Confused with P Channel MOSFET

electronist

New Member

• May 5, 2009

• #1

Could anybody tell me which of Fig A or Fig B is the proper circuit.
The problem is that in Fig A the Battery supply is connected to Drain and in Fig B it is connected to Source.

Please help me out

 

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• fig B.JPG

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Roff

Well-Known Member

• May 5, 2009

• #2

You need to show what's driving the gate in both cases.
Also, you have drawn an N-channel MOSFET.

 

OutToLunch

New Member

• May 5, 2009

• #3

they look like P FETs to me

 

crutschow

Well-Known Member

Most Helpful Member

• May 5, 2009

• #4

P-FETS all the way.

 

Sceadwian

Banned

• May 5, 2009

• #5

He didn't ask if they were pfets, he was confused about which circuit works. But as ROFF said we can't answer the question till he explains what is supposed to be driving the gates.

 

Roff

Well-Known Member

• May 5, 2009

• #6

Yeah, you guys are right, obviously.
They're PFETs. Once again, I posted before my coffee had soaked in.

 

electronist

New Member

• May 5, 2009

• #7

As you can see from the diagram there is a pull up resistor, so if nothing else is driving then the gate is maintained at low or 0V. The other condition is a 5V supply being constant on the gate pin.

The question is Which of the two diagrams have the Drain and Source Pins rightly positioned?

 

Roff

Well-Known Member

• May 5, 2009

• #8

This appears to be a supply switchover circuit. The correct circuit has to be Fig. A. Assuming the 5V is capable of delivering more than a couple hundred milliamps, Fig. B will burn out D5, and then proceed to damage the battery through the body diode.

 

electronist

New Member

• May 6, 2009

• #9

Thank you very much. This seems to make sense