piclist 2003\02\23\075039a >
Thread: Pin won't turn off properly
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face BY : Morgan Olsson email (remove spam text)



Hej David Duffy. Tack för ditt meddelande 10:18 2003-02-23 enligt nedan:
>Are you
>suggesting
>that the LED's are connected to the port pins with no current limiting
>resistors?
>I don't design that way - maybe others do.

Example: driving large MOSFET
In most cases it is desireable to switch on/off fast.
Normally used gate drive resistor is about 30 ohms.
I have not done this with PIC but, I would think it is electrically feasable to connect the gate directly to PIC pin, and the pic internal resistans is adequate.  For the short time the current flows it is no problem with overload.
(yes it might cause transients on pic supply)
The gate har capacitance both to source and drain, which will load the pic output, both at turnon and turnoff, at a voltage that is the fet´s transfer region.  
so, the voltage change is delayed

Also, when the fet is turned off, transients on drain capacitively couples to gate, it the pic then do a RMW on that port it accidentally torns the tranny on, which may be a really bad thing, and might only happen seldom enough it is not noted until it the products are sold...

(for thoose naturally worrying if a high transient might damage the pic: the fet will turn on as normal and conduct to limit the peak at gate to normal turn on voltage.)

/Morgan

--
Morgan Olsson, Kivik, Sweden

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