P channel MOSFET Wiring not working

Thread Starter

teeveeloon

Joined May 29, 2016
3
Hi all,MOSFET Problem.jpg

Essentially I've got a P channel MOSFET that I'm trying to connect.

The voltage of the battery is 4.2ishV (18650)

I have attached my sketch of the circuit

I want the voltage at "D" to be zero when the switch is off and full voltage when it is on.

At the moment, it reads 4.2 with the switch on and anything between 0-4.2V when it is off. How do i get it to be zero the whole time when the switch is off??

Many many thanks, I've tried so hard to figure this out.
 

MrAl

Joined Jun 17, 2014
8,562
Hi,

Connect 10k resistor from source to gate.
You can really use anything between about 1k and about 100k, but 100k might make it turn off too slow so 10k is good.

A mosfet has an internal capacitance that stays charged when the gate is open, so a resistor is required to drain that capacitance to let the mosfet turn off.

Also, a small series resistor in the gate might be a good idea too, like 50 ohms. That is to limit gate current when turning on. Min about 10 ohms, max about 100 ohms.
 

JMac3108

Joined Aug 16, 2010
348
TeeVeeloon,

I had a little trouble figuring out your circuit diagram at first, so I re-drew it in a more standard schematic form. One problem you have is that you've connected your positive supply (4.2V) to the drain of the PFET. Are you aware that FETs have an intrinsic diode? This is an internal diode that is part of the physical construction of the FET, and in the case of the PFET it points from drain to source which is not what you want for your circuit. You need to turn the PFET around. See the attached drawing of your original circuit, with the intrinsic diode shown, and my suggested changes. Also, MrAl is right, you need a resistor from the gate to source. Always use a gate-source resistor on your FETs to ensure they stay off. Good luck with your circuit!




upload_2016-5-28_22-48-40.pngupload_2016-5-28_22-48-40.png
 

dl324

Joined Mar 30, 2015
13,573
Welcome to AAC!
it reads 4.2 with the switch on and anything between 0-4.2V when it is off. How do i get it to be zero the whole time when the switch is off??
As mentioned by the earlier posters, you need a resistor between gate and source.

It would be helpful if you included part number(s) and drew schematics in a more conventional way.

upload_2016-5-29_6-46-56.png

Note that the dashed line for an enhancement mode device is often drawn as a solid line when hand drawing. That can be confused with a depletion mode MOSFET which is on with no gate bias, but they're seldom used.
 
Last edited:

JMac3108

Joined Aug 16, 2010
348
As other said, you might want to try to draw the schematics in a more conventional way. I still can't figure out what you drew :)

I thought the rectangle was a resistor tied to the source, and the half circle with a D in it was the battery tied to the drain!
 
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