Dan Michaels wrote:
>
> Alan B. Pearce wrote:
> >> I'm surprised at this! With all the professionals
> >> on this list, I have become used to the "never go
> >> outside the spec" attitude, like never exceeding
> >> clock frequency etc. I really don't like the idea
> >
> >Is it really that different to driving directly into the base of a transistor?
> >The impression I have is
>
> most
>
> people on this list would do that to drive a relay
> >or other high current load without a resistor between the output from the PIC
> >and the base of the transistor - we get enough messages on the list about
> >reading back the port state when doing this.
> >
>
> I think you should change "most" here to "some".
>
> It always gives me spinal-willies to hear about people doing
> this sort of thing. I always assume [perhaps wrongly] that
> these people are simply not trained as EE's.
>
> - danM


I agree Dan, the bulk of my practical experience is from
repairing things, although I have done a lot of design
over the years. The things I have to repair are always
due to engineers pushing the spec. Like using a 5amp
transistor to switch 5amps.

So if your little car has 150hp you are going to drive
everywhere with the motor screaming at peak hp at
redline towing a big caravan?? How long would it last?
About as long as your new Taiwanese VCR power supply does.

After my initial trade as an industrial electrician
I did a couple of years of industrial electronics,
a very practical training mainly about microprocessors
and how to interface them with everything from special
sensors to huge motors. We were taught that good
design over-rates parts at 10x. So you use a 10w
resistor if it is known to dissipate 1w. This gives
a long term reliability guaranteed to work 24 hours
7 days, even in a hot environment.

So anything less is simply trading reliability for
cost, if you don't care that a large percentage
of your product will fail within a couple of years
that's fine. Some poor sucker like me will end up
being paid to fix it, or should I say improve it,
because I will be putting a decent sized transistor
in it. ;o)

Back on topic, I feel uneasy driving a PIC pin
more than 10mA, I prefer about 5mA if I can.
I don't want it to fail in two years. Driving it
into a short gives me the heebie jeebies. How
much does a resistor cost? 1 cent?
-Roman

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