>> >> What are people using to drive small lamps and solenoids
>> >> in automotive applications? Are logic level FETs the way?
>> >> I need to interface a PIC with an auto-transmission ECU
>> >> and the gear select switch. The current needed to drive
>> >> the ECU and the associated gear indicator lamps looks to
>> >> be in the range of about 500mA but I'm aware of the nasty
>> >> spikes that can occur on automotive wiring so I'd like a
>> >> device that will take it.

Be aware that lamps have inrush currents when cold of many times their
warmed up state.
A factor of 10 is a good rule of thumb. Any switching device should be
able to tolerate such surges.

For ease of use FETs are great - and will usually be robust enough in
an automotive environment with proper design. Avalanche rated parts
helps transient survival and where necessary surge energy suppression
devices can be added. Use inverse protection diodes or similar when
needed. Be aware of the effect of such diodes on solenoid release
times.

If this is a one off or small volume job and cost is not a major
factor then use of protected "smart switches" can be considered. (They
also have a place in volume applications of course but cost more.) As
a trial I have tried turning these on while connected across the
terminals of a car battery. Perfect behaviour - shut down with no
damage. Some also provide fault mode detection by monitoring the
'gate' voltage which is clamped to ground during a fault condition.

For a 1 off job, a 100 volt or 200 volt 10 to 20 volt avalanche rated
FET costs relatively little and can be the basis for a reasonably
bulletproof switch. .

Just found this - looks useful
Types of inrush current and their magnitudes

            http://www.hsinda.com.tw/type%20of%20load%20and%20inrush%20current.htm

        Russell McMahon





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