Impedance Low/High

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 snipped-for-privacy@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"

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.

--
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.

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.

1. 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

1. Reactance - electrical energy stored in electrostatic form (capacitor) or magnetic form (Inductor)

2. Impedance - combination of Resistance and Reactance (vector sum).

Dorian McIntire EE, PE

snipped-for-privacy@bellsouth.net

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.

On Sat, 03 Jun 2006 12:18:54 -0400, Dorian McIntire wrote: ...

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

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

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

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

DC resistance is impedance.

John

the guy

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.

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

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.

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

--- 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.

-- John Fields Professional Circuit Designer

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

Indeed you should:-)

Kevin Aylward B.Sc. snipped-for-privacy@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"

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. snipped-for-privacy@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"

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 snipped-for-privacy@bellsouth.net