# Mosfet Won't Turn Off

#### bumba000

Joined Oct 7, 2015
89
I have been over several threads in several forums including this forum. I can not seem to get the mosfet to turn off. What I have here is really quite simple, but then again maybe I am too. Please forgive my ignorance...

This is my mosfet; www.irf.com/product-info/datasheets/data/irlr6225pbf.pdf

This is what I've done with it

Thank you, John

#### dl324

Joined Mar 30, 2015
12,684
You have source and drain reversed. Body diode always forward biased.

Put in a current limit resistor or something will burn up.

#### bumba000

Joined Oct 7, 2015
89
wow. what an idiot. it works now! thank you.

#### dl324

Joined Mar 30, 2015
12,684
wow. what an idiot. it works now! thank you.
Don't be so hard on yourself. If you think about it, the terminal names don't make sense. Neither do the supply names. In a CMOS inverter, VDD is connected to the source of the PMOS device and VSS is connected to the source of the NMOS; so both supplies are actually source supplies...

#### bumba000

Joined Oct 7, 2015
89
Now it's correct but I need a resistor inline with the gate because my arduino gate trigger won't stop putting out 0.020v. If I use a pull down resistor it stops the led from lightly glowing, but then it doesn't work at all when I press my PWM button. So I've added the 10k inline resistor, but now I don't get enough power to the gate to get all the power I need from source to drain.

What else can I do?

#### dl324

Joined Mar 30, 2015
12,684
The transistor should be off with 0.02V on the gate.

What is your PWM frequency? Might be a problem with the gate cap.

#### bumba000

Joined Oct 7, 2015
89

#### dl324

Joined Mar 30, 2015
12,684
Try putting the 10K resistor between the gate and source. Connect your PWM signal directly to the gate.

Can you slow the PWM frequency to 10Hz? You'll be able to see the LED blink and gate cap shouldn't be an issue.

Have you inserted a current limit resistor for the LED?

#### bumba000

Joined Oct 7, 2015
89
I have tried the resistor from gate to source. It doesn't stop the LED from glowing when not pressing the button. The one thing I know for sure is that yes I can turn the PWM down but the super high PWM frequency is half the key to this whole project. The LED is only there for now so that I can see that I've wired the mosfet correctly. No current limit resistor. A current limit resistor would be on which side of the LED? I'm okay with 50/50's so I'll bite, on the negative side?

Thank you so much for holding my hand through this. I'm sure you have much better things you could be doing right now.

Thanks, John

#### dl324

Joined Mar 30, 2015
12,684
Since you don't have an oscilloscope to look at the signals on the MOSFET, decreasing the frequency would let you confirm visually that the transistor is switching.

Since you're using a power MOSFET, I assume you plan to switch a large current. I don't know what the drive capability of the Arduino is; you might need a buffer that can sink/source enough current to switch the MOSFET at your target frequency.

#### bumba000

Joined Oct 7, 2015
89
I have an oscilloscope, but it's sound card based. I made if from a DIY oscilloscope project I found online. Super simple but can only measure so high... I've turned the frequency down to 350hZ but the LED doesn't blink. It's just on when pressing the button. Yes, I plan to drive an inductor from an 18650 battery. What kind of buffer. Aluminum Polymer cap?

#### GopherT

Joined Nov 23, 2012
8,012
What is the source of your PWM? Is it an arduino or signal generator? Did you connect the negative cable of the signal generator (or arduino) to the negative terminal of your battery?

10k ohms is too high of resistance when running 100kHz to gate.
Try 10 ohms.

#### GopherT

Joined Nov 23, 2012
8,012
@bumba000

Which LED are you using?
What current is it rated?
What is the forward voltage?

What voltage is it at?

#### bumba000

Joined Oct 7, 2015
89
Not sure about the LED type, I pulled it out of some old charger. Battery's at about 4.17 now. Full would be 4.2, dead is 3.7.
Seems to be working with an LED. LED is off when not pressing PWM button. Press PWM button and the LED comes on and I see 4.2V across the circuit (at A & C of LED). Remove the LED and there's always 4.2, pressing the PWM button makes the voltage go down. What the heck?!

What is that you did with the 1hZ. I don't know what that symbol is. I don't think I have one of those... =0)

#### GopherT

Joined Nov 23, 2012
8,012
What is that you did with the 1hZ. I don't know what that symbol is. I don't think I have one of those... =0)
That is a 1Hz Pwm (your arduino)

#### bumba000

Joined Oct 7, 2015
89
That's what I though. Thank you. I'm back to 100kHZ. Didn't want to get too far away and lose me code.

#### GopherT

Joined Nov 23, 2012
8,012
I have been over several threads in several forums including this forum. I can not seem to get the mosfet to turn off. What I have here is really quite simple, but then again maybe I am too. Please forgive my ignorance...

This is my mosfet; www.irf.com/product-info/datasheets/data/irlr6225pbf.pdf

This is what I've done with it

Thank you, John

Connecting the mosfet backwards like this may have fried it.

#### Aleph(0)

Joined Mar 14, 2015
597
I have an oscilloscope, but it's sound card based. I made if from a DIY oscilloscope project I found online. Super simple but can only measure so high
I sorry if out of line but I say you doing things hard way! Good used scope cost less than only 500 dollars so why you having instrument barren lab? The further you go the more you need real test instruments! Just saying