Dave D wrote:

"Sam Goldwasser" wrote in message
...
"Dave D" writes:

A series resistor generates heat, diodes drop voltage and run cooler..
They
can be soldered into the fan leads then covered with heatshrink
sleeving. A
6V supply will speed these fans up.

Not quite. If you have a voltage drop and current flow, you have IV=P
generates the same heat. Doesn't matter if it is a diode or a resistor.
The difference is that the power dissipation is roughly proportional to
current rather than a square function because the voltage drop is more or
less constant.


Exactly, so the diode will run cooler, like I said :-) Naturally, the losses

It'll only run cooler than a resistor in the case of increasing the input
voltage to the fan-diode combination, but then, the fan starts to run faster,
which is goes against what the OP wants, so it's no longer doing its job!
Therefore, it's not efficient. So you add another diode to drop the extra
voltage, and guess what, you're back to dissipating the same as a resistor...
"Efficient" would be if the diode did the same thing as a resistor dropping .6V,
while dissipating less heat, which it doesn't. Given an input voltage, a given
fan, and a given target of reduced RPM, and you only have diodes and resistors
to do it with, you'll dissipate the *same* amount of heat in either case.
There's no 'efficiency' to using a diode... You *will* have to drop the *same*
voltage with either a resistor or a chain of diodes, and the current will be the
same in both cases. There's no magic about this. P=VI.