Moving coil meter - runs away with itself!

Discussion in 'Homebrew and Kit Projects' started by EI6GXB, Sep 24, 2008.

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  1. EI6GXB

    EI6GXB Ham Member QRZ Page


    Seeing as the FT897's meter output jack only puts 1mA at FSD, I built a little meter amp for my 340mA FSD meter from a TIP142.

    10K pot in the I/p for trimming and adjustment. Wiper to base of TIP142 w.r.t ground.

    Question though - When it's set at 'off' from the 897, it's off, no matter what.

    But if I set it at FS, and try and trim, the meter just gradually increases, as if there's bleed from the base through, and when set back to 'sig', nothing will make it move!!

  2. W4HAY

    W4HAY XML Subscriber QRZ Page

    Schematic, please.
  3. EI6GXB

    EI6GXB Ham Member QRZ Page

    going to use a non inverting ld1970 with variable gain instead, as it's far more stable than a darlinton
    Last edited: Sep 25, 2008
  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    CAn you measure your input voltage (output from the rig) and see if it's drifting as well? You might be loading down the circuit slightly.
  5. EI6GXB

    EI6GXB Ham Member QRZ Page

    that's obediently 3v. I think it's something in the darlington. Drift Didn't happen with a pullup resistor, so i'll reinstate that with a 1M pot and go from there. Few smoothing caps also
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    You might also put the meter on the cold end of the Darlington and see what happens
  7. M0DSZ

    M0DSZ Ham Member QRZ Page

    I like your graphics program!

    I made something vaguely similar to provide a 1V reference to give some reasonable current output. Your circuit probably drifts as the transistor warms up so I would suggest an emitter resistor and then perhaps a little more complex base bias including a diode if that doesn't help.

    I appreciate you want to keep it simple but it may be better to use a long-tailed pair with your meter output into one side and the other just DC biassed to create a balance. This would overcome the thermal drift.
  8. KL7AJ

    KL7AJ Ham Member QRZ Page

    By the way, I don't know what your final application for this will be, but one thing that is quite cool is to take the AGC or S-meter output and run it into a data acquisition card. This way you can monitor a station like WWV and plot the relative signal strength over a long period of time. (When you're not hamming, of course!)
  9. EI6GXB

    EI6GXB Ham Member QRZ Page

    The interesting thing is - if I tied the base high with a 10K resistor, the meter sat obediently half way (voltage divider with the pot), and worked perfectly, so I wonder if it just needs a tiny bit of bias to keep it in check
  10. VK2TIL

    VK2TIL Ham Member QRZ Page

    "I wonder if it just needs a tiny bit of bias to keep it in check "

    Yes; that's how transistor amplifiers work.

    I didn't quite understand your initial description of what was happening and I don't know the characteristics of the '897's meter output but it did seem to me that one (or both) of two things is/are involved;

    Beta; beta in an inappropriately-biased transistor is a lucky dip. It varies with temperature and other factors. A Darlington configuration with its high gain is worse than a single transistor.

    Saturation; the transistor could be saturating.

    Correct biasing effectively removes beta from the equation. The explanations of transistor operation & biasing given by W7ZOI in SSD and in EMRFD are very lucid but I'm sure there are others available.

    My solution to providing gain for meters etc is to use an op-amp where feedback stabilises the amplifier.
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