n-chan fet, default state

[justDIY]

is it acceptable to use a "pull-up" resistor on the gate to set the default state of a N-Channel MOSFET to "ON" rather than the common "pull-down" which sets it by default to "OFF"?

mosfet in question IRL3711

"up" = +5V
"down" = 0V

 

[audioguru]

I couldn't find a datasheet for an IRL3711. It might need 10V on its gate to turn on.
A resistor pulling up its gate will cause it to turn on slowly, due to the high gate capacitance that needs to be charged by the resistor. With it turning on slowly, it will get hot if it is switching a load requiring a high current.

 

[justDIY]

sorry, my bad, side effect of posting after midnight

the part is an IRFU3711 .... had it confused with some smd fets I had IRL3714

stats:
Vgs 4.5v with an Rds of 6.7 mOhm (mili?)

I'm using the mosfet to pull-down the MCLR lines on a bunch of pics, keep them in reset until the 'master' pic is ready

The MCLR line is being pulled up to the 5v rail by a 10K resistor, and I have the gate of the mosfet pulled up to the 5v rail also by a 10k resistor, basicly so the master pic doesn't have to expend much current to pull the gate low. when the mosfet is "on" it over-rides the 10k pull-up on the MCLR and forces the line to 0V, keeping the attached pics in reset.

it seems to be working "on the breadboard", but I dont want to continue doing it this way if it's not viable for a long term design (that is, making a PCB, etc)

Since the 3711 is like $2 something a piece and rated for 110 amps, I'll probably be going with a slightly less 'beefy' and less expensive fet for the 'final' design ... or would a regular cheap 'ol silicon transistor work in situation?

IRFx3711:
https://www.electro-tech-online.com/custompdfs/2005/10/irfr3711.pdf

 

[Roff]

How many PICs are on the MCLR line?

 

[justDIY]

Ron:
currently there are just two, but I plan to finish with ten.

Update:

I took out my $2 fet and put in a 2 cent 2n2222 transistor (with a 10k resistor inline with the base... should probably be larger, but I have a ton of 10ks laying around)

the base (and the PIC port it is connected to) are being pulled to +5v by a 10k resistor

when the pic boots, it is able to actively force the port low, which turns off the transistor and the slaves come alive

I don't know how much current it takes to hold a pic in reset, but I imagine it is nanoamps? the current from the pull-up resistor on the MCLR line is really tiny if I did my math right.

I = V/R ... 5 / 10000 = 0.0005 amps?

 

[JimB]

MCLR line, 110 Amp FET? Mmmmm...

"Using a sledge hammer to crack a nut" comes to mind here. :lol:

My choice would be a simple small signal NPN, 2N2222, BC107 etc.

JimB


Edit
Looks like we were typing at the same time.

2N2222, they say "great minds think alike", they also say "fools seldom differ".

 

[Oznog]

I'm not sure why you need a transistor at all. Such strategies of relying on the reset line sound a bit like poor strategy for startup; software should be able to make something less hokey.

There's no need for a transistor as far as I can see either. If you have a 1k resistor between the reset pin and the master PIC's controlling pin, and a 50k from the reset pin to ground, it should do fine. The 50k will keep it low when the master PIC boots and its pins are tristated.