Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: volts and amps



At 08:19 AM 7/14/00 -0400, Wilson J. Gonzalez-Espada wrote:

I always thought that an electric appliance was designed for 4.5 V
(for example) AND a particular current.

That statement is narrowly true, but may be misleading depending on context.

That is why I thought I could not connect a car battery on a 12V handheld
flashlight.

That statement is not correct, thereby illustrating the problem with the
previous statement. In fact, a flashlight battery rated for 12V will work
just fine on a car battery.

The converse would have made a better example: If you take a car headlight
and attach it to a 12V supply consisting of 8 C-cells in series, it isn't
going to work.

As Ludwik pointed out, a power supply can be characterized by its
a) open-circuit voltage, and
b) impedance.

For linear devices, this is equivalent to characterizing it by
a') open-circuit voltage, and
b') short-circuit current.

As they say, two points determine a line..... The word "Thevenin
equivalent" and "Norton equivalent" should be mentioned somewhere near here....

It is standard good practice to use a first-order model to describe real
voltage sources such as batteries. That is, the battery is modelled as an
_ideal_ constant-voltage voltage source in series with an ideal impedance.

.
.
.
V0 R0 . V1
[ideal voltage source]--------[impedance]---X----------|
| . |
| . |
| . [load] R1
| . |
| . |
| . ground |
|---------------------------------X----------|
.
.
.

where everything to the left of the dotted line represents the
battery. The X symbols mark the terminals of the battery, which is a
two-terminal device.

For homework, consider the open-circuit voltage (V0) to be fixed, and
consider the load (R1) to be fixed.
A) Calculate the battery-terminal voltage (V1) as a function of battery
impedance (R0).
B) Calculate the power delivered to the load as a function of battery
impedance.

Explain why, even though this is properly called a "linear circuit", each
of the foregoing functions is nonlinear.

Explain why, as Ludwik said, if the impedance is "low enough" making it
lower doesn't hurt anything.