Using resistors as simple heating elements.

[Dragon Tamer]

You have to de-rate the resistor power massively when they are encapsulated. 5 watt resistors run at 4 watt will melt the solder on their legs!

I would run the resistors at about 20% of their rated wattage, and just use more of them. It won't matter how much you de-rate them the "stone" will still give off X watts heat for X watts of input power.

if he uses more resistors, then the only way to maintain their resistance is to use series parallel resistors. i know from first hand experience that series parallel resistors increases the overall wattage of the resistors, which means less heat produced.

 

[Boncuk]

Sure, but's it's still a poor choice to run the resistors too hot. There will be a much larger thermal expansion and greater difference in hot/cold areas of the cast material, both likely to crack it.

If I understood correctly the OP wants to use the heated rock for insects. The temererature doesn't have to be high though.

The oven I built had a very small volume (48ccm) and the resistors used maintained a constant (air) temperature inside the box of 100deg/C. I mounted them on a PCB at 5mm above the board with 2mm dia brass tubing for the wires (to cool the wire) and nothing burned or melted. The total dissipated power was ~20W for a steep temperature slope and used 6 pieces of 5W resistors of 47Ω.

At 12V and the resistors wired in parallel the power dissipation was 3.06W.

Using resistors to heat a rock for insects I assume a maximum temperature of 37 to 40deg/C which can be achieved by using 1/4W metal film resistors and a total power dissipation of 5W.

BTW, cock roaches don't care for heating.

 

[Mr RB]

...
and used 6 pieces of 5W resistors of 47Ω.

At 12V and the resistors wired in parallel the power dissipation was 3.06W.
...

That's about perfect, you used 30W of resistors with a nice large surface area and run at 3W total (10% of rated W).

...
BTW, cock roaches don't care for heating.

Which probably explains why we don't see many roaches here in the tropics but the city where I used to live (much colder) roaches would run rampant.

 

[Boncuk]

That's about perfect, you used 30W of resistors with a nice large surface area and run at 3W total (10% of rated W).

Slight misunderstanding.

I used 6 paralleled resistors with each dissipating 3.06W. That's a total of 18.36W.

 

[crashmeplease]

Some good advice here, thanks everyone! I can't wait to pinch a handful of resistors from work and start eperimenting! De rating them is because of the smell is something I hadn't considered till I tested a 5w resistor at 4.5w, I usually just tinker with microcontrollers and small currents!

 

[Mr RB]

Slight misunderstanding.

I used 6 paralleled resistors with each dissipating 3.06W. That's a total of 18.36W.

Yeouch! No wonder the roaches didn't want to sit on them!