On Sat, June 18, 2011 9:36 pm, Dwayne Reid wrote:
> At 03:45 PM 6/18/2011, David wrote:
>
>>I wish to drive a IR LED at 40khz (essentially the Sony IR protocol),
>>somewhere between 50mA and 100mA max LED Ifwd.  In testing I have been
>>using a 2N7000.
>>
>>I am confused around whether or not to use resistors, there is a lot of
>>conflicting advice online.  The options seem roughly to be:
>
> Use a series resistor mounted right at the FET if you have long leads
> on either the gate or source.  Reason: the FET will want to oscillate
> at very high frequency and that is destructive if you are switching
> large amounts of power.
>
> If the FET is mounted within a small distance from the PIC, series
> resistor probably not needed.
>
> Use a pull-down resistor if there is any chance that the PIC pin that
> drives the FET will ever be floating for a significant amount of
> time.  Example: pulling the PIC out of its socket for re-programming
> while power is still applied.
>
> If you can guarantee that the FET gate won't ever be left floating,
> the pull-down resistor can be omitted.

I'd like to add- if you have the room on the board, never omit the pull
down. Floating inputs on gates are way too unpredictable.

PICs are getting very fast outputs so "small distance" is getting smaller.
I have measured rise-times on the order of 700ps (yes, *picoseconds*) on
the output of the PIC32. With rise-times that short, ringing *will*
happen, particularly with a capacitive loads like fets, and in the worst
case the ringing will turn the fet on and off. A series resistor gives you
some control over the risetime.

Matt Bennett
Just outside of Austin, TX
30.51,-97.91

The views I express are my own, not that of my employer, a large
multinational corporation that you are familiar with.
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