A E wrote:
Dave D wrote:

a series resistor works nicely, as does
running them from a stiff 6V supply.

A series resistor generates heat, diodes drop voltage and run
cooler.. They

So, you're saying with the same VI, a diode generates less heat
than a resistor???????????


Nope, see Sam's post. It will always drop a relatively fixed
voltage, regardless of current draw, so it is more efficient than a
resistor in this application.

Dave

Makes no sense. You will simply not draw more current through that
diode than with a resistor, because the load is the same... If you
put a resistor in series with the fan, and the resistor happens to
drop .6V (simple to figure out, if you can draw a load line,
otherwise a bit of experimenting), the same amount of current will go
through that resistor as through a diode... Same heat dissipation in
both cases. Don't you think?

The major drawback with resistors is that the v drop depends on current, and
at start-up current is high, so v drop is high - the supply is anything but
"stiff". Aiming for say 7v on a 12v fan, the fan may not even get going.
Diodes drop a fairly steady voltage over the range of fan currents likely.