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slowed-down fan
040
K.S.M. Walraven
If, like many other people, you
have ever been annoyed by the
noise of, say, the extractor fan in
your bathroom, here’s a tip that
may quieten things down a bit.
The fans in bathrooms and
cooker hoods are normally
small ones that rotate at high
speed (but note that many
cooker hoods have a speed con-
trol). The idea is to displace many cubic feet of air at little cost.
Fortunately, the speed of these fans can be lowered fairly sim-
ply by placing a resistor in series with the motor. The impedance,
Z
, of the fan is calculated from
Rv
Z
=
U
2
/
P
,
M
where
P
is the rating of the fan, and
U
is the working voltage, nor-
mally the mains voltage. If, for instance, the rating of the fan is
33 W, and the mains voltage is 230 V, its impedance is 1600
, give
994004-11
or take an ohm.
The series resistor should have a value of about 1/3 of this
value, that is, 470 Ω or 560 Ω . Since it will have to dissipate about
10 W, it is advisable to use two 1 k
resistors, rated at 10 W, in par-
allel. In view of the heat produced in them, it is advisable not to
solder them to the motor connections, but to make the connec-
tions via a three-way terminal block.
[994004]
Elektor Electronics
7-8/99
65
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