p.mulwitz(at)worldnet.att
Guest
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 Posted: Sat Dec 02, 2006 9:07 am Post subject: Ohm's Law, and when it counts. (Caution: Actual electronic |
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Hi Dave,
I am partially responsible for this whole line of discussion, so I
will try to explain the actual truth.
First, Ohm's law always applies to simple circuits like a battery
and light bulb with a switch and/or circuit breaker. The actual Law
is: I = E/R, and power is expressed as P=IE.
The letters are defined: I is Current; E is voltage (electromotive
force); R is resistance; and P is power.
You are absolutely correct when describing a simple circuit -- when
the voltage drops so does the current. The constant in this
discussion is the resistance of the light bulb. The power
specification for the light bulb is given at some specified (or
implied) voltage, but the bulb acts as a simple resistive load for the battery.
A "Switching Power Supply" is a good example of a load that presents
a constant power load rather than a constant resistance load. This
is the kind of circuit the power cord in your personal computer
connects to. It uses a special "Regulator" circuit to produce a
constant voltage output for your computer mother board (and other
devices in the computer box). If you give it a constant load and
reduce the line input voltage then it will draw more current from the
line to maintain constant power output. Similarly, if you increase
the load (for example by turning on a disk drive) and continue to
supply the same line voltage it will draw more current from the power
line to maintain a constant output voltage.
I think it is unlikely that you will find such a complicated load in
an aircraft landing light. Still, the start-up process in the
landing light could require more power and current than the steady
state load. This would probably last less than one second and might
require a larger circuit breaker or fuse for the circuit. This start
up surge in current probably is not significant when considering the
required generator or battery size.
A ballast is actually a simple transformer. You will find one of
these things in a fluorescent light fixture. This is used to convert
alternating current from one voltage to another. It cannot be used
in a direct current circuit like an aircraft landing light.
I only wish I could understand the electrical load presented by an
electric motor. Alas, I never figured out how to deal with that sort of gizmo.
Paul
XL fuselage
(recovering electrical engineer)
do not archive
At 03:38 AM 12/2/2006, you wrote:
| Quote: | maybe I'm missing a crucial point, but if the voltage drops, then so
does the current - Ohm's Law
Dave Johnson
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