Doug Butler wrote:
>> Just to add some old/new words;
>>
>> As a LED switcher, a small transistor should not to be seen as a
>> current limiter or extra power heatsink, for this job a much lower
>> cost resistor is
>> recommended, except of course if the supplied voltage is not known,
>> so the transistor should be used also as a current limit device.
>
> Actually as long as you need a transistor, the incremental cost of
> making it a bigger transistor to handle the power as a current
> regulator is small.  It may be cheaper to use a bigger transistor
> than to add a resistor as a new component.  Also the bean counters
> tell us that reducing parts count improves reliability (though I have
> my doubts that is a usefull rule).
>
> Doug Butler
> Sherpa Engineering


As far as I understand, it will be no extra resistor,
since the emitter resistor would go to the collector,
except of course if you are just connecting the NPN
base directly to the uC port pin, what I would not
recommend at all - a simple short into the transistor
and you will end up with VDC-VLED volts via collector
-base short to the uC port pin, bang and smoke.

For this solution I use to choose a small FET unit,
no need any gate-uC resistor (it is already isolated),
and the rON is lower than a 2N2222.  Of course that
if any 7406/7 gate is available at the board, then
no transistor is needed (if the VDC is higher than
5V - or if 5V than any 74HC04 gate does the job).

W46NER

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