matter what power supply the Arduino has. The 5v pin can’t supply enough current.

And what do you mean by “connected by USB to drive my servo”. As above, the Arduino can’ provide enough power to drive a servo, whether the power comes through the USB or the EXT connector.

Perhaps you mean “…USB to control my servo” - but if there is a USB connection (and a separate servo power supply) why would you need a third connection to EXT?

…R

Thank you for your replies!

I'm sorry, it was badly explained.

Up til now I have used a separate 6V battery-pack to supply my servo. This while supplying the board itself by USB.

Now I wish to keep my debugging-servo and it's dedicated supply (6V) and use the EXT to drive the board and a LED Module while keeping the USB plugged into the computer for software development.

This LED Module has its own 5V regulator so it only needs something above 8V which seems to be available at "Vin" thru the diode, right?

The LED Module draws a maximum of 250mA.

I know now that I mixed things up. I'm sorry for having disturbed you with this.

It would however be interesting to know how much current that may be drawn from the 5V pin. It may however be hard to say depending on USB (which seems to be some 500mA at least) and/or EXT voltage usage.

Best regards, Roger
PS
I do not get how a common ground can control weather USB or the 5V from EXT is being used. To me you would need a couple of diodes where the highest voltage gives the current. But now the voltages are "equal" and we have no diodes...

I do not wish to burn my USB port but on the other hand it has thankfully been explained that it can supply more than I need.

Case closed :wink:

Vin is Aftert the reverse poloarity diode.
Voltage selection is determined by a voltage comparator and a FET.
If Vin/2 is > 3.3V, USB source is cutoff and 5V from regulator is used.
If Vin/2 < 3.3V, USB source is passed thru the FET as the 5V source.

I think I will proceed like this:

Powering up:

  1. EXT (edited, selects itself as current source)
  2. USB (edited, disabled as current source)
  3. 6V

Powering down:

  1. USB (edited, disconnects only data transfer possibillity)
  2. EXT (edited, disconnects power to board and LED)
  3. 6V (edited, disconnects servo supply)

What do you think?

Best regards, Roger

CrossRoads:
Vin is Aftert the reverse poloarity diode.

Thank you, but I know that.

Voltage selection is determined by a voltage comparator and a FET.
If Vin/2 is > 3.3V, USB source is cutoff and 5V from regulator is used.
If Vin/2 < 3.3V, USB source is passed thru the FET as the 5V source.

Very intresting to know. Thank you very much!

Best regards, Roger

I am happy to say that it worked out pretty well!

Now I just need to customize the data with regard to the digit I wish to show.

Arduino UNO rules!

Best regards, Roger