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I need to design a controlling mechanism for heating a nichrome wire of 15 cm (to be specific 36 gauge NiCr wire ).

These are the following constraints

1)The Power source is 5volts from USB in PC.

2)The temperature range of nichrome wire should be between 30 to 60 deg Celcius.

Once the wire heats up to this temperature the power supply should be reduced and if temperature reduces it should heat up and maintain the prescribed temperature.

So i decided to use a basic rapid prototyping board - Arduino to control the supply to the wire via PWM output.

Since the current output from the GPIO pins of Arduino is in few mA ,i should use a driver circuit for heating the wire .

So here are my doubts

1) How to calculate the amount of current required to heat up the NiCr wire to 30 deg C with 5V usb supply.

2) Am thinking to use a mosfet to supply to the wire ,such that i will connect the pwm output directly form aurdino to gate of the mosfet .Is that feasible for these specifications ? Or i should use any other driver IC's

3)To control the pwm output ,i need to measure the current temperature of the NiCr wire and feed it into the ADC of arduino ,and based on the feedback i have to control the pwm output to the driver.

4) To measure the current temperature of NiCr wire ,i cannot use any temperature sensors or thermostats .I have to measure the resistance across the wire and relate it to the temperature.Here am struck with !

How to relate or create a lookup table to determine the temperature of NiCr wire based on the resistance of the wire.

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  • \$\begingroup\$ What resistance should the wire be in an ambient of X? How does this vary with changes to X? \$\endgroup\$ – Andy aka May 27 '15 at 12:00
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    \$\begingroup\$ One of the characteristics of NiCr wire is that the resistance is relatively constant as the temperature changes. That is one of the many reasons that it is used as a heating element in so many devices. You will probably need to actually measure the temperature in some fashion to provide the feedback that your PID controller requires. \$\endgroup\$ – Dwayne Reid May 27 '15 at 12:39
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  1. Try to find some tables or other empirical data that are similar to your application. There are equations for heat loss in various mediums (King's law for heat loss from an infinitely long cylinder comes to mind), but I suspect you'll get better results from testing. You don't need to be exact- just to have more than enough power under the worst-case conditions.

  2. Well, you don't really have a specification until you know the current, which will depend on a number of factors, and the PWM frequency. If your PWM frequency is low then you can certainly switch the 0.5A available directly from a USB (0.1A is all that's really kosher) without a MOSFET gate driver. There is no guarantee that is enough power, however, even if 100% of it makes it to the wire, that's only 2.5W.

  3. You could incorporate a current measuring IC (or shunt resistor and amplifier) that is fast enough to settle during the PWM on-time and measure the resistance. You could also measure the '5V' since it's not likely exact and there may be significant voltage drop in the cable.

  4. Once you have the resistance you can use the temperature coefficient of NiCr and do a single point calibration (measure the resistance at room temperature without allowing self-heating.

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  • \$\begingroup\$ I worked on a project some time back where we used a wire, like the OP has, that was wound in serpentine loops across the back of an LCD display. (This display was expected to work at low outdoor temperatures in freezing cold weather conditions). The temperature of the display was regulated with PWM and the PWM frequency was low enough where it was a simple process to measure the voltage drop across the total wire loop during the ON part of the cycle. A PFET was used to drive the coil so it could be GND referenced. An opamp scaled the voltage for the A/D converter. \$\endgroup\$ – Michael Karas May 27 '15 at 13:06
  • \$\begingroup\$ I worked on a similar LCD temperature control project another time that had a pair of wires wound in serpentine on the heater pad behind the display. In that application one wire was low resistance to be the heating loop and the other wire was much finer gauge and resistance in the 150 ohm range where it could be used in a voltage divider to directly feed the A/D converter without need of the opamp scaling circuit. \$\endgroup\$ – Michael Karas May 27 '15 at 13:10

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