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RLC meters

Discussion in 'Electronic Design' started by Don Foreman, Sep 9, 2006.

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  1. Don Foreman

    Don Foreman Guest

    I'm contemplating design and construction of an RLC meter. Anyone
    know how the commercial ones work?

    I won't go with a bridge design because I don't have access to
    precision C's and L's, nor instrumentation to measure them (yet).
    Readily-available amplifiers have come a long ways since the day of
    the GR and Boonton bridges!

    I do have accurate digital meters and a frequency counter.

    Are the commercial designs simply devices that measure reactance by
    measuring current or voltage when the DUT is suitably excited with AC
    of appropriate frequency? I think I can separate reactive current
    from resistive current with a quadrature osc signal and synchronous

    I'm primarily interested in inductance, in a range of 100 nH to
    perhaps 50 mH. I'll include variable DC current bias to note its
    effects. I'm thinking frequencies of 1KHz, 10KHz and 100 KHz.
  2. It's a lot harder than it looks. Better to buy one on eBay. Otherwise try
    Elektor or "Silicon Chip" magazine (Australia).
  3. jure

    jure Guest

    did you look at the AD7745 ?
  4. Don Foreman

    Don Foreman Guest

    Does "harder than it looks" imply that you know how to do it but would
    rather not tell?

    Opinions about what I "should" do are much less interesting than
    helpful suggestions about how others have done what I intend to do.
    Absent that, I'll do it anyway.
  5. All the kit versions I have seen have very limited ranges and the like.
    Silicon Chip has a magazine format book with designs for a cap meter and for
    an inductance meter. The cap meter measures in three ranges up to 2 uF. The
    inductance meter has two ranges covering 10 uH to 20 mH. I suspect that the
    cost of both kits will exceed the eBay price for a very comprehensive RLC

    I'm an old guy - started with tube radios. Been there, done that, learned
    what's worth doing. I no longer fix popup toasters and pocket transistor
    radios for money and I know what is economically reasonable and what is not.
    But go ahead, good luck to you.
  6. ehsjr

    ehsjr Guest

    # .001 mHy (1 nHy) to 100 mHy (most units measure to 150 mHy)
    # .010 pF to 1 mFd (most units measure to 1.5 uFd)

    PIC based, might be a good idea to take a look at it
    to get ideas.

  7. Don Foreman

    Don Foreman Guest

    Thanks! I also started with tube radios and I no longer do much of
    anything for money. I guess I wouldn't fix a toaster or a transistor
    radio either. However, I'll spend half the winter making something
    I could probably buy, if I think I can make better than I can buy and
    might enjoy doing it -- and probably learn a few things along the way.
    I kinda know what I'm doing some days, having been a practicing
    engineer for 40 years.

    Recent checks on EBay have turned up either meters I wouldn't be
    satisfied with, or instruments way more than I want to afford as in
    graphic user interface yada yada.
  8. Don Foreman

    Don Foreman Guest

    Very interesting. Looks like a kit. It doesn't have all
    the features I want, but if the kit is anything like reasonably priced
    I'd get one anyway. I'll check this out. Thanks!
  9. Perhaps think about using parallel known C's and looking
    for resonance with the scope. Use two constant current
    sources OR'd together.
    +----[CC.1]--->--+--<---o \o--[CC.2]-- DC-set
    ___|___ |
    | | \|/
    |Sig Gen| +-----+-----+------>Scope
    |___ ___| | |
    | | )||
    | C=== )||DUT
    | | )||
    | | |

    [CC.1] is a low power 0-100KHz bipolar CC-source, say
    100mArms or so capability.

    [CC.2] is a high power DC CC-source, adjustable up to
    say 5 to 10A. Switched in/out, as required.

    With no DUT: Scope measurement of the voltage at various
    frequencies gets the Cstray of the test equipment.

    With a DUT: Resonance with various C's and frequencies
    gets the Inductance and the Cstray of the DUT. A guess
    of the Q at resonance gives a rough idea of the losses.

    Looks a little clunky and old-fashioned, but cored inductors
    are better tested with some *welly* going through them.
  10. AndyS

    AndyS Guest

    Andy writes:

    I have an old Heathkit LC meter.

    It uses an internal signal generator to set a frequency
    appropriate to the L range to be measured.

    The unknown L is connected in series with a variable
    capacitor and the C is tuned for a voltage peak at their
    junction. A fairly high impedance detector is used to
    very lightly load the junction.

    The C is calibrated, and the L can be readily calculated.

    In other words, it inserts an unknown with a known, and
    determines the resonant frequency...

    In the past, for SMALL inductance values, I have used
    it in a parallel tank using an MC1648 VCO, and counted
    the frequency of oscilation. That works really well with
    50 nh and up...... Select a frequency range to shoot
    for where the expect inductive, or capacitive, reactance
    is in the ball park of 100 ohms or so. That seems to
    work well for practical values of L and C......

    It ain't rocket surgery, and will give you ample opportunity
    for experimentation... Have fun.

    Andy W4OAH
  11. Dave

    Dave Guest

    My ARRL handbook has several circuits for measuring R, L and C (2004
    edition, section 26.25). Typically they convert the unknown value to a
    voltage that can be measured.

    Very small R, L and C tend to be difficult, but away from that area, 10%
    accuracy can be achieved. If you want better, then bridges are the way to
    go - but as others will point out, you can buy better for less than your
    component cost ..

  12. The Phantom

    The Phantom Guest

    Something like this:

    or these:

    These are low cost units. Really high performance units go for $10K+.

    Tells you something about the difficulty of building one.
  13. Don Foreman

    Don Foreman Guest

    Ordered one today. Thanks for the tip!
  14. Don Foreman

    Don Foreman Guest

    I've ordered a simple low-cost meter for now. Most of the high-buck
    jobs have a lot of features I don't care about -- but I can see that
    building a precise instrument is not a trivial undertaking. Still
    might be fun to try. I'll have the simple one to use while I'm
    screwing around discovering why a better one is so expensive -- or
    perhaps how and why it needn't be.

    Thanks for the leads, Phantom!
  15. ehsjr

    ehsjr Guest

    I think you'll be well pleased. I've read a number
    of reviews on it - all were positive, and I love mine.
  16. The Phantom

    The Phantom Guest

    I see that you're considering a vintage GenRad.

    Have a look at item no. 190028769647 on ebay. I notice that the seller
    says at the bottom of the description that he also has a unit that will
    make 10 KHz measurements. Also look at the seller's other items.
  17. Guest

    You could try finding some downloadable service manuals for some of the
    better RLC meters, and see how they did it.

    Maybe you could find some at .

    If you're not sure what models to download manuals for, you could check
    the archived old catalogs, in the reference library section at (after free registration, which has proven to
    be harmless).

    Good luck.

    - Tom Gootee

    "He who lives in a glass house should not invite he who is without sin."
  18. Ross Herbert

    Ross Herbert Guest

    Ed, I concur...

    I bought Neil Heckt's LC meter kit about 1 year ago and have been
    mighty pleased with it.

    Ross Herbert
  19. Don Foreman

    Don Foreman Guest

    That does look interesting -- and I could have my daughter in the UK
    grab one for me. But I've already ordered the kit that ehsjr
    mentioned and recommends, so I'll see how that works out. Could have
    used it today.
  20. Don Foreman

    Don Foreman Guest

    That site doesn't seem to be working. I also tried it earlier
    looking for a manual or schem on a Tektronix 2235 scope that won't
    display both channels in either alt or chopped though it can add them.
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