>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Jinx
>Sent: 16 January 2007 20:51
>To: pic microcontroller discussion list
>Subject: [EE] B+ disconnect
>
>
>Can anyone recommend an on-line crash-course in FETs ? I 
>hadn't realised how many types there are until I had to look
>
>Attached is what I want to do. Basically the 12F monitors the 
>battery and disconnects it from some loads when too low. An 
>over-ride comes from another part of the circuit to stop the 
>disconnect happening if a particular process is in progress
>
>I found a B+ disconnect here, using the BSP171, described as
>a SIPMOS FET in its pdf
>
>http://www.edn.com/archives/1994/090194/18di1.htm
>
>http://www.edn.com/archives/1994/090194/graph/18di1fg1.htm
>
>My problem is choosing a FET based on what voltage will work
>on its gate. The only other time I used big P-ch FETs was as 
>the top half of a FET H-bridge. In that case 2 x V+ was needed 
>to turn them on
>
>In this circuit I don't want to fiddle about making another 
>voltage for the gate (although the 12F would be capable of 
>that), so would like to be edumacated about FETs so I can pick 
>one the engineer's way instead of wussing out and going 
>bipolar with a TIP

Joe,

You need another transistor to drive that P-Channel MOSFET, otherwise you may never be able to turn it off!  Look at the current path from the 12v battery, through the two resistors connected to the FET's gate, and then into the poor little PIC, where it will switch on the protection diode, i.e. voltage on the PIC pin will be clamped to Vdd+0.6volts.

A small NPN would be suitable to drive the gate, with the collector connected to the existing resistors, the PIC driving a base (via a current limiting resistor) and the emitter grounded.

The figure you are interested in for the MOSFET is the Vgs voltage required for the maximum Drain current you expect to see, with an acceptable Vds (i.e. voltage drop accross Drain-Source).  Any MOSFET datasheet will have a chart showing how these numbers relate.  With 12v to spare, you are in luck as you have enough voltage swing to saturate just about any MOSFET you are likely to use. However, be very carefull not to exceed Vgs(max) which is often in the 10-12v range.  Carefull selection of the two resistors shown in your schematic connected to the gate will limit the maximum gate voltage.

HTH

Mike

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