On Wed, 23 Jul 1997 10:49:11 -0500 stoney writes: >---------- >> From: Craig R. Autio >> To: PICLIST@MITVMA.MIT.EDU >> Subject: Re: Mosfet >> Date: Tuesday, July 22, 1997 9:17 PM >> >> ---------- >> > From: KORKMAZ SAiT ULAS >> > To: PICLIST@MITVMA.MIT.EDU >> > Subject: Mosfet >> > Date: Tuesday, July 22, 1997 8:00 AM >> > >> > Sorry but my electronic knowledge is very low . Anyway I've seen >a >> > digital thermometer schematics using Pic . The pic is using a >2N7000 >> > Mosfet in order to control three 7-segment led . So what is the >use of >> > mosfet here ?? >> > Is there a nice site giving details on electronic components >especially >> > Mosfet and DS Sensors ?? >> > >> Without knowing the configuration, it sounds like ground side >switching >> (enabling) for the display. Assuming other ports are driving the >segments, >> the total current could be significant if all segments are on (I >realize >> "888" should only occur during led test). If you have a bipolar >> transistor, you have to make sure you have enough base drive to >guarantee >> saturation (collector-emitter). In this case, any decent beta >transistor >> will suffice. By using a logic level fet (I think it is without >checking), >> you only provide gate drive and don't worry about anything else. Given a choice, it's usually better to use common-anode displays and pull-up devices for the digit drivers. This is because the PIC port's rated output low current, both per pin (25 mA) and total (150 mA), is more than the output high current ratings (20 mA and 100 mA). This allows higher segment currents and thus a brighter display. Using bipolar transistors (NPN for pull-up, PNP for pull-down) in a voltage follower (common collector) circuit for digit drivers eliminates the need for base resistors as well as making all drive current flow to the display. The drawback of this is about 0.8 to 1V of on-state voltage drop, which can be compensated for with the segment resistors. When driving multiplexed LEDs, turn them off while operating the ADC since all the current is likely to cause the PIC's ground and Vdd to be different from the external Vdd and ground, causing inaccurate readings. Also don't read-modify-write the ports as the heavy load may make the value read back unreliable.