Uriah said:
I seem to be real slow in grasping these simple concepts. Another
question on Impedance.
What is low impedance? and what does it mean? Can you have low
impedance with high voltage or does it only apply to current? Is it
just a relative thing. When dealing with digital circuits does
low/high impedance imply certain things? What things?
thanks again
IMPEDANCE
The name says a lot. Impedance is the tendency of a device or circuit to
IMPEDE or resist current flow. It is not simply resistance but the "sum" of
REACTANCE and RESISTANCE. I put the word "sum" in quotes because impedance
is a vector sum of reactance and resistance not a simple arithmetic sum.
NOT SO QUICK REVIEW
While resistors resist current by dissipating power in the form of heat
(like the brakes in your car), reactive components such as inductors and
capacitors resist current by storing energy. In physics you should have
learned about two forms of energy: POTENTIAL ENERGY and KINETIC ENERGY.
Potential energy is energy possessed by things not due to motion such as the
energy contained in compressed springs, rocks on a cliff or even fire
crackers before exploding. Kinetic energy however is energy possessed by
things only by virtue of their motion such as moving automobiles, falling
rocks or exploding firecrackers. Reactive devices store energy in two ways:
1. Inductors store energy in the form of charge (usually electrons) motion
or current.
-Inductors store kinetic energy.
2. Capacitors store energy in the form of electrostatic force or voltage.
-Capacitors store potential energy.
To understand how a reactive device such as an inductor can impede current
think about a flywheel. A flywheel performs the same function in the
mechanical realm that inductors perform in the electronic realm. If you
attempt to spin a flywheel it resists being turned because you must first
overcome its inertia. Once you get a flywheel spinning however it contains
stored energy and you must first dissipate this stored energy before the
flywheel will stop. In fact you might get hurt trying to stop a flywheel all
at once by simply grabbing it. The same thing happens in inductors.
Inductors resist current flow because they posses a certain amount of
electrical "inertia" in the form of inductance. Because inductors store
kinetic energy you cannot stop the current through an inductor instantly.
Many electronic professionals and enthusiasts are aware of the sparks that
can fly at the switch when devices such as a motors, coils or electromagnets
are disconnected from their power source suddenly.
To understand how capacitors can impede current think about an air
compressor tank. An air compressor tank resists being pressurized because it
is storing energy. This energy can be retrieved later by releasing air from
the tank. You cannot change the pressure in the tank, to some arbitrary
pressure, instantly because you must push air into the tank at some finite
rate to change its pressure and the tank is constantly resisting attempts to
push more air into it. Anybody who's ever watched a pressure gage on an air
compressor, waiting for it to reach a certain pressure, knows you must wait.
All of the hot air above was to introduce the concept of reactance to those
who have problems with the concept. Remember there are three main types of
opposition to current flow:
1. Resistance - electrical energy converted to heat
2. Reactance - electrical energy stored in electrostatic form (capacitor) or
magnetic form (Inductor)
3. Impedance - combination of Resistance and Reactance (vector sum).
Dorian McIntire EE, PE
[email protected]