Calculating Heat

[jonwingfield]

I'm new to the board and circuit design (self taught). Question:

Is it possible to accurately calculate heat ranges (+/- 10-20 degrees) on a DC circuit if voltage and current are known? For example, if I want to light up a 12V DC automobile cigarette lighter but limit it's heated output to a certain temperature range, I assume I could limit output current with a pot but how would I calculate the range limit and associated components. Is there a more efficient way to accomplish this without use of a pot using a momentary switch, a few resistors and/or diodes.

Thanks.

 

[Screech]

The cigarette lighter is a coil of resistance wire.
Tap a wire from earth to anywhere on the resitance wire.

If you tap it half way, then it will conduct twice the current/wattage, but to only half of the resistance wire.

as resistance wire is efficent by it's negative x length squared, at won't be as hot per length\meter.(rember there is less air to cool it.)


you will also reduce the safe rated wattage of your cigareete lighter by a 1/2.

IT will also glow brighter.
just don't drop it between your legs! :shock:

 

[stevez]

I am not sure of what you are asking. My best guess is that you'd like to know if you can adjust the power input to a heater or some other load, to hold the temperature constant. If that is your question then the answer is yes. You could also hold the heat output constant by varying the input - that's a slightly different problem.

Let's look at the heater first. We can use the cigarette lighter to build a simple example. The lighter is often a coil of high resistance material - a ribbon of nichrome or similar conductive material. When 12 volts is applied it heats to red hot. If we lower the voltage it doesn't get so hot - and if we raise the voltage it gets hotter - until it gets so hot it melts. We could control the current allowing more or less current to flow to get the same result. Power is the product of voltage and current.

If we pick some baseline case - let's say 12 volts, red hot, 10 amps therefore 1200 watts and a steady-state element temp of 1350 degrees F - it will help to simplify discussion. Let's say we came to this by carefully selecting the element. I'll abbreviate Base Case with BC.

If you do some math you'll come to the conclusion that the element has a resistance of 1.2 ohms. The resistance of most materials varies with temperature - some more than others. The resistance of the element for the BC is likely quite different cold. It's also true that at something other than 1350 deg F it's different.

The temperature that the element reaches, at steady-state, is the result of the heat input and the heat lost to conduction/convection and radiation. Steady-state is simply the point that everything settles to if nothing changes. This could take a millisecond or several minutes. Let's say that for the BC the element is exposed to free air but the air around it is still except for natural convection.

If we do something to the BC we can understand some of the problems with temperature control. Let's say we blow on the element for a moment. What we'll likely see is the color of the element change slightly in the direction of cooler. What will will have done is increased the heat transfer from the ribbon slightly and it's temperature changed. Since the temperature of the element changed slightly its resistance changed - maybe only slightly but it did change. That means the power input changed.

Lets take the BC and enclose it in a small tube so that we can blow air at room temp in on one end - and let the heated air go out the other. This is essentially a hair dryer. If we start with the BC with no air flow the element will likely get hotter but lets just say we change the element so that we're back to the same conditions. If we add an adjustable fan we can vary the airflow from zero to some high amount. As we increase the flow of air across the element we'll see the element look cooler because it will get cooler. The air flow out of the tube will initially be hotter with less airflow and get cooler as the airflow increases. Approximately the same heat into a greater airflow results in a lower temperature rise.

At some point, with air flowing thru the tube, we could change the input power so that the element gets back to 1350 deg F and red hot. We might measure the element temp with infra-red. We could also measure the airflow and do experiments to understand the correlation of airflow and element temperature. At some point we would have enough information to allow us to stop measuring the temperature - if we know the power input and air flow conditions we'd know the temperature of the element, as long as nothing else changed.

Power input to the heater can be controlled numerous ways. You could control the voltage or current - in linear (smooth) fashion. An adjustable power supply can do this. You could also turn it on or off rapidly - fast enough so you can't see the changes in color or temp. You vary the ratio of on to off time - Pulse Width Modulation. You can purchase controllers to do this - ready made, off the shelf.

You can control the power input by measuring the temperature of the element to hold the element temerature constant. You could also look at other parameters such as airflow and control the power input to hold the element temp constant.

Sorry for the babble but I just wanted to describe a little bit of what goes on to help you get a clearer picture.

 

[jonwingfield]

Thank you for your response. I will noodle on this a bit. I would really like to add some circuitry - resistor / diode - but not to overcomplicate things. My main goal is to limit heat output to < 360 F with 12V 1.2 amp input. I am thinking it is a timing question more than circuitry per se.

YOu guys are great.