quick question regarding addition of a steel plate

[tonz1984]

Hello

I have an aluminium light fitting (see attachment) which has 3 LED's inside. The temperature of the LED's is around 95 degrees under normal operating conditions. When we added in a mild steel plate 0.90mm thick, we expected the temperature of the LED's to rise, but it didn't, the temperature actually reduced!

As the thermal conductivity of steel is less than aluminium, the steel plate should surely act as a thermal barrier, and the temperature should rise. But this is not the case, can anyone provide a short explanation.

It's actually a good thing that the temperature doesn't rise, as the lower the temp of the LED, the longer it lasts. I just don't understand why it went down!

 

[IRstuff]

WHERE DID YOU ADD THE STEEL PLATE?

And why is it surprising? Is the thermal conductivity of the steel worse than that of air? Did the overall surface area decrease?

TTFN

FAQ731-376

 

[tonz1984]

Hi IRStuff, thanks for your help,

The alu body is made from an alu plate 6mm thick, that is connected to the alu finned section, the steel plate was added in between them.

As I understand it, the less heatsinking, the higher the temperature of the LED. But although the K of steel is higher than air, it is less than alu. Even though the surface area is slightly increased, by the steel plate 0.9mm thick, I thought that as there are two additional thermal boundaries, and a material of lower K than alu added to the body, then the temperature of the LED would have increased?

Or am I just not understanding heat transfer basics!!

 

[IRstuff]

Note that you already had one interface, so you only added one additional interface, in addition to the bulk thermal resistance.

What was the change in temperature?

IF you added thermal resistance, then yes, you'd expect a temperature rise. However, if the plate is only .9 mm, that's equivalent to about 3.6 mm of Al, which should not be that drastic an increase, in of itself.

Moreover, it's possible, not knowing the full details of the structure, that the steel is actually improving the transmission of heat from the body to the fins, through a better thermal contact, and/or better heat spreading.

My impression from the drawing is that the body is essentially hollow and the heat is being transmitted to the fins along the rim of the body. If so, then the heat has to work its way back to the center of the heat sink before it can move up to the top. The steel would then reduce the thermal resistance to the center of the heat sink.

TTFN

FAQ731-376

 

[chicopee]

In effect you have slightly increased the surface area exposed to the ambient temprature.

 

[chicopee]

Instead of steel, use a thin phenolic plate if you want to increase the LED temp.

 

[tonz1984]

Thank you both very much, you've been great help!