All,

I have been going round in circles today considering how to drive a=20
logic level FET.  I have tried a few things on breadboard but don't=20
trust the results, many of the weird observations could be to do with=20
the breadboard.

I wish to drive a IR LED at 40khz (essentially the Sony IR protocol),=20
somewhere between 50mA and 100mA max LED Ifwd.  In testing I have been=20
using a 2N7000.

I am confused around whether or not to use resistors, there is a lot of=20
conflicting advice online.  The options seem roughly to be:

A) Connect the gate directly to the PIC pin, no resistor.  Lower parts=20
count, which is always useful.

B) Use a low (100Ohm or similar) resistor in series between PIC output=20
and the FET.  Provides some protection for the PIC.

And for an alternative problem:

Y) No resistor between gate and GND, once again lower parts count and no=20
worry of the PIC having to source more current when pulling the pin high.

Z) A high (>=3D10k) resistor between gate and GND.  Most useful if the PIC=
=20
could ever be disconnected from the FET (which it cannot in this=20
instance).  However, also provides some protection during ICSP, power on=20
(before TRIS is set) and will ensure the FET always turns off.

I am not particularly tight on space nor constrained by a few extra=20
pennies, but it would be good to know which combination of (A|B)+(Y|Z)=20
is correct and for what reasons.

David
--=20
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