how much voltage to gate a pFET: Vsource = 5v, Vgate = 0v/3v

[mikewax]

that's AWESOME i have those parts on hand. now i can proceed with the circuit. THANX a bunch i tried an open drain arrangement with an nFET and couldn't get it to work.
you rock!

 

[audioguru]

Why do so many people look at the "Threshold Voltage" of a Mosfet and think that since it says 2V to 4V then it will conduct lots of current when its Vgs is only 3V??
The threshold voltage is when a Mosfet is almost turned OFF and is barely turned on. If it is 2V to 4V the some of those Mosfets conduct only 0.25mA when the Vgs is 4V.

 

[mikewax]

Can I suggest that...
spec

Hello spec, back from vacation. THANX a bunch the open collector driver worked perfectly to gate the pFET and i got everything working.

 

[crutschow]

Why do so many people look at the "Threshold Voltage" of a Mosfet and think that since it says 2V to 4V then it will conduct lots of current when its Vgs is only 3V??

I've had that same question many times.
Even when you tell them otherwise, that they need to look at the voltage used to specify the Ron value, they still want to refer to Vth as a voltage of interest when using the MOSFET as a switch.
Apparently they think that Vth is some type of digital threshold, where anything above that fully turns the MOSFET on.
Oh well, I guess all we can do is keep preaching..

 

[spec]

Hello spec, back from vacation. THANX a bunch the open collector driver worked perfectly to gate the pFET and i got everything working.

Good news- hope you had a good vacation.

spec

 

[spec]

I've had that same question many times.
Even when you tell them otherwise, that they need to look at the voltage used to specify the Ron value, they still want to refer to Vth as a voltage of interest when using the MOSFET as a switch.
Apparently they think that Vth is some type of digital threshold, where anything above that fully turns the MOSFET on.
Oh well, I guess all we can do is keep preaching..

The maximum gate threshold voltage of a MOSFET is a fundamentally important parameter, just like many other parameters: Rdss, Tjmax, RthJC, Cg, SOA, and so on.

spec

 

[mikewax]

The maximum gate threshold voltage of a MOSFET is a fundamentally important parameter, just like many other parameters: Rdss, Tjmax, RthJC, Cg and so on.
spec

i figure every student reaches a point where they realise that when you're down to discrete components, there's really no such thing as purely digital electronics, unless you're using relays. my understanding (correct me if i'm wrong) is that Rds is a curve and that the Vgs is a key indicator of the shape and displacement of that curve.
isn't it true that sometimes you just gotta check the data sheet and extrapolate from insufficient information?

 

[spec]

i figure every student reaches a point where they realise that when you're down to discrete components, there's really no such thing as purely digital electronics, unless you're using relays. my understanding (correct me if i'm wrong) is that Rds is a curve and that the Vgs is a key indicator of the shape and displacement of that curve.
isn't it true that sometimes you just gotta check the data sheet and extrapolate from insufficient information?

So true. It is rare that any data sheet is complete, and anyway you would need one hell of a load of work to take into account all the variables- which you would do in a worst-case safety-critical design.

For example the Rdss of a MOSFET is dependent on, gate voltage, drain current, drain voltage and channel temperature. As a rule of thumb, you need to double the specification sheet RDss max for a typical power MOSFET application as the channel temperature is liable to be high almost by definition.

The overriding factor is whether you go for a worst case design or a design based on average parameters and accept the penalty that every so often a particular sample of a device will not work.

In the case of this thread, the OP had already stated that that his particular MOSFET was turning on OK with a relatively low gate voltage.

spec

 

[mikewax]

In the case of this thread, the OP had already stated that that his particular MOSFET was turning on OK with a relatively low gate voltage.

it is. i'm now getting the same circuit performance that i had when i had a 5v MCU.

 

[crutschow]

The maximum gate threshold voltage of a MOSFET is a fundamentally important parameter, just like many other parameters: Rdss, Tjmax, RthJC, Cg, SOA, and so on

No argument there.
But Vgs(th) max is not what should be used to determine the voltage needed to reliably use the MOSFET as a low on-resistance switch.

 

[spec]

No argument there.
But Vgs(th) max is not what should be used to determine the voltage needed to reliably use the MOSFET as a low on-resistance switch.

And no argument there either.

spec