Another Function Generator Question...

Discussion in 'The Projects Forum' started by sspil, Mar 17, 2010.

  1. sspil

    Thread Starter New Member

    Feb 12, 2010
    I am developing a lab for my Physics class on RLC circuits. I am running an AC current through a circuit with a function generator. I have chosen the components carefully so I think I have the impedance mismatch problem I originally had worked out. My issue is that when I run the AC current through the resistor alone, I am getting some minor frequency dependence on the resistance using R=V/I where V and I are measured. Theoretically I know that the resistance should be independent of frequency, and I don't know where the error is coming from. Can I be getting some loss in the system that is frequency dependent?

    The overall results of the lab are pretty good, and I think the students will get the idea, but I would like to be able to explain this discrepancy between theoretical and actual on the resistor to my students without using the generic "equipment limitation" explanation. I also would like to understand it for myself!
  2. Ghar

    Active Member

    Mar 8, 2010
    How strong of a dependence do you get?

    Every resistor will have some parasitic inductance and capacitance. You also get some parasitics from the probes especially if you're using one of those alligator clip connections.
  3. SgtWookie


    Jul 17, 2007
    At what frequency or frequency range are you operating the test circuit?

    With DC, the parasitic properties of components are basically inconsequential.

    When you introduce AC, parasitic properties become more significant as you go higher in frequency.

    For example, a speaker might be rated for "n" Watts and 4 Ohms. However, when you measure the speaker coils' resistance, it appears to be a dead short! Wait a minute - it's supposed to be 4 Ohms, right?

    Well, speakers are rated for impedance at 1kHz. At DC, they are practically a dead short from terminal to terminal. As frequency increases, the impedance also increases.

    Not really a great example, but it is something an average person might relate to - as almost everyone knows what a speaker is.

    All components have parasitic properties of R,L,C. However, they have a primary function, and that is mostly what is talked about.

    A resistor's main function is resistance in Ohms. However, it does have a small amount of parasitic capacitance, due to the way that the terminals are connected to the resistive material. They also have inductance, mainly due to the length of their leads. A straight wire measuring 10mm in length has about 15nH inductance. If there any kinks or bends in it, the inductance increases dramatically.

    When you're dealing with high frequencies, the parasitics really come into play. A multilayer capacitor that may measure 47pF at DC levels may measure 180pF at 500MHz.

    Even at lower frequencies, parasitics come into play. I was testing some inductors wound on a material 6 toroidal core. At near-DC (low frequency), they measured around 970nH. Around 10MHz, they measured in excess of 1000nH.

    It is very difficult to account for all of the parasitics.
  4. sspil

    Thread Starter New Member

    Feb 12, 2010
    That makes sense to me. The frequencies I'm operating at are low- in the range of 1K-10K Hz for the lab. I did notice that the resistance does vary more as I increase frequency. The variance is not large, but it is there and I wanted to be able to explain why. I am using alligator clips, an old circuit trainer board and some fairly old probes, so parasitics could be introduced in several places.

    Thank you so much for your help. I always feel better if I'm able to give good practical information to my students along with the theoretical. My students thank you as well!