First, Thanks for your replies. No, it's not for homework :-) it's a final power stage for real circuit that made us crazy for few days and now I understand why..cause of a too much gate voltage needed. that's the problem and you all helped. thanks again. ----- Original Message ----- From: "M. Adam Davis" To: "Microcontroller discussion list - Public." Sent: Thursday, March 15, 2007 2:08 AM Subject: Re: [EE] Simple Mosfet question > Sounds suspiciously like a homework problem... :-) > > The N-Channel mosfet shown switches "on" when a positive voltage > difference exists between the gate (middle terminal) and the source > (bottom terminal). > > When on, the mosfet is essentially a low resistance wire between the > Drain and the Source. (hand waving exists here). > > If the voltage potential between the gate and source is less than the > minimum needed for saturation, it will operate in the linear region, > and the resistance of the mosfet will vary according to the potential. > > So let's apply +24V to the gate of both circuits and see what happens. > > Circuit 1: > The mosfet sees a 24V positive voltage difference between the gate and > the source. It becomes a low resistance between the drain and the > source. > > The light turns on. Since the mosfet is a low resistance, and the > load is presumably a high resistance, then both the source and the > drain are close to ground in voltage potential - nearly all the > current is being consumed by the load. The mosfet stays on as the > gate and drain are still 24V apart. > > Circuit 2: > The Mosfet sees a 24V positive voltage difference between the gate and > the source. It becomes a low resistance between the drain and the > source. > > The light turns on. Since the mosfet is a low resistance, and the > load is presumably a high resistance, then both the source and the > drain *** are close to 24V ***. > > Thus, there is no longer a 24V difference between the gate and source. > > The mosfet therefore starts operating in the linear region - instead > of switching on or off, it's balanced somewhere inbetween. Where it > ends up depends on: > 1) resistance of load > 2) mosfet resistance in linear region > 3) other characteristics of the mosfet and load > > So the mosfet ends up drawing increased power and dumping it as heat. > If the load is reactive, they may start oscillating, perhaps to > destruction. > > Please note that I'm not an analog engineer, so you'll have to verify > everything I've just said, as it may well be incorrect, though the > general idea should be sound. Perhaps I've made your job harder. :-) > > -Adam > > > On 3/14/07, Tal Go wrote: >> Hi All >> >> Can you give me a short explanation why schematic (1) is the correct one? >> and what happen when using schematic (2)? >> >> Thanks >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> >> >> > > > -- > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - > - - - - - - - - - - - - - - - - - - - - - - - - - > Moving in southeast Michigan? Buy my house: http://ubasics.com/house/ > > Interested in electronics? Check out the projects at http://ubasics.com > > Building your own house? Check out http://ubasics.com/home/ > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist