If the FET you are using is a logic level like IRLR3410, you can use a 500
Ohm resistor which would limit your output pin current to about 10mA. Given
a gate charge of 34nC for the FET, the switching time for the FET would be
about 3.4uS (gate_charge/current_into_gate=switching time). If this is too
slow for the application, you can reduce the gate resistor further; the
constraints you have are the peak current your output pin can source (say 20
to 25mA for a PIC16F876). Also keep in mind that the faster you switch you
FET, the more the chances for a di/dt induced false turn on and other nasty
effects.
Madhu

>-----Original Message-----
>From: pic microcontroller discussion list
>
>I'm building something similar and was wondering what resistor to
>use for my gate on the MOSFET?
>
>Thanks,
>Tim
>
>>>> mapr@COMCAST.NET 12/22/03 04:17PM >>>
>Most, but not all, Solid State Relays (SSRs) are optoisolated; essentially
>an opto-isolator with a FET or a transistor output stage. You can get both
>AC and DC flavors of SSRs. Digikey has a bunch of pages of these. If an SSR
>is rated logic level then you can drive them directly from a PIC pin with a
>current limiting resistor. For the Solenoid valve you can just get a N
>channel logic level FEt and with a suitable gate resistor drive your
>solenoid to ground. For the high amp element you might consider a AC solid
>state relay that can be mounted to a panel.
>Madhu
>

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