Unable to get a MOSFET switch working

Thread Starter

deepak4you

Joined May 1, 2010
51
Hi,

I'm stuck with getting a simple MOSFET switch working and I really will be ashamed if I'm missing out on something so very basic.

So I want to create an electronic ON/OFF switch for my 1000W Electric Motor. To do so, I selected IRF4110 MOSFET. I thought it would be easy-peasy to get this work, but my brain has gone porridge to get this to work. Please refer to the diagram attached (excuse me for making it in powerpoint somehow for lack of time).

I built this circuit and tried to test it with LEDs (with a large value resistor) before connecting the motor. What happens is that even without any gate voltage, the LED lights up already. Now, this should not happen, right? The LED should turn on ONLY when I apply gate voltage? Unless like I said I'm missing something very very basic? And I've tried this circuit with IRFZ44, IRF540 and quite a few of them to eliminate any faulty FETs, but all of them are giving the same response.

P.S. I am an Electronics Engineer but not practicing it hence my skills might not be up to the mark, hence a possible simple basic fundamental miss in my design and approach.
 

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crutschow

Joined Mar 14, 2008
26,968
Yes, you appear to have made a classic newbie mistake with MOSFETs. ;)
No input is not a zero input.
The gate input of a MOSFET looks like a very high impedance capacitor, thus if there's no voltage applied, the gate can assume any value as determined by leakage currents, or just stay at the last voltage applied.
And, as you've determined, they all exhibit this behavior.

So you must either apply a voltage (high or low) or, if you want it off with no input, tie a resistor from gate to source (10kΩ for example) to drain off the gate charge and make Vgs =0.
That should solve your problem.
 

Thread Starter

deepak4you

Joined May 1, 2010
51
Ohh man! What the heck! :D I have been frying my brains over this. And none of the material on the internet mentioned this pull-down resistor! You can't believe what a relief this will bring to me when this gets working. Thanks a million tons!
 
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