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[on:]

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.

[Reply #1 on:]

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?  0.025Ω or 0.25Ω ?
0.5A * 0.025Ω = 12.5mV  (0.0125 V)

[Reply #2 on:]

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.

[Reply #3 on:]

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.

[Reply #4 on:]

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

[Reply #5 on:]

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.

[Reply #6 on:]

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.

[Reply #7 on:]

Thanks Mark. Actually, it's 0.25R. I worked the numbers again and realised that I had misread the tables at Wikipedia.