Possible to “Superrate” Resistors With Cooling?

If I move a huge amount of air with several powerful fans, could I push 5W and 2W carbon composition and carbon film resistors maybe 20% beyond their power rating? If so, how else could I improve cooling? I'll be mounting the resistors on plastic with hot glue. Voltages between components could get up to 600V, so I don't feel safe mounting them on an aluminum plate.

Hi Cody

I think the real answer is to buy resistors that have heat sinks such as supplied by RS components 188-289 which have a large heatsink about the resistor so as to help in the colling process and then to usea fan but since you are mounting these on plastic id be very cautious with regards to this ensuring that you use stand off resitors on a stip bour or similar so that no direct contact is made with the plastic casing at all

Strict;y speaking you would possibly get away with 2+- 20% but because of tempreature co efficiants Id be inclined to move to the next power rating myself This is far safer and snures resistor stays cooler

See link below for details
http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0188289/

or

http://scoraigwind.co.uk/using-a-high-power-resistor-as-a-dump-load/

It would be a thick sheet of plastic with the resistors open to the air, not a case. Larger resistors go up in price rather rapidly, so I'm not willing to purchase huge resistors, such as those built into heatsinks. I'll suck it up, take your advice, and buy larger resistors, though. I'll also make sure there's some spacing between the resistors and the plastic. Thank you.

Make your own heat sinks then Bit of tin old tin can pair of scissors if you have too wrap it wound the restors with an extended fin

Solder it together round the resistor and bingo , "Save yourself some money another idea for you "

Yes forced convection cooling (a fan) will increase resistor power capability.

http://www.ohmite.com/techdata/res_select.pdf

Mounting the resistors with glue to plastic changes the free-air power capability. This can be calculated to see if it increases or decreases it. Or, you can run a resistor at its free-air at rated power and measure its surface temperature. Then mount the resistor to the plastic with glue and measure the resistor surface temperature. Change the power to achieve the power found by the free-air test and that is the "new" power rating.

How to measure the resistor surface temperature? Due to the small component a thermocouple can have a significant error but because the error is the same for either case (free-air and mounted) the method above will work. A thermal camera can also be used and then there's always your finger. With both tests running move your finger from one to the other and you will be able to gauge when they are close to the same temperature.

Every piece of advice you give me is ingenious. What do you think about Mark's idea of wrapping each resistor in aluminum from soda cans? I'd sand the outsides and use glue as an insulator between the aluminum and the resistor leads.

Marks idea is good and is sort of like the ubiquitous Dale power resistors. They look to be leaded resistors press fit in an aluminum heatsink.

Another trick is to cut a copper penny in half and solder a half to each end of the resistor. I don't know how much this increases the power rating.

Carbon film resistors are used in Amateur Radio oil cooled RF loads. The mineral oil increases the dissipation rating by 10X or so.