At 07:16 PM 12/5/2009, you wrote:

>This polarity reversal of the FET was covered in the very clever 2
>transistor plus FET circuit cited in this thread. Note that the 2N7000
>MOSFET that they used is a very low spec device - it has highish gate
>turn on voltage and undesirably high on resistance. As a demonstrator
>of the concept it may be OK but, if you are serious about using that
>circuit to get a near zero voltage PV panel switch, then there are
>many many better FETs available.

Careful. I would suggest that the only reason it (typically) works is
*because* the 2N7000 is so crappy. The reverse current through the PV
times the Rds(on) must be high enough to reliably switch the 2N3906 off-
say +20mV assuming minimal temperature gradient between the PNP and NPN
(not necessarily a good assumption). If the design
limit for reverse current is 10-20mA then the Rds(on) must be no *lower*
than 0.5 - 1 ohm. Min Rds(on) is not usually specified, but *typical* value
for the 2N7000 is about 2 ohms with 4V Vgs.

IOW, it's barely bad enough. ;-)

Put a decent MOSFET in there and it will most likely never switch off!

Of course it would be much more controlled if a good low-voltage logic-
level MOSFET was used with a series resistor.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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