# Impedance Low/High

Discussion in 'Electronic Basics' started by Uriah, Jun 3, 2006.

1. ### UriahGuest

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

2. ### Kevin AylwardGuest

Impedance, simplified, is just a characterisation number of a component
that determines what current is caused by an applied voltage.

It is resistance (impedance) R = V/I, V is the applied voltage, I is
the current.

There is a little complication for AC circuits, which is where
resistance -> impedance, but this does not change the basic concept of
resistance/impedance.

Impedance can be any value from 0 - infinity. So, yes you can have low
impedance and high voltage. This will produce a high current.

Impedance can effect the speed and frequency response of a system. The
basic figure of merit is T=RC, with F~1/T. C is the capacitance. Lower
impedances imply faster speed, and more current.

There is a general principle here. That is, for a given accuracy, if you
increases speed, you need to increase the current. This is expresed by
F(error, speed, power) = 0

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

3. ### Ralph MoweryGuest

It is a relative thing depending on who is doing the talking and what the
subject is. For audio circuits (microphones) low impedance may imply a mic
that has a dynamic element and an impedance of under 50,000 ohms, more like
under 1000 ohms. A high impedance may imply a crystal element and an
impedance of over 1 meg ohms. Lots of times you can drive a high impedance
load with a low impedance source and it will work. If you try to drive a
low impedance load with a high impedance source, the source will be
overloaded and not do very well. You may think of it as a battery. If you
put several AA batteries in series to get 12 volts and compare it to a car
battery at 12 volts , the car battery will have a lower impedance than the
AA batteries as it can deliver more current due to the internal impedance of
the batteries.

You can have high voltage and high current at the same time, but that takes
lots of power. Usually low impedance indicates low voltage and high current
and high impedance indicates the opposite.

With digital circuits it does come into play. You may see it as fan in and
fan out. A low impedance output can drive a small amount of low impedance
loads, but a greater number of high impedance loads. As a general rule the
low impedance circuits are less subseptical to random noise than the high
impedance circuits.

4. ### John FieldsGuest

---
Impedance is nothing more than resistance to the flow of charge.

Likening it to the flow of water in hoses with equal lengths but
different diameters connected to sources of water with the same
heads, the smaller diameter hose will allow less water to flow,
therefore it presents a higher impedance to the source than the
larger diameter hose.

5. ### Dorian McIntireGuest

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

6. ### John PopelishGuest

Impedance is a word that means the frequency dependent ratio of the
voltage applied across two nodes divided by the current that voltage
forces through those two nodes. If frequency doesn't make any
difference, the impedance is a resistance.

Low(er) impedance implies a relatively lower volts per ampere, while
high(er) impedance implies a relatively higher volts per ampere, at
some frequency.

For example, an 8 ohm impedance speaker takes 8 volts across it to
force an ampere through it at some specified audio frequency, perhaps
1 kHz.

7. ### Rich GriseGuest

.
Dewd! I like this a _lot_!   The way I heard it once, "An inductor opposes a change in current, a
capacitor opposes a change in voltage".

Thanks!
Rich

8. ### Dorian McIntireGuest

I wanted to add to my last post that impedance and reactance, as a general
rule, are only observed in AC and not DC circuits. Resistance alone is only
observed in steady state (nothing changing) DC circuits. Reactance and
impedance effects can only be observed in DC circuits during transients such
as when power is first applied or the voltage is changed suddenly.

The key thing to remember is that changes have to occur in a circuit for
reactance or impedance effects to be observed.

Dorian McIntire

9. ### John LarkinGuest

If there's a voltage source of X volts, and you hook a load to it, if
the voltage droops just a little, it's a low impedance source. If it
loses a lot of voltage, it's a high impedance source.

If you connect your car battery to the starter motor, it might drop
from 12 to 8 volts with the 100 amp starter load; that's a source
impedance of 4/100 = 0.04 ohms (it lost 4 volts when it was delivering
100 amps.) If you connected a string of eight AAA batteries (also 12
volts at no load) to the starter motor, you'd see near zero volts,
maybe an amp or so of current, and the car *would not* start.

Imagine a tiny motor spinning 1000 RPM, and a huge motor also spinning
1000 RPM. Now imagine grabbing the shaft of the little one, then
grabbing the shaft of the big one. You could feel the lower mechanical
impedance of the big motor; it might rip your hand off, but it
wouldn't slow down much; it just doesn't care. It's the difference
between being hit by a pillow going 30 MPH, or a cement truck going 30
MPH. Same thing.

John

10. ### Pooh BearGuest

You wanted to *add* some more ? Goodness.

My definition of impedance for newbies is 'like resistance but takes into
account the effect on ac signals of inductance and capacitance too'. If the guy
wants a lecture he can always ask.

Graham

11. ### John LarkinGuest

DC resistance is impedance.

John

12. ### Ralph MoweryGuest

DC resistance is resistance, not impedance. AC has impedance. If you have
a coil, it will have a DC resistance and an AC impedance which will
normally be much differant.
The same with a capacitor, it will have a DC resistance (normally very high
for low values) and an AC impedance.

13. ### John LarkinGuest

Gosh, that's not what they taught me in engineering school. They
claimed that impedance is the vector sum of resistance and reactance.

Thanks for straightening me out. I'll ask for my money back.

John

14. ### John PopelishGuest

I think you have misunderstood impedance. Impedance is a two
dimensional value and frequency dependent. Resistance is a special
case of impedance, where the second dimension (the imaginary part that
is frequency dependent) is zero. Pure inductance or pure capacitance
have an impedance that has no real part (that dimension has a zero
value), but only an imaginary part (that is frequency dependent).

Impedance is a general concept that includes all combinations of
resistance and inductive or capacitive reactance. Impedance carries
an implied if not stated frequency dependence, except for the case of
pure resistance, which is frequency independent.

15. ### Dorian McIntireGuest

Yes, learning is like that. You must use at least a few words to explain
things and I choose very few words for this particular topic. You haven't
experienced a lecture if you think my post was a lecture. If people want to
learn they have to watch, listen and realize there are no short cuts.

If the original poster or any others for that matter only want quick,
useless, opinions about electronics they just need to let me know and I'll
stop posting.

Saying something takes into account the "effect" of something doesn't quite
explain what the "effect" is. Does it?

Since you mentioned it the effects are as follows:

The impedance for a perfect capacitor (no leakage) approaches infinity (open
circuit) as the frequency goes to zero (DC) and its impedance approaches
zero (short circuit) as the frequency approaches infinity.

What the above really means is that a capacitor is an open circuit for DC
and just the opposite for extremely high frequencies.

The impedance for a perfect inductor (zero resistance) approaches zero
(short circuit) as the frequency approaches zero (DC) and its impedance
approaches infinity (open circuit) as the frequency approaches infinity.

What the above really means is that an inductor is a short circuit for DC
and just the opposite for extremely high frequencies

In practice both capacitors and inductors have resistances and other
properties that make them less than perfect.

Dorian

16. ### John FieldsGuest

---
Wrong. Impedance is defined as:

Z = sqrt (R² + (Xl - Xc)²),

and, since capacitive reactance is defined as:

1
Xc = -------
2pifC

and inductive reactance is defined as:

Xl = 2pifL,

at DC, then, the "f" term will be zero, so Xc will be infinite and
Xl will be zero. Plugging that into:

Z² = R² + (Xl - Xc)²,

gives us:

Z² = R² + (0 - infinity)²,

= R² + infinity

= infinity,

So Z = sqrt (infinity)

= infinity.

Looking at it another way, with DC in the circuit the capacitor will
not pass any current other than the initial transient and whatever
leakage current manages to get through it, in series with the
resistance of the resistor and the inductor, so the
resistance/impedance of the circuit at DC is essentially infinite
ohms.

In a parallel circuit, DC will flow through the inductor and will be
limited only by the resistance of the inductor in parallel with the
resistance of the resistor and the ESR of the capacitor, so the
impedance/resistance of that circuit at DC will be essentially zero
ohms.

17. ### Dorian McIntireGuest

John,

Looks like your second post and my third post almost crossed in the system
and say essentially the same thing expressed two different ways.

Dorian

18. ### Kevin AylwardGuest

Indeed you should Kevin Aylward B.Sc.

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

19. ### Kevin AylwardGuest

Er... this is not accurate. Impedance is indeed observed in DC circuits.
Impedance is a *generlised* term that covers both DC and AC at *any*
frequency. The DC condition is when f=0.

To wit:

Z = R + jXz

Now set jXz to zero.

Resistance is an impedance, but all impedances are not resistances.
Er... no...see above.
The key thing is remembering the actual definition of impedance Kevin Aylward B.Sc.

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

20. ### Dorian McIntireGuest

Setting the imaginary term to zero leaves only resistance which is what I
stated can only be observed in DC circuits. This is merely a semantic
argument so I'll leave you with the Wikipedia definition for impedance (not
my first choice but readily available):

"Electrical impedance, (sometimes Electric impedance) or simply impedance,
is a measure of opposition to a sinusoidal electric current. The concept of
electrical impedance generalizes Ohm's law to AC circuit analysis. Unlike
electrical resistance, the impedance of an electric circuit can be a complex
number. Oliver Heaviside coined the term impedance in July of 1886"

Dorian McIntire EE, PE

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