Here is my solution based on your numbers. Here are the assumptions:
1. Rwire = 6.3 Ohms at 250C
2. Iwire in still air = 100mA
3. Pwire in still air = 63mW
4. Power range: 10 to 1. Max Pwire is set at 650mW
The power dissipated by the wire (which will be a function of ambient temperature and air speed) can be computed as illustrated on the schematic.
The wire will be maintained at 250C until the max current is exceeded (by Q2).
The Vo in still air will be about 1.630V. The Vo at 650mW will be about 5.155V.
The current into the wire will be clamped (by Q2) at about 700mA.
U1 *must* be a very low input offset op-amp, or you will get *large* errors. I recommended a chopper stabilized part (though it is surface mount only). The common-mode range should include ground, and the output needs to swing from about 1.5 to 7.5 volts. Max output current is no more than 2 ma.
You'll get best accuracy by using 0.1% resistors where indicated.
Choose the Q1 darlington for at least 5W power dissipation, and a current gain of at least 1000.
Please note high power resistors R1 and R5!
Keep lead lengths short to your hot wire...they will add errors!
The circuit will be unstable without the R6/C1 filter network. This is because there is a thermal lag between current and temperature in the wire. I guessed at the proper values. Your mileage will vary.
The circuit is *fully* protected from faults...you shouldn't have any problems. Good luck!
EDIT:
Also, you can confirm the operating temperature of your wire by putting it in an oven with a thermometer and raise the temperature. Plot P vs. T. The operating temperature is where P is projected to cross 0 mW.
EDIT 2:
The Op-Amp I specified achieves low offset errors by using internal digital switches and an oscillator. You will need an 0.1uF cap across the power supply close to the chip to keep noise from being injected into the signal.
1. Rwire = 6.3 Ohms at 250C
2. Iwire in still air = 100mA
3. Pwire in still air = 63mW
4. Power range: 10 to 1. Max Pwire is set at 650mW
The power dissipated by the wire (which will be a function of ambient temperature and air speed) can be computed as illustrated on the schematic.
The wire will be maintained at 250C until the max current is exceeded (by Q2).
The Vo in still air will be about 1.630V. The Vo at 650mW will be about 5.155V.
The current into the wire will be clamped (by Q2) at about 700mA.
U1 *must* be a very low input offset op-amp, or you will get *large* errors. I recommended a chopper stabilized part (though it is surface mount only). The common-mode range should include ground, and the output needs to swing from about 1.5 to 7.5 volts. Max output current is no more than 2 ma.
You'll get best accuracy by using 0.1% resistors where indicated.
Choose the Q1 darlington for at least 5W power dissipation, and a current gain of at least 1000.
Please note high power resistors R1 and R5!
Keep lead lengths short to your hot wire...they will add errors!
The circuit will be unstable without the R6/C1 filter network. This is because there is a thermal lag between current and temperature in the wire. I guessed at the proper values. Your mileage will vary.
The circuit is *fully* protected from faults...you shouldn't have any problems. Good luck!
EDIT:
Also, you can confirm the operating temperature of your wire by putting it in an oven with a thermometer and raise the temperature. Plot P vs. T. The operating temperature is where P is projected to cross 0 mW.
EDIT 2:
The Op-Amp I specified achieves low offset errors by using internal digital switches and an oscillator. You will need an 0.1uF cap across the power supply close to the chip to keep noise from being injected into the signal.
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