power resistor as heater

[tanzil_dhk]

Hi,
i want to use power resistor to heat up a surface from (30~70 degre celsius).
the area is quite small 3x3 cm and so i want to use single small power resistor
and a control circuit.
Can someone recommend me a power resistor for this purpose?
i want to use it with a supply around 12 v from dc supply.

[FvM]

You can use a 10 - 20 W aluminium housed resistor, e.g. Arcol HS10, 15 ohms. http://www.arcolresistors.com/resist...housed-resiso/

Similar types available from many manufacturers.

[tanzil_dhk]

can you tell me about the temperature issue. To my knowledge higher the power dissipation , higher the temperature.
But how would i know the power vs temp. relation.....
Also how to decide whether i should buy a high resistance or low resistance power resistor

[FvM]

Thermal resistance of your setup (surface - ambient) depends on
- backside thermally isolated or exposed
- emission coefficient (e.g. black metal, painted, blackened)
- horizontal or vertical position
- air velocity
- heat sink by mounting

A rough calculation gives about 60 K/W for a 3 x 3 cm surface, so you don't need more than 1 W in steady state. Resistance an be calculated respectively.

[BradtheRad]

can you tell me about the temperature issue. To my knowledge higher the power dissipation , higher the temperature.
But how would i know the power vs temp. relation.....
Also how to decide whether i should buy a high resistance or low resistance power resistor

Are you heating a metal surface? Consider using a mosfet/transistor in a TO-220 or TO-3 package. It has a metal tab for heatsinking. (You would insert a plastic or mica insulator.) Its heating effect is more efficient than you will get with a power resistor.

You can vary the resistance by varying the bias. You could adjust it to dissipate a lot of power or a little, until you obtain the heating effect you want.

[FvM]

Although I don't see the point of a transistor being "more efficient" than a resistor (how? implementing overunity efficiency?) I agree that it's a straightforward solution if the transistor is acting also as controlling element.

[tanzil_dhk]

Are you heating a metal surface? Consider using a mosfet/transistor in a TO-220 or TO-3 package. It has a metal tab for heatsinking. (You would insert a plastic or mica insulator.) Its heating effect is more efficient than you will get with a power resistor.

You can vary the resistance by varying the bias. You could adjust it to dissipate a lot of power or a little, until you obtain the heating effect you want.

the surface i would be heating has plastic layer (on the heating side) and then metal layer (on top of plastic). i will check the package. faster heating is also a key for my design.

- - - Updated - - -

Thermal resistance of your setup (surface - ambient) depends on
- backside thermally isolated or exposed
- emission coefficient (e.g. black metal, painted, blackened)
- horizontal or vertical position
- air velocity
- heat sink by mounting

A rough calculation gives about 60 K/W for a 3 x 3 cm surface, so you don't need more than 1 W in steady state. Resistance an be calculated respectively.

so, that means 1 W should be enough to raise temp. to 70 degre. and i should calculate resistance using
the formula, dT = power/ resistance, right??

[FvM]

the surface i would be heating has plastic layer (on the heating side) and then metal layer (on top of plastic). i will check the package. faster heating is also a key for my design.

That's no what I would expect for a reasonable heating zone design.

It might be necessary to place a metal sheet as heat spreader below the plastic layer and e.g. attach it with silicon rubber. The heating power must be limited not to melt the plastic layer. Applicable temperature depends on the material which wasn't mentioned yet.

[tanzil_dhk]

That's no what I would expect for a reasonable heating zone design.

It might be necessary to place a metal sheet as heat spreader below the plastic layer and e.g. attach it with silicon rubber. The heating power must be limited not to melt the plastic layer. Applicable temperature depends on the material which wasn't mentioned yet.

I know the heating on top of the plastic will be less than required. And i can not use a metal sheet.there are ofcourse other issues that are not discussed here. I should make it more clear by saying i am right now focusing on generating a heat of around 70 degree on the bottom of the surface. A power resistor that dissipate enough energy to raise temperature into the air under small area of 3x3 cm would be enough.

[FvM]

Too many arbitrary constraints to my mind...

Why don't you simply make some experiments with a power resistor and a variable lab supply?