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Hot-Wire constant voltage Anemometer

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
 
J

John Popelish

Jan 1, 1970
0
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.

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.
 
F

Fred Bloggs

Jan 1, 1970
0
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.

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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