About electrical impedance and resistance! - Page 1
Forum section: electronics / beginners

author: Ghitza
author_rank: Contributor
timestamp: February 05, 2019, 08:10:58 pm
content: Hello guys,since this is my first post here I salute you all!First of all please accept my apologies for my bad english.I'm an ultra biginer in electronics and surelly there are a lot of unclkear things for me.As the title say, I am a little confused about the resistance and the impedance.As you can see this:"Electrical impedance, or simply impedance, describes a measure of opposition to alternatingcurrent (AC). Electrical impedance extends the concept of resistance to AC circuits, describingnot only the relative amplitudes of the voltage and current, but also the relative phases.When thecircuit is driven with direct current (DC) there is no distinction between impedance andresistance; the latter can be thought of as impedance with zero phase angle."I understood about impedance that is specific to an alternating signal but, where it say upper there is no distinctionbetween those two in DC, is it OK for me to say "impedance" instead "resistance" when I do a measurement in a circuit?The complete article is here:http://bmia.bmt.tue.nl/people/BRomeny/Courses/8C120/pdf/ElectricalImpedance.pdfAnd to expres myself clearly I have made a measurement with a Philips PM2403 on an arduino board.With the multimeter seted on x10Ω I receive a resistance value of about 570Ω.I know the measurement it is made in DC but can I also say there is an impedance of 570Ω instead of resistance?Thank you very much guys!

author: The Soulman
author_rank: Super Contributor
timestamp: February 05, 2019, 08:38:08 pm
content: Quote from: Ghitza on February 05, 2019, 08:10:58 pmNo, when measuring with dc you only know resistance.Impedance is always related to frequency, and therefore that frequency should also be stated, i.e. 100 Ohm at 100Hz.Or more common: a nominal or average impedance across a certain frequency range.

author: IanB
author_rank: Super Contributor
timestamp: February 05, 2019, 08:50:07 pm
content: Impedance has two components, resistance and reactance. When energized with AC both resistance and reactance are present and impedance is the result. When energized with DC the reactance component disappears and only resistance remains. Under these circumstances impedance is identical to resistance.

author: The Soulman
author_rank: Super Contributor
timestamp: February 05, 2019, 08:58:43 pm
content: Attached is a plot for a small woofer (loudspeaker), it is advertised as 4 Ohm "nominal" dc resistance is actually 3,4 Ohm.

author: Ghitza
author_rank: Contributor
timestamp: February 05, 2019, 09:14:52 pm
content: Quote from: IanB on February 05, 2019, 08:50:07 pmThank you very much Sir for the explanation.So in my case with the multimeter can I say there is an 570 ohms impedanceor it is mandatory to use the resistance term every time?Kind Regards!

author: The Soulman
author_rank: Super Contributor
timestamp: February 05, 2019, 09:17:31 pm
content: As can be seen from the plot the impedance of the loudspeaker varies wildly with frequency, a typical1/8 Watt metal film resistor would do much better in the audio spectrum and would produce a near flat line,however at (much) higher frequency's it will also start to curve.Quote from: IanB on February 05, 2019, 08:50:07 pmYes, using the perfect resistor.In all other cases it is inaccurate to say that the dc resistance equals the ac impedance.

author: IanB
author_rank: Super Contributor
timestamp: February 05, 2019, 09:33:27 pm
content: Quote from: The Soulman on February 05, 2019, 09:17:31 pmYour statement is true, but that is not what I said. What I said was that when measuring with DC then the impedance is identical to the resistance. In other words the DC "impedance" equals the DC resistance. This is true for all devices, perfect or otherwise.Another way of saying it is that the impedance becomes equal to the DC resistance as the frequency tends to zero.

author: The Soulman
author_rank: Super Contributor
timestamp: February 05, 2019, 09:34:10 pm
content: Quote from: Ghitza on February 05, 2019, 09:14:52 pmNot impedance, resistance would be correct but not always necessary to type.For instance:"The meter in the picture is reading 14 Ohms"Or:"The meter in the picture is reading 14 Ohms DC"Or:"The meter in the picture is reading a resistance of 14 Ohms"I would count all of those as correct.

author: The Soulman
author_rank: Super Contributor
timestamp: February 05, 2019, 09:40:36 pm
content: Quote from: IanB on February 05, 2019, 09:33:27 pmAh yes, that way around it is correct.But for completion I'd say impedance @ 0 Hz.That would take a mighty long time to measure with fft tho.

author: Audioguru
author_rank: Super Contributor
timestamp: February 05, 2019, 11:26:40 pm
content: A capacitor has an impedance that is determined by its value and by the frequency. A fairly high capacitance value and a fairly high frequency results in a very low impedance, almost a short circuit.But its resistance is infinite.

author: nick_d
author_rank: Regular Contributor
timestamp: February 06, 2019, 03:04:22 am
content: To understand about impedance properly, consider a circuit such as this one:Vin/|\||[] R1=10k|*------ Vout|[] R2=10k||---0VAt the point marked Vout, you will see a voltage of Vin R2 / (R1 + R2), this is the ordinary resistive divider formula.Now consider what happens when we replace one of the resistances with an impedance:Vin/|\||[] R1=10k|*------ Vout|== C1=10u||---0VThe way to analyze this circuit is firstly to choose a frequency (omega, in radians per second), and then set Vin to a sine wave at that frequency. Since the capacitor acts kind of like a battery, it will not allow the voltage at Vout to change instantaneously. Instead, like a battery it must be charged and discharged, which is a gradual process. This means in practice that a sine wave will appear at Vout, but it will be smaller than the Vin sine wave (how much depends on the values of R1 and C1) and it will be delayed with respect to the Vin sine wave (how much again depends on the values of R1 and C1). These parameters are called the amplitude and the phase shift of the network.The point of impedances is they give you a convenient way to calculate the amplitude and the phase shift as if both components were resistors. You set Z1 = 10k, i.e. the purely real impedance of R1, and you set Z2 = 1 / (j omega 10u), i.e. the purely imaginary impedance of C1. Then you are in a position to calculate as before, Vout = Vin Z2 / (Z1 + Z2). Here the Vin and Vout are complex quantities whose magnitude represents the amplitude of the input and output sine wave, and whose angle represents how much delayed (in radians) the sine wave will be. Since Z2 / (Z1 + Z2) is a constant with a given magnitude and angle, we can say that these are the amplitude and phase of the network.Note that these calculations have to be done separately for each value of omega, which makes sense: fast sine waves will be delayed by a large fraction of their cycle time and reduced considerably in amplitude, whereas slow sine waves will be hardly affected by the network.cheers, Nickedit: swap Vin, Vout in the second divider formula

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 06, 2019, 03:44:05 am
content: When you are measuring on a PCB you always have to be aware of the limitations of your equipment.For example, when you are measuring a diode in the ohms range, your meter may settle on some value, but it will be utter nonsence.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2019, 05:52:14 am
content: Quote from: Doctorandus_P on February 06, 2019, 03:44:05 amThis is not a good example. On some meters you actually measure diodes on the ohms range by design and the displayed value is the forward voltage.