I am trying to predict the approximate temperature that will be generated through a ceramic wire-wound resistor (those little white blocks that are about 10x10x50mm), so that I know if it needs to be enclosed within a case to prevent accidental contact with things (like my fingers), and/or need a fan to assist with cooling.

It is a DC circuit running 12V, and I am trying to achieve a load of around 5W (it is a load ballast for an ATX PSU conversion).

Using the normal V=IR and W=IV calcs, I've worked out that I need roughly 29 ohms to achieve 5W, and will use a 10W rated resistor. A 27 ohm resistor should give around 5.3W.

I figure that by using a couple of resistors in parallel, it would halve the current (and therefore heat generated) in each resistor. A couple of 47 ohm resistors in parallel, would be 23.5 ohms and generate 6.1W, but only a little over 3W per resistor, and therefore be a cooler (safer) solution. Am I correct in my understanding so far?

If everything above is correct, then the last thing I want to know is how much of a temperature rise 3W is likely to create, or how hot these things will get. I found some articles that say to calculate this, I need to know the 'thermal resistance' of the resistors - but I am struggling to find this info, and unfortunately, the suppliers don't have datasheets for them. Do these devices all have a common approximate thermal resistance, or can they vary wildly depending on their source? Is there a general figure I can use to base my calcs on, or will it be a case of 'suck it and see' (though not literally - obviously!!).

Thanks
Graham

(edit)
If I took this a step further, and used 3 x 100 ohm resistors in parallel, then I'd get around 4.3W in total, but less than 1.5W per resistor (so even cooler?) - and I could get away with using the much cheaper 5W resistors, and buy all three for the same cost as a single 10W resistor...??

 

I thought I might find you some useful info at Ohmite, but the datasheet for their TUW series isn't very helpful - apart from a graph that shows the power rating should be derated by 50% at an ambient temperature of 175°C.

With some resistors heat conduction via the leads is important, but with the type in question that will not be very significant. Using two resistors in parallel will result in lower temperature, but it depends on the spacing between them. If they are very close, you will get about 6/8 of the effective surface area of a single resistor.

Here's a Yageo datasheet with a little useful info on temperature rise: http://www.yageo.com/NewPortal/yageodocoutput?fileName=/pdf/throughhole/Yageo_LR_SQP_NSP_2013.pdf

 

Here's a Yageo datasheet with a little useful info on temperature rise: http://www.yageo.com/NewPortal/yageodocoutput?fileName=/pdf/throughhole/Yageo_LR_SQP_NSP_2013.pdf

Many thanks ebp. If I'm reading those graphs correctly, it appears that even at as little as 40% of their rated value, temperatures can be anywhere between 50C and 150C - warm enough I think to warrant trying to position them inside the case, and probably close to the fan so that they get a good airflow across them. The upside is that I can use the fan exhaust to warm my fingers up during cold winter evenings tinkering with my projects out in the garage...

Cheers
G