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Hot-Wire constant voltage Anemometer

Discussion in 'Electronic Design' started by [email protected], Aug 7, 2007.

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  1. Guest

    Hi,
    I am trying to build the circuit in Figure 1 in this link:

    http://www.taosystem.com/products/HWCVA-TN.pdf

    I want to get Vw= 0.4V or higher (0.4-0.8 V). Rw has a small value
    between 4-8 Ohms. The idea of the circuit that when an air flow goes
    by the hot wire resistance Rw, the hot wire resistance decreases,
    while keeping the voltage constant across Rw (Vw should stay
    constant).

    My problem is to choose an Op Amp that will keep the voltage constant
    across Vw, while giving an output current of over 80mA. I tried many
    Op-amps for that matter, even the high output power op amps but
    nothing works for that design. Would someone help me with that.

    Thanks
     
  2. Since the hot wire is always driven with the same polarity,
    you can add an emitter follower current booster to the opamp
    output to provide most of the heater current. The output
    voltage signal is also taken from the follower. You might
    need a heat sink for that transistor.
     
  3. Fred Bloggs

    Fred Bloggs Guest

    The schematic in the application note is not to be taken literally, the
    op amp symbol is merely an articifice used to illustrate the voltage
    controlled regulator. And they make a few statements that make me
    wonder, such as the capability for indefinite coaxial cable lengths from
    the Vw tap to the hot wire sensor. At a minimum this will introduce a
    voltage divider action causing a deviation from true CVA operation, and
    for reasonable vales of Rw and R2 will introduce a destabilizing phase
    lag into the feedback loop. But then again they are only interested
    accurate measurement of the hot wire current out to something like
    500KHz 3dB bandwidth, and they do seem to have a handle on the concept
    of deconvolution of the hot wire response, so maybe they can deconvole
    other effects, but the physics of the heat transfer does not change in
    that respect. There may be practical advantages to the circuit as well
    such as the maintainace of Vw at the sensor node whether it is connected
    or not.
     
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