First you can use a resistor in series with the load. What size resitor is
needed: There are two equation you need to use: Ohm's law (U=R*I)and the
power equation ( P=U*I). (*) states multiply. If the fan use 5Ampere on 12
Volts then the internal Resistance is 2.4 ohm (12 over 5). Since the
voltage on which you are going to use the fan is 24 volts, you need a
resistor that is 2.4 ohm. When this resistor is connected in series with
the fan, it uses an equal amount of energy as the fan. This energy has to
be removed. The Power is 5A multiplied by 12Volts equals 60Watts. That is
you need a resistor 2.4ohm and at 60Watts or more.
An other way is to use a voltage converter 24 to 12V. The converter must be
bigger in watts than the load. These converters use a switching technique:
It connects 24 volts to the fan for a very short period of time
(microseconds) and then it shuts off the 24 volts for an equal amount of
time. The converter does this repeatedly. Then the average voltage to the
load is 12 volts.
>From an energy point of view, the second method is the best. The efficiency
is about 80-90 percent. The first method is the cheapest, but it vastes
energy, and the generator has to produce more power.
Torfinn Sornes
42 Ford GPW
tsornes@online.no
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