27 Jun
2012
27 Jun
'12
4:53 a.m.
On 06/27/2012 12:42 AM, Chris Tofu wrote:
So in laymans terms you vary the speed by narrowing or
widening the
pulses delivered on the positive lead (duty cycle). A steady 12vdc
and you get max air flow/speed? The tach lead is registered by
something on the mobo, and otherwise has nothing to do with the
rotation of the blades?
This is usually the case, yes. Visualize PWM...perform an integration
over several cycles, to take the area under the curve. The energy
delivered to the load varies with the pulse width. But the switching
devices (usually MOSFETs) are going from cutoff to saturation and back,
never operating in active mode for more than a few microseconds at a
time, and so dissipating very little power, thus running VERY efficiently.
Many Sun systems drive their fans this way. Actually many high-end
server systems from many manufacturers drive their fans this way, as
does some PC hardware. It's very common.
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA