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Topic: 5V at the USB port without connection to PC (Read 8862 times) previous topic - next topic

deSilva

#15
Sep 19, 2010, 10:20 am Last Edit: Sep 19, 2010, 10:22 am by mpeuser Reason: 1
Needless to say, I second your statement and I can feel with your troubles.

In one of my standalone versions the supply has to be external, as also higher voltages are involved. When connecting to a PC the 5V logic becomes activated but not the rest of the system. This had led to problems, as it is sometimes forgotten to switch the mains on, as it seems to work, though with unclear limitations. To prevent extra indications and checks I have used the (alas I have to say: better designed) Seeeduino boards. But it seems their newer versions also sport auto-power selection (?)

Nevertheless - to my limited understanding - the use of a very low voltage MOSFET without changing the comparator to a rail-to-rail type is a mis-design. I am very unhappy about the reaction from the Arduino team....



David Cuartielles

Guys,

please be patient and let Gianluca time to answer this thread, we are all working around the clock for the official presentation in NYC on September 23rd.

That is why I wrote an answer quoting Gianluca the other day.

As mentioned, more news to come on this topic, please stay tuned.

/d

Gianluca

Hi All,
Sorry for the long time.
- The mos is an FDN340P.
It replaced the NDT2955 in order to benefit of its lower Rdson (110mOhm) at an lower Vgs (-2.5V), in order to work better with a non rail to rail OpAm
Will do some test today and show up with results tomorrow.

Thank you for pointing this out. Please send me any suggestion or special request about this in order to fix it as soon as possible,

Ciao
    Gianluca

deSilva

Quote
Please send me any suggestion or special request about this in order to fix it as soon as possible,

Thank you, Gianluca - but we have already.
A very low voltage P-MOSFET will in fact NOT work better with a non-rail to rail comparator. As the current comparator will never exceed 3.7V the FET MUST close at -1.3V which the new one does not!

James C4S

I'm seeing a similar issue in the opposite direction.  When connected to USB I am seeing voltage on Vin.

When only connected to USB, I measured measure 5.06V on the 5V pin.  The Vin pin measured 4.54v.  This happens on the Uno and my 2009-board.  An older Diecimila I have laying around (that uses the jumper pins instead of auto-switch) does not have the problem.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

apollo21

Hi
Did you watch the following?
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1286627335
Please write your thoughts.
Kyoji

Alan Burlison

Quote
I'm seeing a similar issue in the opposite direction.  When connected to USB I am seeing voltage on Vin.


Yep, I see exactly the same thing.  I have a high-current draw external load (HL1606 LED strip) that I'm powering with an external 5V PSU, which also supplies 5V to Vin.  If I forget and disconnect the PSU whilst leaving the USB cable connected, the Vin pin supplies 5V back out to the LED strip, but it can't supply the current needed (up to 2A), and the strip goes haywire.  I haven't toasted my Duemilanove yet, but trying to draw 2A from it clearly isn't a good idea.  Seems like a design flaw to me.

apollo21

Hi Alan

I wrote the same thing in two places Chigau words.
Because both write, and write your response here and there.

555 has an internal network with 100kohm
The window comparator. (18kohm Another theory says).

Power and network they are connected to GND,
2 / 3 of the "control" is terminal.
Meanwhile, the "trigger", "threshold" between networks and terminals 100kohm
Equivalent circuit will have no hysteresis.

Used to detect when the control terminal, the protective resistance 100kohm used.
The temporarily stable,
Network "3 / 3-2 / 3" Resistance and 100kohm
50kohm and parallel equivalent.

Therefore, the "threshold" terminal "3 / 3-2 / 3" is close to the resistance 51kohm used.

Instability because the show circuit simulator,
1nF capacitor connected to both.
Please try to prove too.

kyoji

WalterM

#23
Oct 17, 2010, 11:59 am Last Edit: Oct 17, 2010, 12:19 pm by 3WalterM4 Reason: 1
I found the same behaviour of the Vin pin like James and Alan.

At a similar application like this:
http://www.david-laserscanner.com/forum/viewtopic.php?p=12366#p12366

....a permanent connection via USB is necessary to send motor trigger impulses in synchronism with the captured camera frames. I noticed that even if the wall power supply wasn't plugged in the laser and the stepper motor could be powered from Vin.
To avoid this lack I have inserted a diode (1N4001) in front of the Vin pin. (Of course kathode to Vin).
A test of Vin as an output revealed:
Without any load >> 4.5V
      At I = 42mA >> 4.2V
        I = 100mA >> 4.1V
        I = 180mA >> 3.9V
        I = 360mA >> 3.6V

But this effect has nothing to do with the wrong replacement of the FDN340P for the NDT2955. It is caused by IC4, the voltage stabilizer MC33269D-5.0.

In the meantime I have removed the FDN340P from all my boards and substituted them by the old and fine working NDT2955.

Some posts above Gianluca Martino has promised to send results of his tests. Unfortunately he has forgotten us users.  :(

James C4S

Quote
n the meantime I have removed the FDN340P from all my boards and substituted them by the old and fine working NDT2955

I wish I could find a NDT2955 to do the same.  :(
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

WalterM

#25
Oct 21, 2010, 08:50 am Last Edit: Oct 21, 2010, 09:39 am by 3WalterM4 Reason: 1
James,
I ordered my NDT2955 from RS Components:
http://de.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=ndt2955&x=32&y=14

And I think they have affiliated companies or distributors all over the world:
http://www.rs-components.com/index.html

James C4S

Thanks Walter.  I don't know why I didn't notice before, but Digi-Key did have them in stock.  I thought I had already checked there and didn't find them.  Order placed, looking forward to making the change.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

pak

Hi dude,
    I've same problem with 5V at gold fuse without USB cable supply. But one of the UNO board is doing well, then I inspected the board and found different usage of LMV358 and LM358. So the UNO with LMV358 is doing well. This IC is supported to be rail-to-rail output but LM358 does not. I'm still looking for the replacement of LM358.

Hope it will help.

pak

PS: New UNO is not using 16MHz crystal anymore. both are resonators.

sburlappp

Would a USB cable with the power wire cut work around the problem? You could then power the Arduino Uno from an external source while still talking to your PC, without modifying the board itself.

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