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Gain 100 microAmp into 200 microAmp

Discussion in 'Electronic Repair' started by Bo-Lennart, Mar 29, 2013.

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  1. Bo-Lennart

    Bo-Lennart Guest

    Hi....I have a broken AF-test unit with an analog 100 miroAmp meter. And that meter is broken. I have a 200 microAmp-meter that can suit my needs. I just replace the scale from the broken meter to the 200 micro-meter. But if I feed the instrument with a signal that should make 100% FSD, it just make50% (of course)of FSD.
    Is ther someone that know about a circuit that can gain those 100 microamp to 200 microamp, in a linear way.

    Would be very greateful
    Bo-Lennart Karlsson
    Falun, SWEDEN
     
  2. JW

    JW Guest

    Should be able to do that with an op-amp.
    http://en.wikipedia.org/wiki/Operational_amplifier
    See the third circuit down and read the description.
     
  3. N_Cook

    N_Cook Guest

    Hi....I have a broken AF-test unit with an analog 100 miroAmp meter. And
    that meter is broken. I have a 200 microAmp-meter that can suit my needs. I
    just replace the scale from the broken meter to the 200 micro-meter. But if
    I feed the instrument with a signal that should make 100% FSD, it just make
    50% (of course)of FSD.
    Is ther someone that know about a circuit that can gain those 100 microamp
    to 200 microamp, in a linear way.

    Would be very greateful
    Bo-Lennart Karlsson
    Falun, SWEDEN

    ++++++

    Isn't it always that way around. Is there a dropper somewhere that you could
    , as suck it and see initially, put a preset over to adjust . Unlikely as
    they would have used a 200uA movement originally but you never know, the
    calibration R may have enough play with any luck.

    In a sense I'm looking forward to the day that I try to rewind an unobtanium
    meter movement for something worth repairing, like a valve tester. I've
    previously swapped a phosphor-bronze hairspring which is a daunting part of
    such a job
     
  4. Jamie

    Jamie Guest

    You will need to get the schematic for that area of the meter circuit
    so that a resistor can be recalculated for it.

    It's more than likely the original meter was shunted with an R to
    do both a current scale and act as a damper for the needle movement.

    The driving circuit most likely has more than enough to accommodate
    that 200ua meter after recalculations.


    Jamie
     
  5. Jamie

    Jamie Guest

    All general analog coiled meters are calculated via current and a
    voltage together regardless of their use. Even the iron vane meters
    are treated this way. Etc.

    For exact calculations you need to know the coil R and meter's
    current so that voltage can be removed from the driving source to give
    you the final R required to place that meter of 200ua' to full scale
    when the reference is %100

    Only the dedicated types, which I am sure he does not have, are
    designed to directly accept the name plate energy sources, current or
    voltage.

    There are some meters like VU types for example, have a (I) requirement
    point at 0 db scale, not full scale. The network needs to be calibrated
    to fit that requirement and not full scale. It depends on the face scale
    design.

    I don't know what it is these days but we used to calibrate meters at
    around 75% deflection.

    Jamie
     
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