Go Down

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

Lydecker

Thanks for the replies.

I understand and agree with the description of the operation you have both given, however the dispute I have is with specific terminals of the P MOSFET and how they are connected. When using a P MOSFET as a switch, the load (or circuitry to be switched) should be connected to the Drain of the MOSFET which has not been done in this schematic (they have connected the load to the Source).

The MOSFET switches using the Vgs (Voltage between the Gate and Source). I.e in this case the voltage between the output of the comparator and what should be the voltage of the USB 5V (however this is NOT what is drawn).

Does that make sense?

Thanks

vasquo

Quote
The MOSFET switches using the Vgs (Voltage between the Gate and Source). I.e in this case the voltage between the output of the comparator and what should be the voltage of the USB 5V (however this is NOT what is drawn).


What I understand you're saying above is you think T1 selects whether the voltage comes from VUSB or GATE?
Like an SPDT switch?
No.

The mosfet is just being used as an on/off switch, i.e. whether to disconnect or connect VUSB, and allow it to flow or not to U3.

The output of the comparator (gate) just "flicks" this switch on or off. 
(when comparator is low, powered from VUSB. When comparator is high, disconnected from VUSB.

retrolefty

Someone once posted on this forum in detail explanation about the specific usage of this FET switch in the arduino auto-voltage switching application and it involved the body diode of all things? So while it might be an unconventional set up it appears to perform the function that is required.

Lefty

CrossRoads

There is a big discussion of it in the old forum, I didn't attempt to find it tho.
Operation is as vasquo says - USB not connected to 5V when comparator output is high, and USB becomes the source for USB when comparator output is low (generally, powered from PC and nothing connected to barrel jack).
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Lydecker

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

Go Up