Re: Mosfet switch issues


"ian field" <dai.ode@xxxxxxxxxxxx> wrote in message
news:KFNVi.11323$6v.8868@xxxxxxxxxxxxxxxxxxxxxxx

"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> wrote in message
news:qkMVi.15926$lD6.4028@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I setup a mosfet to switch power to a pin on a PIC. The circuit works
only for low R.

Vcc ---> R ----> Drain + link to pic
Source ---> GND
Parallel port --> Gate

Gate voltage is about 3.5-4.5V.

Everything works fine with R about 2k but with 4.7k or higher it does
not. Vcc is 5V.

Why would this be the case? Surely 4.7k or even 10k isn't all that high
in this case? The pic(link to pic attached to the drain) is a schmitt
triggered input. So it shouldn't be drawing all that much current?

I'm not sure how to interpret the data*** but it has a section for
timing conditions and a schematic for "load conditions" with a pullup of
464 ohms and bypass cap of 50pf.

I imagine that if the capacitance of a pin is about 50pf, and lets say I
double it to 100pf, then a resistance of R = 4k gives me a 400ns rise
time and 1k is obviously 100ns. Surely both are reasonable?

Here I'm neglecting the speed of the parallel port but I imagine its
independent of what R is?

So what I'm really interested in is what is going on? What am I not
taking into account that is causing timing issues? I imagine it might be
the capacitance but I'm not sure. I just can't see why I would need to
supply so much current to something let a mosfet gate which is what I
imagine the input pins are. (I realize that for high speed switching it
requires more current but its a schmitt trigger so I should be able to
run it as slow as I want?)

Thanks,
Jon

Did you select a specifically "logic level" type MOSFET?

?

Its 2N7000. Shouldn't any general purpose small signal mosfet work?

It seems that the resistors are the key to getting it to work(in the sense
that it doesn't work if R is to large).

I can't understand why the resistance has to be so low in this case ;/ I can
get the circuit to do its job but its taking about 30mA max(on average its
probably only about 5mA I suppose) and that seems like way to much(To
program a pic using mosfets) ;/



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