For low speed switching like your circuit shows, you need to pick a 
device based on:
	a) max current
	b) max voltage
	c) Rds-on

Rds-on will tell you dissipation for given load current. Get one with 
low Rds and the world is your oyster.

One thing your circuit needs: an output transistor to isolate the 
off-state gate voltage from the pic.  The gate needs to pull up to your 
SLA voltage (12?) to turn off.  If by some chance your SLA is much 
higher, you need to used resistors (as you showed) to limit gate voltage 
to something well under the typical rating of 20V.

By the way, I just implemented a similar circuit to cutoff loads to 
prevent excessive battery discharge.  I used a MAX8212 and a FDS6975 
fet.  The fet is rated at 6 amps, 30 volts, and about 40 mOhms.  So at 1 
amp, it dissipates 40 mW.

Jinx wrote:
> 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
> 
> TIA
> 


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist