MOSFET Always On??

Thread Starter

Joeyhodson32

Joined Feb 6, 2021
4
Hi, I am having a problem where my MOSFET instantly starts conducting when I connect a power supply. I have the gate signal from an Arduino Nano, which is tied low. I am nearly 100% sure I have the drain and the source wired correctly. I have created somewhat of a rough schematic which I have provided also. The load draws A LOT of current (8A). I'm not sure what is causing this. Any input would be greatly appreciated. Inked_Rough_Schematic_LI.jpg
 

ericgibbs

Joined Jan 29, 2010
12,908
hi Joey,
You cannot control a MOSFET with a Load in the Source, from a 5V Gate voltage, it will never turn off the MOS.
E

The problem is as the MOS turns On, the voltage drop on the Source Load causes the Gate to Source voltage to reduce.
 

Thread Starter

Joeyhodson32

Joined Feb 6, 2021
4
hi Joey,
You cannot control a MOSFET with a Load in the Source, from a 5V Gate voltage, it will never turn off the MOS.
E

The problem is as the MOS turns On, the voltage drop on the Source Load causes the Gate to Source voltage to reduce.
That's interesting, I wasn't aware. How would you suggest I power my microcontroller? With another power supply?
 

Thread Starter

Joeyhodson32

Joined Feb 6, 2021
4
Hi Joey,
This sim shows the problem.
Note the Gate to Source volts, ,, try the Heater in the Drain circuit.

E
Updated;
Added a editing drawing and sim, using a 12R as the Heater
Wait is my heater not already in the drain? I am confused. The picture you added looks as if my heater would be in the source with the microcontroller, am I mistaken?
 

ericgibbs

Joined Jan 29, 2010
12,908
hi,
This image is a N MOSFET.

What is the part number of your MOSFET.?

Is it this type.? [ref image]

E


Update:
Is it possible you have the MOSFET wired upside down.?
If Yes, the internal diode will forward biassed hard ON.
 

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Thread Starter

Joeyhodson32

Joined Feb 6, 2021
4
hi,
This image is a N MOSFET.

What is the part number of your MOSFET.?

Is it this type.? [ref image]

E


Update:
Is it possible you have the MOSFET wired upside down.?
If Yes, the internal diode will forward biassed hard ON.
Yes, that's the one. I made a mistake on the schematic but not my hardware. I accidentally have the MOSFET upside down, creating more confusion. So the schematic shows me having the heater in the source (on the bottom) when in reality the MOSFET should be flipped, flipping the drain and the source. If you do that, that is how my hardware is actually setup. The image below should be correct now.fixed_mosfet.PNG
 
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Audioguru again

Joined Oct 21, 2019
3,186
Your new schematic shows an N-channel Mosfet upside down but if it was a P-channel Mosfet it will turn on when its gate gate to ground and turn off when its gate goes to +12V which will never happen.

I wish there was only one symbol of a Mosfet instead of many different ones.
 

BobTPH

Joined Jun 5, 2013
3,315
If this is an N channel MOSFET, you need to connect the source to ground, the drain to the heater snd the other side of the heater to V+. That is the only way it works without additional circuitry.

An N channel MOSFET has the arrow of the source pointing in. And an enhancement mode MOSFET shows the channel as three dashes. The symbol on your original schematic shows a P channel depletion mode MOSFET. Wrong on two of the two attributes that determine the four basic types of MOSFETs.

Bob
 
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