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Topic: FDN340P Drawn incorrectly on schematic? (Read 2 times) previous topic - next topic

retrolefty


Hi,

No I do not think it acts like a SPDT switch! Like I said, I am happy with the 'descriptive' purpose and operation of the MOSFET - i.e it is switching USB 5V either on or off. What I am not happy with is the way the terminals of the MOSFTET are drawn in the schematic.

Please see my attached image.

Circuit A is what has been drawn on the schematic.

I believe that what should have been drawn is Circuit B.

Any comments?

Thanks


Yes, don't guess or postulate. Get the datasheet for the specific FET and see it's terminal pin out and then look on your board to see if it's wired as drawn or not. The fact that it works as presently installed on arduino boards is not in question, just how the FET might be being used. Mosfets are able to conduct current in either direction to and from drain and source, that is also known.

Lefty

vasquo

Ahhhh i see what you mean. Why didn't you say so in the 1st post. Your beef is with the drawn diode! :)

Looks like the internal diode for the mosfet symbol is drawn the other way around.

Per datasheet, figure B mosfet symbol is correct.

Lydecker

Cool, thank you vasquo! Good to know I'm not going insane!

yes my beef is the way the diode is drawn and hence ALSO the drain and source terminals.

In their schematic they have drawn the circuit symbol for a P MOSFET such that the Drain terminal of the mosftet is connected to the USB 5V when in fact they should have drawn it such that the source terminal is connected to USB 5V. Basically the MOSFET symbol is drawn back to front in their diagram.

I thought it was just a simple error in the schematic drawing, however I was doubtful given this error has been present since the Duemilanove schematics back in 2009.... I can't believe this hasn't been picked up on to date?

retrolefty

#13
Feb 12, 2013, 10:47 pm Last Edit: Feb 12, 2013, 10:52 pm by retrolefty Reason: 1

Ahhhh i see what you mean. Why didn't you say so in the 1st post. Your beef is with the drawn diode! :)

No, diode is built into fet and is drawn correctly in both examples and just like the datasheet shows cathode of diode connected to source terminal. His issue is how the fet is wired into the larger circuit, which side should be drain and source terminals.

Looks like the internal diode for the mosfet symbol is drawn the other way around.

Per datasheet, figure B mosfet symbol is correct.

vasquo

#14
Feb 12, 2013, 10:49 pm Last Edit: Feb 12, 2013, 10:53 pm by vasquo Reason: 1
[font=Verdana]The drain/source is drawn correctly and works correctly.
It's just the drawn diode that is wrong in the schematic.... see datasheet for correct diode orientation. [/font]


FYI, in Eagle, you can draw the schematic any which way (heck, you can just show the mosfet as a box with 3 legs and a smiley face) and as long as you assign and label the pins, and connect them to the correct device pins, it will work.  

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