Connect with us

Can you measure Impedance with an Ohm Meter..

Discussion in 'Electronic Basics' started by Denny B, Jul 17, 2003.

  1. Denny B

    Denny B Guest

    Is it possible to measure Impedance with an ohm meter.
    For example can you measure the Impedance of a 4 ohm
    car speaker with an ohm meter?

    When I tell people they cannot measure Impedance with an
    ohm meter, you need an impedance meter and the impedance
    must be measured with the speaker in operation, with a signal generator
    supplying a fixed frequency, this always leads to disagreements
    with the person who demands an ohm meter to measure what he calls
    Impedance, when I tell him an ohm meter can only measure the resistance
    of the coil there is always disagreement.

    I tell them Impedance is AC resistance and what they measure
    with an ohm meter is DC resistance. Disagreement here again.

    Will somebody take me up on this and clarify the situation.

    Thanks in advance
    Denny B
     
  2. Denny, you are perfectly correct on all points, impedance is an AC
    phenomenon and can only be measured using AC test signals. A component of
    the resulting value will be dependant upon the DC resistance of the coil,
    since AC signals are affected by DC resistance, but there will be a
    component of the result that shows only with AC stimulation and this
    component will have different values at different frequencies. A well
    designed speaker in a well designed enclosure will exhibit a more or less
    constant impedance across the intended range of working frequencies, but
    this value will not be the same as the DC resistance of the coil. There are
    a couple of ways that you may try to justify this (apart from the simple
    "try it and see").

    1) The speaker coil has resistance and inductance and capacitance. A DC
    measurement will sense the resistance but not the inductance and capacitance
    of the coil, however an AC measurement will sense all.

    This is a gross oversimplification but illustrates the point that there
    *must* be more than resistance involved.

    2) Imagine applying DC only to the speaker. The speaker cone will deflect
    by some amount and then remain stationary and silent. There will be a
    standing voltage and current which will allow the calculation of a
    resistance. We are supplying electrical energy to the speaker which is all
    turning into heat in the coil (there is nowhere else for the energy to go)
    and this is a resistance phenomenon. Now imagine applying a pure AC signal
    to the speaker (no DC). The speaker cone will vibrate and is therefore
    radiating sound energy continuously. As in the previous case we are
    supplying electrical energy to the speaker, and as in the previous case some
    of that energy is heating the resistive component of the coil in the same
    way as before, but the speaker is now also radiating energy in the form of
    sound and this energy must be coming from the electrical stimulation we are
    supplying.

    In the first case the electrical energy is dissipating in only one way, and
    we can express that as a resistance only. In the second case the electrical
    energy must be dissipating in the resistance in exactly the same way as
    before, but we are now also dissipating energy from the AC source in the
    form of sound and this energy must be coming from the electrical input. The
    additional energy demand to suppply the sound "appears" to the electrical
    input as an impedance (more correctly a dynamic impedance, since it is
    necessarily a phenomenon which only shows when the cone is moving) which is
    in addition to the resistance that is "seen" in the DC case. The impedance
    of a speaker will always be greater than the resistance, to account for the
    additional energy that must be supplied to make the sound.

    The above simplifies the physics, since there are other effects: the coil
    and wiring will hace some capacitance but this is very small; some energy
    will also be lost to the physical movement of the cone (frictional lossess
    and the energy needed to flex the cone suspension which will appers as heat
    in the cone and cone suspension); inductive coupling between the coil and
    surrounding metalwork (including the magnets) will induce currents which
    will dissipate as heat in the metals. There are probably more I have not
    your thought of. Well designed speakers in well designed enclosures will
    minimise the other effects, and in fact an ideal speaker would have dynamic
    impedance and no resistance (since resistance represents wasted energy and
    thus inefficiency).

    Chris
     
  3. Ratch

    Ratch Guest

    You are correct. Impedance is the vector like sum of resistance and
    reactance. Reactance is 90° out of phase with respect to resistance. A
    battery driven ohmmeter can only measure DC resistance. It could be said
    that it measures the impedance at 0 HZ where the reactance is zero.
    Impedance is not AC resistance by itself. AC resistance occurs at higher
    frequencies when the charge flows more near the surface of the conductor and
    less at the center. This reduces the cross section of the conductor and
    thereby increases the resistance. The resistance and reactance both reduce
    current, but in a different way. Resistance is caused when the charge
    carriers collide with the material lattice causing it to vibrate faster.
    This causes a loss of drift velocity and kinetic energy for the charge
    carriers, and a increase in kinetic energy for the material lattice. The
    overall effect is to increase the temperature of the material and decrease
    the current. Reactance is caused by a back voltage occuring, which
    decreases the current and stores energy in either a electromagnetic or
    electrostatic field. The energy is taken and returned with no loss during
    parts of the AC cycle. If the coil is attached to a speaker, the impedance
    will change because energy is being used to produce sound. This is called
    acoustical coupling. Ratch
     
  4. Dan Fraser

    Dan Fraser Guest

    No you cannot measure impedance with an ohm meter but in the case of
    speakers, you can guess reasonably well.

    8 Ohm speakers often have a DC resistance between 4 and 6 ohms.

    4 ohm speakers have DCR between 2 and 3 ohms

    and so on.

    The speaker impedance will always be higher than the DCR but not a lot
    higher.

    Therefore I can make a good guess about the impedance of a speaker by
    measuring the DC Resistance [DCR] with an ohm meter.
    --
    Dan Fraser

    From Costa Mesa in sunny California
    949-631-7535 Cell 714-420-7535

    Check out my electronic schematics site at:
    http://www.schematicsforfree.com
    If you are into cars check out www.roadsters.com
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-