Simple Mosfet (irf610) Switch is always on

Thread Starter

kavkav

Joined Jan 1, 2013
64
My irf610 just completes the circuit regardless of the gate signal. My LED indicated if it's flashing at the frequency of the gate signal. The gate signal is about 10 volts peak to peak AC square wave. I have tried off setting the gate signal so that it would be DC pulse but still nothing. I thought maybe I burned my mosfet so i tried a brand new one, still didn't work. My signal is coming from a function generator and its negative terminal is grounded to the power supply's ground which is grounded to the wall outlet.

my switch is wired with the simplest setup possible:
http://www.electronics-tutorials.ws/transistor/tran18.gif

My power supply is 24 volts and i have 110 ohm resistance so it should be giving approximately 20mA current.

It worked 3 weeks ago. I have no idea why it's not working now. I've been at this for 2 hours. It's very frustrating because I am trying to get this done so I can move forward with the circuit I am suppose to turn on and off.

Please don't give me a long pdf of everything about transistors I am only wondering in simple terms what the problem is.

Thanks.
 
Last edited:

#12

Joined Nov 30, 2010
18,217
That is not a circuit, it does not depict a MOSFET, and 24V/110 ohms is not 20 ma, it is 218 ma.

You can make it simpler by posting a circuit that has a mosfet in it. Preferably the circuit you are using.
 

#12

Joined Nov 30, 2010
18,217
It worked 3 weeks ago.
That tells me the circuit is valid. What changed? You already tried a new MOSFET. Maybe a wire became disconnected?

I am concerned that you do not show a resistor from the gate to ground to keep the gate turned off when not in use. That can allow static electricity to get in, and the weather has been winter dry lately where I am.

My approach is that something about the assembly of the circuit changed and you must find what changed.
 

Thread Starter

kavkav

Joined Jan 1, 2013
64
Is there a minimum current that must go through the drain source. Or is there a minimum frequency? The last time I did this it worked with 0.5hz and higher. I can't find what changed. I made another mistake though. My resistance is actually 182 ohms. But I don't see how that would cause it not to work unless there is a minimum current that must travel through the mosfet. Maybe my funciton generator is not giving out enough current to the gate. I kind of burned it and it doesn't give much of an output with high frequencies but but lower frequencies it works fine. Not as clean as it used to be but clean enough especially at frequencies below 500 hz. On top of that is gives 10V peak to peak. I tried DC pulsing it but that didn't make a difference either. Maybe to be on the safe side I will try my other function generator.
 

Thread Starter

kavkav

Joined Jan 1, 2013
64
leaving the gate open like that is just asking for things to go wrong....have a look here how you can drive your fet.....
I don't want to sound stubborn, but it just frustrates me as to why the simple set up does not work. I don't want to do anything fancy or difficult. I just want to turn something on and off. Then once I have mastered the simple setup. I will move on the something more difficult like what you have posted.
 
Are all of your grounds connected somewhere, that is the generator ground and circuit ground must be tied together? Your mosfet source should go back to the supply negative terminal. Your signal generator should also go back to the negative terminal of the supply.

Is that a big power resistor? What is it for? A previous post suggested a resistor from gate to source to turn mosfet OFF but that is way too low a value for that.
 

Thread Starter

kavkav

Joined Jan 1, 2013
64
Ground of the power supply is connected to the ground of the wall outlet. The source is connected to the negative terminal. The signal generator originally went back to the GROUND of the power supply. I tried the negative terminal instead but still didn't work. A resistor from gate to source, I just tried that with 300 kilo ohms. Still didn't work. The power resistor I put because the power supply is 60 watts and the originally I didn't have the other resistor so it was 7 watts.

MAYBE both mosfets are messed up. I'm afraid to try my last brand new mosfet because if it gets ruined somehow then I gotta go get new ones.
 

ScottWang

Joined Aug 23, 2012
7,004
When you using in 24V then the Rgs can be 20K.

You can do a testing that you can using a 3V/20mA LED in series with a 1.2k Resistor to replace the motor, and using a 1K resistor connecting to G, and another pin of resistor connecting to 24V, and disconnected from 24V to check the MOSFET does it working ok?
 
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sheldons

Joined Oct 26, 2011
613
I don't want to sound stubborn, but it just frustrates me as to why the simple set up does not work. I don't want to do anything fancy or difficult. I just want to turn something on and off. Then once I have mastered the simple setup. I will move on the something more difficult like what you have posted.
all you need is 1k-10k from the gate to ground,if needed a 0.1uF in parallel with the resistor and that will prevent the circuit from misbehaving with no signal applied
 
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