# Resistance Vs Impedance

I need answer for the folllowing questions relevant to printed circuit board technology.

What is the exact difference between resistance and impedance?

If impedance mean for the restriction of AC current, why we have to consider it for DC.?

Thanks
BS

## Replies

• technospartan
FOR THAT YOU MUST FIRST KNOW WHY WE CONSIDER ONLY AC FOR CONDENSORS & INDUCTORS.thats because we have the phase difference in voltage-current only in case of ac. inductor is an element which OPPOSES THE CHANGE of current.while a resistor is an element which OPPOSES THE FLOW of the current.now an impedence can be said as a combination of any kind of load. just consider a wire (conductor),when only the opposition of flow is concerned (which happens to be the case of dc) its resistance is only considered for calculation and optimisation purposes. but when the change of current(AC!!!!!!) exists we have certain magnetic effects (we call it electromagnetism!!!!!!!!) due to which change of current (inductance) has to be calculated along with flow opposition (resistance).in such a case (of AC) we have the concept of IMPEDANCE.

"ITS NOT SURVIVAL OF THE FITTEST
ITS LIVE TO THE FULLEST."
• balaseven
thank u folk.....

but, in PCB technology,we have to maintain impedance for each traces....For example,we have to maintain 50 ohm impedance for single ended nets.and 100 ohm differential impedance for differential pairs....In most of our board,we are using DC power supply.In that case,why we have to consider impedance(50ohm,100ohm)......we should consider resistance...isn't it?

Thanks
BS
• just2rock
for embedded design yes you can choose resistance diff. But why generally you consider 50 ohm/100ohm impedence is only due to alternate flow of current through track of PCB and to prevent the damage of the same
• balaseven
Hi just2rock,

Thanks
BS
• sauravgoswami
well do u mean different ratings of current flowing????as far as i knw,impedance has a real part and an imaginery part while resistence is a pure form!!!Any doubts!!
• reachrkata
For calculation purposes you can of course consider the track resistance only for DC applications. But strictly speaking, on a PCB, the track need not be straight and could be winding around. This constitutes an Inductance like behavior. And if you have a ground layer below the route, this also constitutes a capacitance behavior, where the route and GND act like the terminals and the PCB material as the dielectric. For a differential track also the 2 parallel tracks act like a capacitor.

Considering all these scenarios, in general the term impedance is used.
But for DC systems, it is safe enough the measure resistance alone.

-Karthik
😁
• sauravgoswami
As far as i know,maximum active and passive devices work on DC....pls clarify more
• umashankar.s
Can you tell me why we are using 50 ohm impedance for single ended?
Is it because of cable having 50 ohm only?

also the reason for using 75, 90, 100 ohm impedances ..

y not we use 100 ohm for USB?
• valter
hey thanks!!!!
• umashankar.s
Hi all

you folks have any valid justifications with you for impedance...
i have found one..

• Basically the impedance is depending on the thickness of the conductor. If the thickness decreases then the impedance increase. We can not reduce the thickness further after the certain limit. Since if we do then the conductor strength become weak. It can be cut at any point. So in general all the manufacture makes the conductor at constant thickness. For that constant thickness the impedance will be 90 ohms so that only we are asked to maintain 90 ohms on our PCBs to avoid impedance mismatches between on our PCBs and on the Cable.
-Bye Folks
• umashankar.s
one more....

The arithmetic mean between 30 ohms (best power handling) and 77 ohms (lowest loss) is 53.5; the geometric mean is 48 ohms. Thus the choice of 50 ohms is a compromise between power handling capability and signal loss per unit length, for air dielectric.
• KINETIC_JOULES
Sorry. . . please explain your question more clearly. Are you JUST wanting the difference between impedance and resistance?

"If impedance mean for the restriction of AC current, why we have to consider it for DC.?"
Please use better English. I don't mean to sound rude. I'm just having trouble following your question.
• KINETIC_JOULES
balaseven
I need answer for the folllowing questions relevant to printed circuit board technology.

What is the exact difference between resistance and impedance?

If impedance mean for the restriction of AC current, why we have to consider it for DC.?

Thanks
BS
Impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases.
• balaseven
Hi kenetic,

R u a PCB designer?Do u have any experience with PCB technology?Then only yoy can easily understand the question.I dont want general concepts ans diffrence b/w Resistance and impedance.The answer should be related to PCB.
• Kaustubh Katdare
easy there, balaseven. Kenetic is one of the youngest members on CE and an aspiring electrical engineer. Read her introduction here: #-Link-Snipped-#

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Let the original discussion continue. 😀
• KINETIC_JOULES
The_Big_K
easy there, balaseven. Kenetic is one of the youngest members on CE and an aspiring electrical engineer. Read her introduction here: #-Link-Snipped-#

-----------------------------------------------

Let the original discussion continue. 😀
Thank you. I understand I may not know much. I'm trying. I'm still in high school for peat sake!
• balaseven
Bigk

Tyler

me too joined here to learn something,go ahead......

bye...............
• neerajdhawan
Hi, Impedance and resistance are same for any circuit for DC but when we talk about AC signals they may be different as impedance has to components Resistive and Reactive and we right these components to show impedance Z as Z=R+iY. R is Resistive and Y is reactive component. Impedance of a give circuit may be different for different frequencies.
Hope this replies your question partly for more details please let me know
• KINETIC_JOULES
balaseven
Bigk

Tyler

me too joined here to learn something,go ahead......

bye...............
Glad to know I'm not alone! ^_^ I'm still young. I've got time to learn everything!

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