Trinket Pro and MOSFET THAT WON'T TURN OFF!!!!!!

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
I know this has been talked about to death, but I can't seem to find an answer despite looking thru the better portion of the internets, and having an EE friend of mine review it.

I have a super simple circuit. Microcontroller to mosfet that isn't switching a load per say. It's kind of a cascading network of FETs. Eventually the FET will have a load, but not right off the bat.

I've attached an example of how I'm controlling just one FET in the network. The gate goes high and turns the N channel mosfet on. When the gate goes low however, the mosfet stays on despite having a pull down resistor.

The mosfet is a logic level so i'm driving it directly from the micro. I've attached some docs for more info.

Any help is greatly appreciated. My apologies for the crudeness of the schematic.
 

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Alec_t

Joined Sep 17, 2013
12,064
Welcome to AAC!
If the FET source is left floating, or goes sufficiently below ground, then the FET won't turn off.
Does the 'system' share the same ground as the micro?
The pull-down resistor needs to be between the gate and source.
 

Papabravo

Joined Feb 24, 2006
16,107
I know this has been talked about to death, but I can't seem to find an answer despite looking thru the better portion of the internets, and having an EE friend of mine review it.

I have a super simple circuit. Microcontroller to mosfet that isn't switching a load per say. It's kind of a cascading network of FETs. Eventually the FET will have a load, but not right off the bat.

I've attached an example of how I'm controlling just one FET in the network. The gate goes high and turns the N channel mosfet on. When the gate goes low however, the mosfet stays on despite having a pull down resistor.

The mosfet is a logic level so i'm driving it directly from the micro. I've attached some docs for more info.

Any help is greatly appreciated. My apologies for the crudeness of the schematic.
Without knowing what is connected to the Source and the Drain of the Logic Level MOSFET your circuit is ambiguous. Voltages on a device are with respect to each other not some absolute reference point. What is it about that concept that you do not fathom?

When the MOSFET is "ON", what is the voltage difference between the Gate and the Source. When the GATE signal is removed, what is the voltage difference between the GATE and the Source?
 

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
Thanks for the swift replies.

The source and drain will essentially be rerouting power in a bank of batteries (sorry that is about as much as I can say :(
Once the network is configured correctly the batteries can be discharged.

Yes they share the same ground

I fathom it all. I've never used a FET in this way and don't know why its not working. The Gate voltage is roughly 4.75v when on, and .01v when off (with respect to the micro ground.
 

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
Welcome to AAC!
If the FET source is left floating, or goes sufficiently below ground, then the FET won't turn off.
Does the 'system' share the same ground as the micro?
The pull-down resistor needs to be between the gate and source.

So currently, I'm merely trying to get a network of FETS to configure. No load on the circuit at all. If the pull down needs to be between the gate and source, then I need to complete the network and have a load in place for the FETs to OPEN correct?

FYI...I'm and ME by profession and an avid, though shade tree EE. I'm boot strapping a company and trying to work out a patentable idea. My apologies for the subject matter. I'm sure some of you are tired of explaining this.
 

Papabravo

Joined Feb 24, 2006
16,107
Thanks for the swift replies.

The source and drain will essentially be rerouting power in a bank of batteries (sorry that is about as much as I can say :(
Once the network is configured correctly the batteries can be discharged.

Yes they share the same ground

I fathom it all. I've never used a FET in this way and don't know why its not working. The Gate voltage is roughly 4.75v when on, and .01v when off (with respect to the micro ground.
Apparently you fathom very little. THE FET DOESN'T CARE ABOUT GROUND WITH RESPECT TO THE MICRO! Using a MOSFET in this way is probably NOT going to work. It is the WRONG type of part for the stated application. Too bad your secret project is unlikely to get off the ground if you cannot reveal the details.
 

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
THE FET DOESN'T CARE ABOUT GROUND WITH RESPECT TO THE MICRO! Using a MOSFET in this way is probably NOT going to work. It is the WRONG part.
SO WHAT WOULD YOU RECOMMEND?

The idea will get off the ground.

It took about 10 posts for the CAPs to come out. I hate "technical" help forums.

Sometimes you have to be ambiguous and confidential. It's called IP. It's what keeps us from becoming China.
 

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
You mean the technology leader in practically every field that matters?
No, I was referring to the fact that our IP is constantly being stolen. I understand they are the technological leader, that's not due to intelligence, its a lack of creativity and drive and the rampant negative criticism that come with interacting with other intelligent individuals.
 

Papabravo

Joined Feb 24, 2006
16,107
No, I was referring to the fact that our IP is constantly being stolen. I understand they are the technological leader, that's not due to intelligence, its a lack of creativity and drive.
I think that is a myth from the last century. People have been stealing things since the invention of the atomic bomb. What else is new?
Secrecy only confers a temporary advantage. Coming up with more and better ideas faster is the only lasting solution.
 

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
ha. no. I can jump online right now and order a Video slot machine I designed 3 years ago, from some ambiguous manufacturer in China for 1/10th the price. They have all the source material after 1 visit and one round of quoting with a stateside representative.
 

Papabravo

Joined Feb 24, 2006
16,107
ha. no. I can jump online right now and order a Video slot machine I designed 3 years ago, from some ambiguous manufacturer in China for 1/10th the price. They have all the source material after 1 visit and one round of quoting with a stateside representative.
Too bad. I guess it sucks to be you.
 

Thread Starter

Lithium Man

Joined Feb 15, 2021
9
Hey, thanks for all of the "technical" help with my question Pap Bravo.

I knew signing up for the site, I was kind of asking for it. Engineers are good for 2 things. the first being extremely helpful, innovative, collaborative. The second being elitest, condescending and arrogant to a fault.
 
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