(new to this list, so I hope I'm not answering a Q that
someone has already answered)

The IRF510 requires 10V to guarantee full turn-on.
It may turn on somewhat at 5V... you will be better off with
an IRL510 (100V, .54 ohm) or an IRLZ14 (60V, .24 ohm), which
are "logic-level" FETs that turn on fully at 5V. (And IR even guarantees
on-resistance for some of them at 4V.) These are jellybean
MOSFETs available from DigiKey for 60-70 cents in single quantities.

Technically the "max" capacitance in the specs refers to
the max. capacitance you can hook up to the pin that Microchip
will still guarantee normal operation. A small MOSFET running
from 6V w/ gate tied directly to a PIC output shouldn't hurt a PIC
(although it's outside the mfr-guaranteed operation of the PIC), but
it's a larger-than-normal capacitive load; the PIC will most likely
provide as much output current as it can to pull the gate up,
and it'll just mean a longer than normal rise time.

(this is true for pretty much any digital chips w/ output tied directly
to a FET gate)

Strictly speaking, though, if you want the the behavior of the part
to be guaranteed correct, you should use a resistor or a MOSFET driver
to ease the load on a digital output. (i.e., what Tjaart said.) The
larger the resistor the slower the turn-on time; it should be
in the 100's of ohms.

Check out "The Art of Electronics" by Horowitz & Hill:
they are a great source for how to use FETs (and just about any other
electronic component) & will tell you stuff like putting a freewheeling
diode to prevent inductive kickbacks from the motor from damaging
your circuit.

Jason Sachs
Electrical Engineer

Deka Research & Development
340 Commercial Street
Manchester, NH 03101
(603) 669 5139 x327

> -----Original Message-----
> > > ---------- SNIP-------------
> > > I used the IFR510 to control a small RS motor from the basic stamp
> pin.
> > > Output was 5v and I put a 500K resistor in series with the gate. A
> > "micro"
> > > ampmeter did not show any gate current at all ???. I used a 6v
> lantern
> > > battery for motor power..... ==Mac==
> >
> > Why you need _SERIES_ resistor in MOSFET ???  You may shortly
> connect PIC's
> > output to gate.
>
> Nope, you can't. The maximum capacitance _directly_ connected to an
> output pin
> is something like 50pF (or was is 25pF - I can't remember). The inrush
> current
> into the MOSFET gate could cause a few nasties. A small series
> resistor keeps this
> down to an acceptable level. 5V/20mA = 250ohm minimum, so use a 270
> Ohm.
>
>