# Resistance And Frequency

Discussion in 'General Electronics Chat' started by Teslafan, Nov 17, 2014.

1. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
Hi everyone,
I am trying to simulate a circuit containing resistors, inductors and capacitors. When i calculate the Impedance of the circuit i find it to be for example 150+100j for a frequency of 5Hz. When i run the same simulation for a frequency of 15 Hz i find it to be 162+134j. I understand that inductive impedance increases with frequency but what about resistance? It cannot be independent since it changes too! Is there any possible relationship formula connecting these two parameters?
Thank you

2. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
the resistance does not change with frequency. the inductive impedance is a combination of inductance and resistance at a frequency. are you including the capacitive reactance in your calculations?

Jul 18, 2013
10,571
2,382
The resistance of the inductor is considered in series with the Inductive reactance and does not change with frequency.
The resistance influences the total Q of the coil.
Max.

4. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
Thank you for the response. My question is how can i calculate the inductor series resistance so i can be able to calculate the total resistance of the circuit (ohmic resistance and inductive resistance)

5. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
Alfacliff thanks for the response. Lets say i have an RL circuit containing only a resistance and an inductor. When i simulate that with LTSpice the total impedance is changing with frequency. My question is if i can have the impedance data for 10 Hz using field measurement, could i possibly calculate the impedance at 50 Hz using some kind of relationship formula between frequency and impedance or resistance?

6. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
use the formula for inductive impedance..

7. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
the formula only gives me the imaginary part and not the real one.

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,963
1,098
Here you have

$Z = sqrt{R^2 + XL^2}$

XL = 2 * pi * F * L

9. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
When you enter an Inductor into LTSpice, you can decorate the model with all sorts of parasitic series and shunt resistance, interwinding capacitance, etc. Read the help file... it goes on for several pages...

10. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
Let Z=R+jX. That formula is for converting in polar coordinates and give the magnitude of impedance.

11. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
Yes indeed MikeML i tried to eliminate all parasitic resistances in order to see if the results are still the same. And the total resistance of the impedance kept changing with frequency..

Oct 2, 2009
5,450
1,066

13. ### #12 Expert

Nov 30, 2010
16,355
6,852
Z^2 = R^2 + L^2
Z^2 - L^2 = R^2
Insert the value of L and the value of Z
Run the numbers through your calculator to get to R^2
Square root the R^2 result and you will find the full amount of resistance, which includes the internal resistance of the inductor. Subtract the resistor you know about and the results will be the resistance that is in the windings of the inductor.

14. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
That is my file.

File size:
4 KB
Views:
23
15. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
Ok the resistance that is in the coil is constant or frequency related?

16. ### #12 Expert

Nov 30, 2010
16,355
6,852
Constant. Consider it to be an imaginary resistor in series with the inductor. It's really a distributed resistance, but it works like a single resistor in series.

Jul 18, 2013
10,571
2,382
Constant, the resistance of the coil depends on material wound with, the diameter and number of turns, i.e. length of the conductor.
Or measured in a traditional way with DC resistance meter.
Max.

18. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Here is an LTSpice sim of a simple RLC network. I show that the Real part of its input impedance is constant with frequency, while its Imaginary part (and sign) varies with frequency.. Note that inductive reactance cancels capacitive reactance at only one frequency (resonance), leaving only the resistance...

Note how I found the impedance and that it would normally be a vector (magnitude & angle). Note what I did to plot the Re and Im parts of the impedance separately.

Ain't LTSpice great? Hey Cruts... Lets see you do this with Multisim

Last edited: Nov 17, 2014
19. ### Teslafan Thread Starter New Member

Mar 8, 2011
25
0
MikeML thank you for the sim info it was very helpful. If you try and do that for the asc i attached you will see that the magnitude is not the same and varies with frequency. That is my original question

20. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
That is a consequence of the parallel branches in the network.

Even this trivial network shows a variation in both the Magnitude of Zin and the Real term of Zin...

Last edited: Nov 18, 2014
Teslafan likes this.