Military Vehicles, May 1997,: [MV] Re: Voltage reduction
[MV] Re: Voltage reduction
dunlops@racotek.com
Fri, 02 May 97 08:19:51
In general the techniques are the same as running 6V stuff on a 12V
system.
Non-critical stuff such as fans, wiper motors, lights, horns, and so
on, that always use the same amount of electricity, are best handled
with a dropping resistor as described in the earlier post.
The high power dissipation requirements for the resistors needed make
them hard to find from electronics suppliers. I have found that the
best bet is to get an ignition ballast resistor from the local NAPA,
Carquest, or what have you. They are rated in ohms, typically
anywhere from .75 ohms up to 2 ohms. Any ignition ballast will handle
enough power to drop the voltage from 24 to 12 (or from 12 to 6)
without overheating.
To get the right resistance, you can use two of them and hook them up
either in series (double the resistance) or parallel (half the
resistance). Sometimes you might need three hooked up together.
You can also get dropping resistors from the J C Whitney catalog where
they are sold as "voltage converters" even though they don't actually
sense voltage.
I usually start with a high resistance, hook everything up, and check
the voltage with a meter. If it is too low I start subtracting
resistance. This saves having to measure the amperage requirements
for the device, which requires an accurate and high capacity ammeter.
It also saves some math.
Be careful mounting the resistors. They get very hot. This is
especially a problem for things like fans that are run continuously
and less of a problem for things like horns that run intermittently.
Dropping resistors won't work for variable loads such as radios, car
phones, and so forth. You'll have to get an electronic converter.
Good suppliers for these include alternative energy suppliers (the
wind generator and photovoltaic crowd have the same problem as many of
their systems are 36 volt), and marine equipment suppliers (many boats
are 24V).
For the fan motors, you may be able to find a 24v motor that will fit
from an auto electric shop.
-Steve Dunlop
Minneapolis, MN
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Subject: [MV] Re: Voltage reduction
A voltage reduction from 24 to 12 volts can be obtained in two ways:
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).
....
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