David wrote:
> 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 PI=
C=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.
>  =20
You should always add a pull-down if you need the FET to be off during=20
initial power-up. Mosfet gates are effectively capacitors and therefore=20
leaving them floating is a bad idea if you want predictable behaviour.
> 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

--=20
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