I recently made a box with power resistors in it.
I have 10A flowing in it with 13Volts. I have 400W resistor in the path.

But its heating damn hot. I have no provision for a fan.

1 Why does it get so hot when i have high wattage resistor?
2. WHat is a good solution for this issue?

thanks
jis

 

You have to dissipate about 130 watts of thermal energy! Do not be surprised if the box is becoming hot, you should worry if it stay cold!

Explain what you want to achìeve, for possible solution.

Cheers

Alberto

 

i am doing a test for 2-3 hours continuously and my box with resistors act as load. More like a test which tell my product is reliable.

But the box which i use for testing is not reliable due to heat.
I want to keep the box from breaking down, so i need to keep my heat down.

thanks
jis

 

If you pump 130W into a resistor it still has to dissipate 130W regardless of the wattage rating of the resistor.

It is a question of how rapidly you can remove the heat. If the resistor is in a closed box a 1000W resistor will still get hot.

There are only three ways or removing the thermal energy:

1. conduction
2. convection
3. radiation

 

You should mount your resistor on a large metal surface to act as a radiator and dissipate quickly the thermal energy produced. A blower properly placed will help.

Cheers

Alberto

 

The power rating of a component is the amount of power it can handle, not the amount of power it can handle without getting hot. 100W into a fixed resistor is exactly like 100W into a light bulb - it's gonna get hot. A light bulb is designed to dissipate the heat without some source of external cooling other than direct radiation into the surrounding air. A large power resistor is not, and the data sheet probably has information about a maximum case temperature. It's your job to make sure that max temp is not exceeded. You need either an air stream or a cold conduction surface if it is to run at anything near its rated power level.

The very first audio amplifier I built I made the same mistake. A 2N3055 transistor is rated for 115 W, so I thought it would handle 25 W with no problem and no heatsink. oops.

ak

 

i am doing a test for 2-3 hours continuously and my box with resistors act as load. More like a test which tell my product is reliable.

But the box which i use for testing is not reliable due to heat.
I want to keep the box from breaking down, so i need to keep my heat down.

thanks
jis

You need to find a way to remove the heat (as MrChips has indicated), or build the load box to withstand the heat....

 

You need to add a fan/vent plain and simple
50cfm should be plenty..

http://www.hoffmanonline.com/stream_document.aspx?rRID=233309&pRID=162533

 

If the voltage is low just dump the resistors in a pail of water.

 

or oil. oil cooling helps the heath cantenna disipate a lot more power.

 

Use one of these resistors bolted to an Aluminum plate that is exposed to the outside of the enclosure.
Max.

 

i am doing a test for 2-3 hours continuously and my box with resistors act as load. More like a test which tell my product is reliable.

But the box which i use for testing is not reliable due to heat.
I want to keep the box from breaking down, so i need to keep my heat down.

thanks
jis

If you haven't already done so - you can get aluminium cased resistors with one flat surface and bolt-holes for mounting to a heatsink.

Bolting these to the inside of a metal box would at least ensure the minimum thermal resistance to free air outside - you could reduce the thermal to ambient even further by bolting extruded (finned) aluminium heatsink to the outside, overlayed with resistors inside.

 

While making the same mistake, bolting a triac to a sheet metal box, I found that the metal was too thin to move the heat. I bolted on a slab of aluminum as a heat spreader and got it running at, "just warm".