At 08:19 AM 2/4/2004 -0800, you wrote: >Spehro Pefhany wrote: > > You need +10V (with this kind of MOSFET) from gate to source in order to > > turn the MOSFET on fully. To use a non-logic-level (10V) MOSFET as a high- > > > 2) Connect the load from the drain to +24, and use a logic-level > > MOSFET for direct-drive from the PIC (a gate resistor of ~1K > > is not a bad idea). This kind of MOSFET is specified with > > Vgs = 4.5V or lower. (look at the Rds(on) spec on the datasheet. >I'm looking at a Vgs(th) spec of 2.0-4.0 for the device I'm using, and I've >configured it as you've described above, and it does work. It *may* work, but it is ABSOLUTELY not guaranteed to work. Look more carefully at what Vgs(th) really means- it's specified as 2-4V (maximum) Vgs for 250 *microamps* of drain current. Admittedly, I have made the rash(?) assumption that your solenoid draws more than a few tens of uA. >FYI, in the working configuration (high side load) I measure a Vds voltage >of only about 150mV when it's ON (conducting), so I'm convinced the Vgs >threshold is met. See above. It's very poor design practice to use a device like this (outside the guaranteed area of operation) which is why I'm being a bit hard on you. Vgs(th) is more useful as a guarantee that it will NOT work (turn the solenoid on) with Vgs > 0, than it is as an indication that it will work with a power load. >My question was: why does this exact same drive configuration NOT work IF I >simply move the load to the "low" (i.e. Source) side of the MOSFET. I think I explained that, since the load voltage is subtracted from the Vgs.. but Michael Rigby-Jones' explanation maybe was better for you. 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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body