Mosfet latching

Thread Starter

dvd280

Joined Nov 5, 2020
4
I am quite new to dealing with hardware at this level, so excuse me if my question is easily researchable - its just that I am not sure what to even look for.

I am working with an arduino uno, and trying to implement a button switch which allows me to control a current from an external 9v battery, by pressing a button.
I know that this could be done with a much simpler design (without the arduino at all), but I am mainly interested in understanding how mosfet works.
In the circuit photo (circuit works, pressing the button does activate the motor, and releasing does turn it off) , I have a resistor at the top left part of the circuit, my question is :
what is the function of this resistor?

I have tried removing it, and when I did it seemed like the transistor got stuck as as "on" (motor was still pulling power from the battery), even though there was no voltage coming from the "+" rail - which means that the connection was supposed to be severed. So I don't really understand how attaching a 10k resistor actually changes how the circuit behaves. The input voltages are 5V for the arduino, and 9V for the battery.
 

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jpanhalt

Joined Jan 18, 2008
11,088
Welcome to AAC.

A schematic would be easier to discuss. From what I can see, however, that resistor looks like a pull down resistor. That is, when the button is released, it pulls the gate back down to ground (-) and turns off the mosfet. Without that resistor, when you activate the button, you turn the gate on at 9+ volts, Releasing the button make the gate "float," and since the gate is a capacitor, it stays charged and on until it slowly discharges.

Put a small wire to the gate pin, and touch it with your finger. See if that turns it off when in the stuck on position, Your body capacitance should discharge it. Or, just touch that wire to your ground rail. If you do that, be sure the switch is not pressed.

What you describe is perfectly normal for an N-mosfet and with slight modification can be used for testing them. Search on how to test a mosfet.
 

Thread Starter

dvd280

Joined Nov 5, 2020
4
Welcome to AAC.

A schematic would be easier to discuss. From what I can see, however, that resistor looks like a pull down resistor. That is, when the button is released, it pulls the gate back down to ground (-) and turns off the mosfet. Without that resistor, when you activate the button, you turn the gate on at 9+ volts, Releasing the button make the gate "float," and since the gate is a capacitor, it stays charged and on until it slowly discharges.

Put a small wire to the gate pin, and touch it with your finger. See if that turns it off when in the stuck on position, Your body capacitance should discharge it. Or, just touch that wire to your ground rail. If you do that, be sure the switch is not pressed.

What you describe is perfectly normal for an N-mosfet and with slight modification can be used for testing them. Search on how to test a mosfet.
Thank you for the detailed reply, it clarified everything for me.
 
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