Impedance Help!

[windozeuser]

Hello again fellow board members

I have a problem I never Really got the concept of Impedance I was wondering if someone could put it into clear easy language

Thanks!

 

[samcheetah]

i hope thats a typo and you meant "hello" :lol:

impedance is the amount of opposition that a network offers to the flow of current. impedance is just like resistance but impedance is a function of frequency. the frequency dependency comes from the fact that capacitors and inductors are frequency dependent resistors. so if you have a network comprising of two or three resistors the equivalent will be called resistance. but when there are reactive components in the circuit the resistance will have a real part and an imaginary part. the real part is the one that dissipates power while the imaginary part (due to the reactive components) is the one that just stores the energy. that resistance which has both a real and an imaginary part is named resistance.

ohms law states that the ratio of voltage and current is a constant. now if the network under consideration is resistive, that constant is called resistance. and if the network has reactive components as well as resistive components then that constant is called impedance.

 

[williB]

I first heard of impedance as it relates to Speakers.
a speaker with an 8 ohms impedance does'nt have a resistance of 8 Ohms.
it has an impedance of 8 ohms at a certain frequency.

 

[windozeuser]

lol I fixed the typo yes I meant "hello" hehe

THanks for your reply I kinda understand it but I don't understand the frequency part

 

[checkmate]

Impedence is defined simply as the ratio of voltage against current. It is generally a complex number, with the real part being the resistive component, and the complex part being the reactive component. So a resistor is a special case with a purely real impedence. The reactive component of impedences are most often a variable of frequency.

If you express it in terms of euler's reppresentation, there is an amplitude and phase component. The amplitude is the magnitude ratio of voltage vs current. Thge phase is the phase difference between them. As you notice, this simply translates to a complex version of Ohm's Law. Given an AC voltage or current characteristic with a certain frequency, you can simply use the impedance and the modified "Ohm's Law" to derive the other component.

 

[williB]

that resistance which has both a real and an imaginary part is named resistance.

sam meant impedance there

 

[williB]

lol I fixed the typo yes I meant "hello" hehe

THanks for your reply I kinda understand it but I don't understand the frequency part

Ok take a transformer
it is a short to DC but as you increase the frequency it is no longer a short..

 

[Oznog]

It's a simple number in DC circuits and AC circuits with negligible inductance/capacitance. It is a complex number for AC with inductance/capacitance. Since the impedance of an inductor or capacitor varies with freq, you have to either state the impedance for a given freq or engineers may actually give a complex number that nobody else would understand. The 8 ohm rating of a speaker includes the inductance at a given freq.

At least when I was in school, "resistance" is only appropriate for simple ohmic resistance. The complex number would be called "impedance" or less commonly "reactance". Or often less formally just termed "load". All commonly use ohms as a unit.

 

[checkmate]

The easiest way to accept this idea is to simply throw out the idea of resistance. As I said, resistance is only a special case of a purely real impedance, and Ohm's law applies just as before.

The electronics world does not merely comprise of resistors. Inductive and capacitive components are all over, and they all have frequency dependent impedances.

In reality, everything has its own resistance, capacitance and inductance, including real resistors, wires, and even the copper traces on your PCB.

 

[samcheetah]

that resistance which has both a real and an imaginary part is named resistance.

sam meant impedance there

thanx for pointing that out williB

about the frequency part. well thats easy. you see the inductive reactance of an inductor is given by

XL = 2 * (pi) * f * L

and the capacitive reactance of a capacitor is given by

XC = 1 / ( 2 * (pi) * f * C )

and the impedance of a series RLC network is given by

Z = R + ( XL - XC )i

so you see the inductive reactance and the capacitve reactance are functions of frequency. the inductive reactance increases with and increase in frequency and the capacitive reactance decreases with frequency. so capacitors and inductors can be thought of frequency dependent resistors. that means that capacitors and inductors oppose the flow of current but that opposition is dependent on frequency.

now as you can see that the impedance Z has all the components. the R is the real part and XL - XC is the imaginary part. and therefore impedance is also a function of frequency.

and as pointed out by others that resistance is a special case of impedance. that will be when either XL = XC or XL = 0 and XC = 0.

i hope that helps

 

[Nigel Goodwin]

I was wondering if someone could put it into clear easy language

I get the impression that he's now even more confused?, after all these explicit, complicated replies.

Put VERY simply, resistance is a DC value, impedance is an AC value, both are effectively the same thing - but measured at DC or AC.

A speaker is always a good demonstration, as you already know, the DC resistance is less than the AC impedance.

 

[pebe]

i hope thats a typo and you meant "hello" :lol:

impedance is the amount of opposition that a network offers to the flow of current. impedance is just like resistance but impedance is a function of frequency......

Why this use of lower case letters all the time? The first word of a sentence should always be in upper case. Or have the rules of grammar been changed?

 

[Nigel Goodwin]

Why this use of lower case letters all the time? The first word of a sentence should always be in upper case. Or have the rules of grammar been changed?

Considering his location is clearly filled in as Pakistan, I don't think it's really worth criticising his English grammer!.

I'm presuming your 'Pakistan' grammer is probably going to be a lot worse?, and "samcheetah's" English is quite excellent!.

 

[checkmate]

I've always thought Mathematics was the universal language, not English.

 

[pebe]

Why this use of lower case letters all the time? The first word of a sentence should always be in upper case. Or have the rules of grammar been changed?

Considering his location is clearly filled in as Pakistan, I don't think it's really worth criticising his English grammer!.

I'm presuming your 'Pakistan' grammer is probably going to be a lot worse?, and "samcheetah's" English is quite excellent!.

My knowledge of the Pakistan language is zero! My comment was aimed at his criticism of windozuser's correct use of an upper case letter to start a sentence. I should have thought that with so much 'sloppy' English on forums, good grammar should be applauded - not criticised.