On Thu, 9 Apr 1998 23:39:05 EDT FScalini writes: > Am I >safe to attach a PIC output pin directly to the gate of the mosfet or >is there >a safer way to do this. [...] >with the mA units (Id) so I'm guessing that's the drive current. But >those >values are ranging from about 100 to over 1000mA so I'm reluctant to >experiment with my one remaining functioning PIC. Id is the *drain* current, the current that the FET can control on the output side. The drive current at DC is very close to zero. But, there is a large capacitance from the gate to the source and drain which mean that rather heavy transient drive currents are required to turn a MOSFET on or off quickly. Usually a small resistor (22 ohms) is connected in series with the gate to help reduce instability from feedback through the internal capacitances. The series resitor will also reduce the current surge from the PIC pin when the logic level changes. >One more question: The MOSFET lowers it's resistance somewhat when I >apply 5V >to the gate but doesn't seem to saturate until i hit around 10V. The older type MOSFETs (such as the IRF510) require about 10V to turn on fully. "Logic Level" MOSFETs have been developed which require only 5V. For example, the IRL510 is similar to the IRF510 but has a lower turn-on voltage. It will work with 5V drive supplied directly from the PIC. The logic level devices tend to be a little more expensive and aren't available in the highest voltage / power ratings. >Whats the >typical (or most efficient/lowest cost/lowest hassle) way to use the >PIC and >the MOSFET to their full potential. Set the IRF510 aside and order some logic level FETs (as well as more PIC chips). Connect a resistor of about 22 ohms in series from the PIC to the gate. Also connect a resistor of about 10K from the gate to source (ground). When the PIC is first reset, the pins are inputs so the drive voltage to the gate is indeterminate (it is floating). The resistor to ground ensures the MOSFET will stay off until the PIC starts to drive it. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]