piclist 2008\07\24\180854a >
Thread: FET question
www.piclist.com/techref/index.htm?key=fet+question
face picon face BY : Sean Breheny email (remove spam text)



Hi Mauricio,

A few questions:

1) Besides 3.3V and the mosfet drain, what else is connected to the
rabbit module? (I suspect there may be some kind of ground leakage
path)
2) What voltage is your PIC running at? (to determine how it is
driving the FET gate) I assume that the PIC shares the same ground as
the FET?
3) Have you tried measuring the voltage across the rabbit module's
power input (pos to neg) when the FET is turned on? (trying to
determine whether there is a static on-resistance problem with the
FET)
4) Does the rabbit module have large capacitors on its input or a
switching power supply which may have a high inrush current? (This may
cause the voltage to drop sharply at startup, which may in turn
trigger the module to shut itself down immediately)

Also, I'd suggest replacing the rabbit module with a 7 ohm resistor
(approx) to simulate a 450mA load, then turn on your FET and measure
how much voltage there is across the FET and across the resistor.

You might also try removing the 5.6K resistor and just driving the FET
gate directly from the PIC.

Sean


On Thu, Jul 24, 2008 at 5:50 PM, Mauricio Jancic <listasEraseMEspam.....janso.com.ar> wrote:
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