How can I quantify the change in resistance value due to the heat generated by the circuit?

blue777777

How can I quantify the change in resistance value due to the heat generated by the circuit? [Copy link]

 

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The potentiometer I use has a resistance range of 0~50Ω and a temperature coefficient of 100ppm/℃. It is generally powered on for 24 hours under working conditions, and the working current is calculated as 4mA DC.

I calculated that the heat generated is about 70J, but the temperature coefficient seems to refer to the effect of ambient temperature.

Is there any relationship between the two?

qwqwqw2088

4mA is very small, it mainly depends on the ambient temperature.

What is the rated power of this potentiometer?

blue777777

qwqwqw2088 posted on 2022-2-17 09:37 4mA is very small, it mainly depends on the ambient temperature and the rated power of this potentiometer.

Rated power 0.5w (70 degrees Celsius)

But if you work for 24 hours, 70J of work is not a small amount.

chunyang

According to the temperature coefficient calculation in the manual, the temperature change is determined by the heating of the device, the heat dissipation conditions and the change of the ambient temperature, which are all variables in the calculation. If you want to calculate, the biggest difficulty is to obtain the thermal resistance parameters, which is impossible under non-professional conditions. However, considering that the potentiometer is generally not small in size, the temperature rise caused by the 4mA current can be ignored, so only the impact of the ambient temperature change can be calculated. If it is a miniature potentiometer, the temperature rise can be measured in a stable environment, and then estimated by linear accumulation.

chunyang

blue777777 posted on 2022-2-17 10:09 Rated power 0.5w (70 degrees Celsius) But for a long working time of 24 hours, 70J of work is not small.

How did you calculate it? List your formula and calculation process.

blue777777

chunyang posted on 2022-2-17 10:27 How did you calculate it? List your formula and calculation process.

So... Q=I2Rt

maychang

"I calculated that the heat generated is about 70J."

70J is calculated based on 4mA, 50 ohms, and 24 hours.

How much heat is generated is not important. What is important is the power. 70J is the heat generated in 24 hours. Spread over 24 hours, the power is only 0.8mW, which is much smaller than the rated power of the potentiometer, 0.5W (a difference of three orders of magnitude). Therefore, there is no need to consider the heating of the potentiometer itself, just consider the change in ambient temperature.

blue777777

chunyang posted on 2022-2-17 10:25 Calculate according to the temperature coefficient in the manual, and the temperature change is determined by the heat generation of the device, the heat dissipation conditions and the change of the ambient temperature, which are all variables in the calculation. You want to...

Thank you! So we need to test and measure the temperature change first before evaluating the feasibility of the design?

blue777777

maychang posted on 2022-2-17 10:37 "I calculated that the heat generated is about 70J." 70J is the result of calculation based on 4mA, 50 ohms, and 24 hours. How much heat is generated is not important...

OK, thanks!

chunyang

blue777777 posted on 2022-2-17 10:35 Just... Q=I2Rt

The key here is to calculate the dissipated power.

blue777777

maychang posted on 2022-2-17 10:37 "I calculated that the heat generated is about 70J." 70J is the result of calculation based on 4mA, 50 ohms, and 24 hours. How much heat is generated is not important...

I would like to ask again, why is power important? If the power is too high, will the temperature change be great?

chunyang

blue777777 posted on 2022-2-17 10:39 Thank you! So do we need to test and measure the temperature changes first before evaluating the feasibility of the design?

Calculate the estimate first to see if you need to test first.

blue777777

chunyang posted on 2022-2-17 10:44 The key here is to calculate the dissipated power.

How to calculate this

chunyang

blue777777 posted on 2022-2-17 10:44 I would like to ask again, why is power important? If the power is too high, will the temperature change be very large?

First, understand the relationship between heat and power.

chunyang

blue777777 posted on 2022-2-17 10:35 Just... Q=I2Rt

The reason I asked you how you calculated it is to remind you that you have got the basic concepts wrong and used the natural formulas incorrectly.

chunyang

Think about why I said in the post on the 4th floor that the temperature rise caused by 4mA current can be ignored.

maychang

blue777777 posted on 2022-2-17 10:44 I would like to ask again, why is power important? If the power is too high, will the temperature change be very large?

70 joules is the work done by the current in 24 hours, which is the heat generated by the potentiometer. If the same amount of heat is generated in 1 minute, the potentiometer will obviously be very hot in this 1 minute, that is, the temperature will rise a lot. If it is generated in 24 hours, because the potentiometer is also constantly dissipating heat in 24 hours, the temperature rise will be very small.

chunyang

blue777777 posted on 2022-2-17 10:45 How to calculate this

We all learned this in junior high school: P=UI=I^2R=U^2/R.

maychang

blue777777 posted on 2022-2-17 10:45 How to calculate this

Power is the calculation formula you listed on the 6th floor minus the time t, which is the current squared multiplied by the resistance.

blue777777

chunyang posted on 2022-2-17 10:47 The reason why I asked you how you calculated it is to remind you that you have got the basic concepts wrong, and the natural formulas and so on are also used incorrectly.

Emmm, how about you explain it in detail? If the dissipated power you just mentioned refers to thermal power, then isn't it just my formula divided by time? The energy change brought by 24 hours of cumulative work is 70J, then this heat will change the ambient temperature through heat transfer, I don't know if my understanding is correct