5V at the USB port without connection to PC

Now I have exchanged the 340A for a DMOS transistor BS250. It was just lying around.
And now it works. The only lack due to its relative high R-DS-on is that the 5V on the board is only 4.6V.
But for my application the board is externally powered anyway. And then the 5V are really 5V.
I will try to find a transistor with a lower on-resistance than the BS250. But not something in SMD.
Do you know one?

By the way: A better controller than that one of the link above is completed. It has a connection via USB to receive motor impulses from the PC in synchronism with the captured camera frames.

Maybe you find something here? P-MOS is awkward.... It has to switch through at 4.5 volts!
http://www.mikrocontroller.net/articles/MOSFET-Übersicht#P-Kanal_MOSFET

According to SMD Codebook Page 3 (useful site!), an SOT23 marked "340" should be a FDV340P, which is indeed a P-channel MOSFET, according to many web pages. But I can't find an actual datasheet! (and no mention of "340A")

See my posting #5..
And both should work... Or is the gate voltage situation (-1.3 volts to close it) to tight??

Thanks guys for your help.
I will try to find a suitable FET for a replacement. Unfortunately the best suitable transistors are SMD or TO-220 and not like TO-92.

I have taken up the thought of deSilva and investigated the switching behaviour of the MOSFET ...340A.
The Drain voltage was measured with respect to the applied Gate voltage. The Source pin was connected to +5V. Two measuring series were made. One with 100[ch937] load at the Drain to Gnd and the other without any load.
With load:
Gate voltage Drain voltage
4.36 0
4.11 0.55
4.08 1.07
4.04 2.12
4.01 3.14
3.6 4.96

Without a load:
4.68 0
4.43 1
4.18 4.99

This means that the MOSFET on the Arduino cannot switch off with a Gate voltage of 3.75V from the comparator LM3580.
In the datasheets I found for the NDT2955 a Gate Threshold Voltage of typical -2.6V and for the FDN340P -0.9V. But it isn't clear that a FDN340 is on my board.

So this looks like an awkward design fault. They overdid in the strive for low gate voltage :slight_smile:

I just wonder why the comparator outputs those 3.75 volts rather than something closer to 5 Volt...

edit

I see.. the LM358 is not a true rail-to-rail opamp. Is goes down to ground but not to Vcc.. Max input is Vcc -1,5 volts = around 3.5volts, which is than buffered by the voltage follower..... This works in combination with a "typical" logic level MOSFET but not with that super low gate voltage type.. As they say: "The better is the enemy of good " :slight_smile:

So another workaround would be to change to a pin compatible rail to rail dual opamp....

Now we have an official statement from Gianluca Martino, a member of the five-headed Arduino team.

I had written this request to the Arduino Blog:

[ch8222]Since the Arduino Duemilanove is equipped with a (FDN)340 instead of the original MOSFET NDT2955 the switching-off of the 5V isn[ch8217]t still working. See my posts:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284034373
I have noticed it at 5 boards, labeled: Made in Italy, designed by tinker.it[ch8220]

Gianluca Martino replied:

[ch8222]The power Switch is intended for switch the supply from one power source to another and not to remove the 5v from the USB port. This is necessary to keep the ftdi live for an insertion of the USB.[ch8220]

Let me sum up:
The fact is, the precursors of the Duemilanove, the Diecimila Arduino and the NG Arduino were equipped with Power Jumpers:
http://www.ladyada.net/learn/arduino/lesson0.html
And it was necessary to set it in the right position by hand before starting.

The new Duemilanove has an automatism which should do this hand job for the user by switching a MOSFET. And the producer of the boards wouldn't had introduced it if it wouldn't be necessary.
And as long as they have used the NDT2955 all was working perfect. But now with the FDN340 it doesn't work! And we know the reason why. Then it would have been cheaper to substitute the MOSFET by a simple wire bridge.

It would be interesting to know the opinion of the hardware specialists of this forum regarding the statement of Gianluca Martino. And of course of mine.

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....

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

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

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!

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.

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

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.

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

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. :frowning:

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. :frowning:

James,
I ordered my NDT2955 from RS Components:

And I think they have affiliated companies or distributors all over the world:

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.