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Topic: Heating Nichrome wire - power, current, voltage requirements? (Read 4673 times) previous topic - next topic

geoland

Mar 25, 2012, 05:47 am Last Edit: Mar 26, 2012, 02:26 am by geoland Reason: 1
Hi. I want to calculate the energy requirements and temperature increase of 3 inches (75mm) of 22AWG Nichrome wire.

Referring here. 3" of 22AWG Nichrome, requires approx 0.5 amps to increase its temperature by 20C (68F). The resistance through the wire is ~0.25R, and increase in R due to heating up to 20C very small - assuming zero.

E = IR
E = 0.5*0.25= 0.125v, is the voltage required to provide 0.5amps to a wire with a resistance of 0.25R and heat it 20C (68F) from 0C (32F). In which case given the very small R values in question, raising the temperature of the wire by 20C from any other temperature state, will require the same current and voltage.

P = IE
P = 0.5*0.125 = 0.0625 Joules 0.0625watts.

It makes sense that the energy required to heat 3" of 22AWG wire will be quite small. I'm wondering if my calculations are correct? Not included, is the additional energy to warm a small flat glass component, 22mm x 15mm x 0.8mm to which the Nichrome will be attached.

The plan is to schedule power using PWM switching a logic level mosfet.

Runaway Pancake


Referring here. 3" of 22AWG Nichrome, requires approx 0.5 amps to increase its temperature by 20C (68F). The resistance through the wire is ~0.025R, and increase in R due to heating up to 20C very small - assuming zero.

E = IR
E = 0.5 * 0.25 = 0.125v, is the voltage required to provide 0.5amps to a wire with a resistance of 0.025R and heat it 20C (68F) from 0C (32F). In which case given the very small R values in question, raising the temperature of the wire by 20C from any other temperature state, will require the same current and voltage.


Which is it?  0.025? or 0.25? ?
0.5A * 0.025? = 12.5mV  (0.0125 V)
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geoland

#2
Mar 25, 2012, 09:30 pm Last Edit: Mar 26, 2012, 02:28 am by geoland Reason: 1
Thanks. It's 0.25R ((1.0015R/foot of wire @ 20C)/4 =~0.25R for 3 inches).

Intuitively, this seems like too little power. I want to be sure that these calculations are ball park.






Grumpy_Mike

Quote
Intuitively, this seems like too little power.

Yes but that includes no cooling or heat transfer to other things, and 20 degrees hotter than ambient is not very hot.

GoForSmoke

Doesn't the resistance of most wire increase as it heats up?
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geoland

#5
Mar 26, 2012, 02:22 am Last Edit: Mar 26, 2012, 03:43 am by geoland Reason: 1
Yes, but it's negligible in the desired temperature range ~20C.

To raise temperature of the wire by 20C requires 0.5A and guess a linear amp/heating scale is probably close enough for this application.

The basic circuit will be an11R, in series with the Nichrome element and +12v supply. PWM the voltage through a logic level n-channel MOSFET Gate, -ve side of heating element to Drain and Source to GND. This gives ~1amp to play with, but I need to calculate the requirements for raising the temperature of the glass. A variable resistor in place of the 11R will provide more power flexibility.

MarkT

Quote

If your heater resistance is really as low as 0.025R you will have trouble with a MOSFET as they usually have an on-resistance of about 0.1R. Therefore most of your heating energy will be going to heat up the MOSFET.


Find a better supplier of MOSFETs, 0.005 ohm is not too hard to find, 0.01 ohm is pretty commonplace these days (unless high voltage parts). They can be commoned up for higher currents, and frankly when we are talking 62.5mW its not a great problem.
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geoland

#7
Mar 26, 2012, 02:57 am Last Edit: Mar 26, 2012, 03:27 am by geoland Reason: 1
Thanks Mark. Actually, it's 0.25R. I worked the numbers again and realised that I had misread the tables at Wikipedia.

Dew321

Hi Everyone,

    I am currently working on a project that requires the use of arduino to heat up a nichrome wire attached to it, and I am using PWM (through the analogwrite function) to regulate the temperature. However, when I set to analogwrite function to around 80 or more, the serial port will suddenly disconnect. I have verified this, as it works when the alligator clips connecting the nichrome wire to the arduino wire are not attached to it but will disconnect right when the clip meets the wire. Does anyone know what is happening here?

      Thanks

Paulcet

Are you driving the wire directly?  Or are you using a MOSFET?  Show us your circuit and possibly your code.

CrossRoads

Dew321, don't crosspost. I already answered this in the other thread.
You should really have started your own topic on this instead of jumping into two others'.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

far_1

Nichrome resistivity changes very little  with temperature. From my experience it's incredibly small.

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