AVR-Chat

At 01:49 AM 7/21/06 +0000, Ryan wrote:
>First, thank you for the incredible reply!  It will take me a day to
>chew it over.  I will use a shottky diode as you suggest.

First a Schottky diode will delay the turn off not quench the current.  The 
Schottky has a lower voltage drop and thus the current decays less 
quickly.  They do usually have faster reverse recovery characteristics but 
that really shouldn't come into play here.  For anything up to a 2-3A coil 
something simple like an MR852 works just fine, at least as long as you 
don't want MHz switching.  Faster turn-off or quench can be had with a 
snubber or a zener diode and regular diode connected back to back.

>The solenoid draws approximately 1 amp continuous.

Calculate your thermal budget.  That will be your killer more likely than 
the current ratings.  There are advantages to using a big FET and just not 
having to worry about it overloading.  At this kind of current a polyswitch 
should be ample short circuit protection.

>I have 3 questions for you or anyone else...
>
>1) What is the purpose of the 10 ohm gate resistor?

Limits current to the gate.  The FET gate is essentially a capacitor, 
limiting the current
         - reduces strain on the driver
         - controls (somewhat) the turn-on time of the FET and thus EMI 
from the switching
         - with paralleled FETS it is also used to prevent the FETS from 
oscillating at turn-on


>2) What is the purpose of the ferrite bead?

They can be used for EMI, I've not used one myself.

>3) Are there any benefits towards using a "mosfet driver"?  (When
>would one be required?)

Depends, what are you using to drive it now?  The biggest advantage is you 
get a pre-packaged high current drive optimised for driving FET gates, 
sometimes including overcurrent protection.  The disadvantages are space 
and cost.  If you are not PWMing it high current drive is less important 
since you are less likely to need high edge rates.  Slow edge == less EMI 
== more heat on turn-on and turn-off

Robert

http://www.aeolusdevelopment.com/

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