Floating Mosfet Control?

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fred_b

Member
My circuit has a mosfet to switch a small fan on and off.

The problem is that the fan will sometimes come on when the control wire is not connected. (My design requires it to be disconnected at times).

I tried using a 100K and even a 33k resistor to pull down the pin on the mosfet but it will sometime still run the fan when the control wire is removed.

Could someone have a look at the circuit?
 

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B

BeanieBots

Moderator
As far as I can tell (no pinouts for FET and type not indicated) from that diagram, it looks OK.
If you have a long wire connected to the FET gate input it could be picking up enough noise to switch on with just 100k pull-down.
Try adding a 10k pull-down on the connector side (NOT a potential divider) of the 10k currently shown in series with the gate.
 
F

fred_b

Member
Try adding a 10k pull-down on the connector side (NOT a potential divider) of the 10k currently shown in series with the gate.
Click to expand...
Do you mean to replace the 100k with a 10k?

I now have a 10 k between the mosfet gate and ground. It works- sometimes. Other times, the fan just runs no matter what the signal.

Edit:

It seems to be consistently working now. Thanks for your advice.
 
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J

Jaguarjoe

Senior Member
fred_b said:
Do you mean to replace the 100k with a 10k?

I now have a 10 k between the mosfet gate and ground. It works- sometimes. Other times, the fan just runs no matter what the signal.
Click to expand...
No, I think he's saying put the 10k from pin6 of the 'axe to ground. Putting it where you did makes a voltage divider which cuts the mosfet drive in half making it perform marginally when told to turn on.
 
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Jaguarjoe

Senior Member
If you've got the 10k from pin 6 to ground you won't need that 100k at all. You can get rid of the 10k feeding the gate of the mosfet too.
 
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