Using a P MOSFET as an an/off switch for heating pad

Thread Starter

jonm2012

Joined Mar 22, 2017
3
Hi Everyone,

I am trying out the following project: https://onion.io/2bt-pid-control-python/
The part that I am having trouble with is using the P type MOSFET as a switch.

So far, I have made every connection as indicated in the circuit diagram for the project, however, the heating pad is always on and never switches off regardless of the signal being sent to MOSFET Gate. I think there might be something wrong with the circuit layout offered in the project. Any help is appreciated.

 

DickCappels

Joined Aug 21, 2008
5,875
Your MOSFET is "upside-down". The source should be connected to the positive power supply terminal then the drain can connect to the grounded load.

For this you would need the gate voltage to switch from the positive power supply to a voltage negative enough from that voltage to make the MOSFET conduct sufficiently. If the magnitude of yourPWM signal is not sufficiently large you can try a circuit like the one below.
upload_2019-10-6_13-24-45.png

Since you are using PWM a better approach would be to use a high-side driver to assure that the MOSFET switches fast enough.
 

Thread Starter

jonm2012

Joined Mar 22, 2017
3
Your MOSFET is "upside-down". The source should be connected to the positive power supply terminal then the drain can connect to the grounded load.

For this you would need the gate voltage to switch from the positive power supply to a voltage negative enough from that voltage to make the MOSFET conduct sufficiently. If the magnitude of yourPWM signal is not sufficiently large you can try a circuit like the one below.
View attachment 187380

Since you are using PWM a better approach would be to use a high-side driver to assure that the MOSFET switches fast enough.
____________________________________________________________________________________
Right now my PWM is sending a Voltage signal that oscillates between 0 & 5V. How can I make the signal coming out of the PWM have negative voltage?
 

DickCappels

Joined Aug 21, 2008
5,875
In this case, it only needs to be negative with respect to the + power supply (and ground qualifies). Pay attention to the maximum gate-to-source voltage and make sure that the gate-to-source voltage is enough to assure adequate current to your load -you will find this in the MOSFET's datasheet.
 

Thread Starter

jonm2012

Joined Mar 22, 2017
3
In this case, it only needs to be negative with respect to the + power supply (and ground qualifies). Pay attention to the maximum gate-to-source voltage and make sure that the gate-to-source voltage is enough to assure adequate current to your load -you will find this in the MOSFET's datasheet.
Thank You.
 

BobTPH

Joined Jun 5, 2013
2,017
Using an N MOSFET is far simpler. Your original circuit works with an N MOSFET. With a P, you need a level translator like the circuit in post #2. So why do you want to use a P MOSFET?

Bob
 
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