Help with simulating the winding resistance of an inductor.

[cvriv]

I attached an image. The circuit on top is the actual circuit. One of the two circuits below is the equivalent circuit with the winding resistance. I'm not sure which one is right.

I originally thought that the first circuit, below the original, was the right one, but when I did an AC analysis on it I found that the roll-off at lower frequencies was different. It's supposed to be about 40db/ decade. I know that the winding resistance is in series with the inductor.

The second circuit, below the original, has the same roll-off at high and low frequencies. Also, I built this circuit on a protoboard and after messing around with it I found that the actual roll-off is very similar to the second circuit down.

I measured the winding resistance of the 10mH inductor Im using and acquired a reading of just about 250ohms.
 
I noticed that when using the second circuit down with a winding resistance of 250ohms, the analysis gives results that do not reflect the results of the actual circuit. The roll-off is right but the values are off. To get the results to line up I had to change the winding resistance to 110ohms.

Which is the right winding circuit and why are the results off when I enter the real winding resistance?

Thank you.

[fourtytwo42]

A picture of your actual circuit might help but also how are you measuring voltages in your practical circuit and the simulator, perhaps one is rms and the other p-p ?

[ogden]

Don't you have access to the internet? Use search.

There's inductor model shown and some more info for you: https://wiki.analog.com/university/courses/electronics/comms-lab-isr

[cvriv]

I attached a image of the circuit.

Right now I'm using a benchtop DMM to take circuit measurements and I'm using Multisim for the simulation.

I have my signal generator giving me 2Vp-p. When I dial in the signal generator to take a measurement at the center frequency, the DMM is reading 125mVrms. When I do the same thing in Multisim, the DMM, for both of the circuits, are reading about 56mVrms. Why is this?

When back the frequency down to 1kHz, my DMM reads 1.3mVrms. One simulated DMM reads 3.9mVrms and the other reads 944uVrms.

Also, the center frequency of the real circuit is about 1kHz higher than the simulated. Why?

[Benta]

Are you certain your DMM will accept frequencies over 1 kHz?

[ogden]

Reading is much more effective way of gaining knowledge than experiments and questions in forum. You shall familiarize yourself with properties of ideal LC circuit, then approach circuit of real inductor. 47K is way too high signal source impedance. Decrease it to let's say 4.7K so your voltages are not so low.

[cvriv]

I have a Rigol DM3058E. I checked the specs and I think I'm good from 20Hz - 100kHz.

[cvriv]

Reading is much more effective way of gaining knowledge than experiments and questions in forum. You shall familiarize yourself with properties of ideal LC circuit, then approach circuit of real inductor. 47K is way too high signal source impedance. Decrease it to let's say 4.7K so your voltages are not so low.

Ugh. This is a class assignment. I can't change the value of the resistor because this is what the professor assigned as a resistor value. I would love to read all about this but the book for this class doesn't mention anything about this or much of anything of what he talks about in class. He isn't really teaching by the book. This is in fact talked about in my old EE book, which I have been reading through to try and figure this out. His assignment simply says measure the resistance of the inductor and add this resistance to the simulation and run it again.

As simply as wrote this assignment out.... I want to as simply add the resistance to the simulated circuit so I can run it again. I don't know how to add it to the simulation, I dont know why the real circuit has a center frequency that's 1K higher, I don't know why my real values are that much higher. I know that the simulated stuff is IDEAL and the real stuff is NOT. The difference between them is BIG and I don't know why. I could probably spend the whole weekend reading and still not figure out why. I have a lot of homework to do and don't have time for that. If someone here can't give me a quick and simple explanation as to what I'm doing wrong if I'm even doing anything wrong.... then I'll just draw up a report with my weird results and just say I don't know. I am seriously frustrated here because my professor is no help to me, the damn book is no help, and everyone here is telling to read read read. I know! Forget it. I'm just going to finish this with what I got and move on.

[ogden]

This is a class assignment. I can't change the value of the resistor because this is what the professor assigned as a resistor value. I would love to read all about this but the book for this class doesn't mention anything about this or much of anything of what he talks about in class. He isn't really teaching by the book.

Oh, I see. What kind of signal source do you use? Make sure it is pure sine wave. Square wave will not work well. Not with most DMM's. DMM shall be "true RMS" for AC voltages. You shall sanity-check your generator and DMM by simply measuring 1V signal source output w/o circuit first, only then work on this weird circuit with stupidly oversized 47K resistor which brings voltages to very low levels where DMM's are not that good.

[edit] Regarding "True RMS" - Rigol DM3058E is good for the job.

[ogden]

I have my signal generator giving me 2Vp-p. When I dial in the signal generator to take a measurement at the center frequency, the DMM is reading 125mVrms. When I do the same thing in Multisim, the DMM, for both of the circuits, are reading about 56mVrms. Why is this?

In multisim you are using 1vpp instead of 2vpp of circuit. Look, you shall be precise & consistent how you do things and how you report your results. Electronics is exact science.

[T3sl4co1l]

Ugh. This is a class assignment. I can't change the value of the resistor because this is what the professor assigned as a resistor value.

...

I would love to read all about this but the book for this class doesn't mention anything about this or much of anything of what he talks about in class. He isn't really teaching by the book. This is in fact talked about in my old EE book, which I have been reading through to try and figure this out. His assignment simply says measure the resistance of the inductor and add this resistance to the simulation and run it again.

Then you're done.  You've followed the letter of the assignment; it's your prof's own problem that they designed a poor experiment.  Usual school rules apply: if they're expecting a polished turd, that's unreasonable -- complain to dept head, and if that gets no traction, complain to registrar or higher up.

For the record, "add to the simulation" is the key: an inductor's inductance and DC resistance add, in series.

Also for the record, that you've got measurements well within 5% of the simulation, using 10% standard components, is quite satisfactory.

Practical, accurate inductor models are more complicated than this, having a parallel resistance component (effectively, the inductor winding acts as the primary of a transformer, and various losses act as the secondary of that transformer -- thus, in parallel), as well as frequency-dependent components (skin effect and core loss), and capacitance.  Capacitance is usually modeled as a parallel C, or R+C, but in reality, the C is distributed along the winding.  What level of model do you choose?  Depends on how wide a frequency range you need to cover.  For narrow frequency ranges (say, a high-Q resonator), it actually doesn't much matter where the resistance is placed, as long as it is present!

Tim