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Topic: 5V at the USB port without connection to PC (Read 8 times) previous topic - next topic


Sep 09, 2010, 02:12 pm Last Edit: Sep 09, 2010, 02:16 pm by 3WalterM4 Reason: 1
When the Arduino Duemilanove is connected via USB to my PC and the board is externally powered at the same time, the PC cannot boot.
I have measured the voltage at the fuse F1 and found +5V even if the PC is disconnected. That means the MOSFET NDT2955 is always switched on. This was noticed at my 4 Arduino boards. Two of them were built-in, but the others were never used before, only now for this check with a not-switched 9V wall power supply.
The mesured voltage at the Gate of the MOSFET was 0V when USB was connected and 3.75V when only externally was powered.
The labeling on the MOSFETs are 340 A. Was the NDT2955 replaced?  


Sep 09, 2010, 03:53 pm Last Edit: Sep 09, 2010, 03:54 pm by mpeuser Reason: 1
As the discussion in the German forum showed, this is strange.

I just had a look at two of my boards:
Using 7.5V at the barrel connector closes the transistor reliably (15MOhm)
When I go down below 7 Volts the regulator is no longer able to uphold the 5 volts, the system voltage goes down to to 4.7 volts and the transistor switches through.

This is as exspected, as the comparator sees Vin =(barrel voltage -0.6)/2 against 3.3 Volts.
Having a very high load (500mA) your barrel voltage might fall below 7 volts.....
It seems there isa grey zone between the the regulator working correctly and the comparator already giving up between 6.8 and 7.1 Volts


Thanks for the tests.
What is the labeling on your MOSFETs? Perhaps they have changed the transistor and nobody has noticed a misoperation up to now. But this is incredible. So many boards worldwide in action.  


Sep 09, 2010, 05:15 pm Last Edit: Sep 09, 2010, 05:16 pm by mpeuser Reason: 1
It is a RZY2955 on my boards (China made) note the number 2955 can also refer to a bipolar transistor, but this one is fine.

When you said 340A I became a little bit uneasy...
The only thing I find is an IRF340 ... This however is a N-channel MOSFET, whereas the 2955 is a P-channel MOSFET...
Can you find out what is really on your board? Where did you get them from in the first place...


By means of a magnifying glass I have again looked on my four boards. All MOSFETs are labeled [ch8222]340 A[ch8220]. And their housing isn't a SOT-223 with one or two Drain pins. I don't know the type of package, it is 3.1 mm x 1.4 mm.
On the backside of the two Arduinos which are [ch8222]free[ch8220] and never used before they have glued on a paper or plastic sticker. On the first is printed CM6865 and on the second CB1280.
All my 4 boards were bought here:
But the IRF340 is a N-channel Power MOSFET and comes in TO-3.

Do you have an idea how we can proceed? If I would exchange the 340A for a NDT2955 it would only be a half solution. And I am planning to build further on controller like this:

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