Power Resistor As Heating Element

[muashr]

Hi,

A 25W/12 Ω resistor is to be used as a heating element. It is desired to get it hot up to 100°C. The questions are:

1. Can it be connected directly to the output pin of a microcontroller (which can only output upto 5V) or an additional cicuitry is required (for more voltage and current)?
2. How much current and voltage is required in general for such purpose?
3. Which quantity plays more important role in such cases: current or voltage?

In case of additional circuitry please mention it or provide schematic.

 

[kubeek]

Sounds like a homework to me. Try showing some effort on your side.

 

[Pommie]

Those are just stupid questions.
1. No.
2. Depends how well it's cooled or the thermal mass it's required to heat and how that is cooled.
3. They are related by an equation so both are equally important.

There will be correct answers to these questions but only because they have turned the subject into rote learning rather that pure science.

I remember my daughter coming home from school in the mid 90's and asking if a computer disk was serial or parallel. I couldn't answer as as I had no idea of the correct answer. At that time (before Sata) the data came of the disk serially and was transferred to the processor in parallel.

Still no idea of the correct answers to any of the above questions.

However, I loved the 6 orange balls question. Just wonderful.

Mike.

 

[audioguru]

The datasheet of the microcontroller shows that its output goes as high as 5V when it has no load. Its maximum allowed output current is shown to be 25mA but then its output is not 5V, it might be only 3V.
A simple power calculation shows a maximum output power into a load is 3V x 25mA= 75mW which might be able to make a 1/10th Watt resistor 100 degrees C but nothing must be near it.

The datasheet for the 25W/12 ohm resistor shows how much power it needs for it to reach a certain temperature with a certain size of "heatsink". You forgot to tell us what the heating element is heating. A gallon of water?
The thermal energy required to heat a certain amount of water is found on the web and is measured in btu's.
Then you look up the formula to convert the btu's into electrical heating Watts.

Then you find a power transistor or Mosfet to produce the correct current into the resistor and calculate the voltage required.