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Topic: [answered] What number tells you a MOSFET is Logic Level (Read 18796 times) previous topic - next topic

KeithRB

Which is why they invented the Vgs(on) spec which specifies (at a certain drain voltage and current) the voltage when the MOSFET can be considered "on".

Vgs(th) is more suited to SPICE modeling.

Jiggy-Ninja

Which is why they invented the Vgs(on) spec which specifies (at a certain drain voltage and current) the voltage when the MOSFET can be considered "on".

Vgs(th) is more suited to SPICE modeling.
I just checked all of my MOSFET datasheets* and none of them have that spec. Are you confusing it with RDS(on)?

*2N7000, DMG6968, DMG3415, BSS84, BSS138, FQP27P06 and FQP30N06L

MrAl

Only if you ignore the time that sunrise and sunset take. The transition between "unambiguous day" to "unambiguous night" actually takes time, and between those states there will be a period of uncertainty when some people might say it's day, and others say it's night.

Digital is an abstraction that we can use to simplify reasoning about certain kinds of systems that operate with specific constraints, but the underlying reality is always stubbornly and messily analog.

Transistors are analog devices. It is possible to use them in a digital manner by forcing them to always operate in saturation or cutoff, but in order to do that you need to know the analog boundaries between these modes and the linear region. The Threshold Voltage only gives you the upper bound of the cutoff region, it doesn't tell you the lower bound of saturation.

Stop saying stupid and wrong crap. MOSFETs do not suddenly transition from OFF to ON as the gate voltage rises, and teaching that to someone who doesn't know any better is just going to make them drive it incorrectly and needlessly destroy their parts.It's like you're not even trying to understand what Mike is writing.

A FET with a 5V threshold would indeed be completely unsuitable for Arduino use. I believe most logic level FETs actually quote a 4.5V theshold, and the graphs will show how much current they can actually handle with the extra 0.5V gate voltage.
Hello,

Some of your statements are not very reasonable.

In your first paragraph you state that you sometimes cant tell the difference between night and day.  So you are willing to NEVER be able to tell the difference between night and day just because in the morning we might have 1/2 hour where we might call it either and in the evening for 1/2 hour we might be able to call it either.  So yeah, let's say that we can never tell the difference because of those two times which total about 1/24 which is less than 5 percent of the time.
You see how ridiculous that looks now?

In your last paragraph you void what you said in the first paragraph by ACTUALLY BEING ABLE TO DETERMINE EXACTLY WHAT I HAD SAID TO START WITH while saying you cant do that.  Either you can tell the difference, or you cant, which is it?  You looked at Vth and determined that the device was unsuitable, yet you and Mr Grumps state that you CANT DO THAT FROM Vth.
So you are stating that you CANT do that, then you actually DO THAT.
So make up your mind, can you or cant you do that?

You see why i have to chuckle a little.

Also it would be nice if Mr Grumps would be a little more civil in his conversations.  If someone does not have respect for me FOR ANY REASON including but not limited to being wrong on some topic, then i cant have respect for them either.

What else you can do after you look at Vth is look at the other spec we had been talking about, which is the Vgs at various current levels.  I dont think anyone would disagree with that, but maybe you can find something wrong with that too :-)

You two better study this well because there will be a test on this on Friday :-)


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