On Mon, 06 Nov 2006 07:46:14 -0800, T-n-T wrote:

In article , PeterD
wrote:

On Sat, 04 Nov 2006 15:09:41 -0800, T-n-T wrote:

Thinking more about it now I do want to err on the side of not killing
my terrarium inhabitants. So a wall wort transformer is the way to go.
the purpose of using wire and a resistor is so the wire can be sealed
in silicone tubing for fish tanks and siliconed on the ends., it just
seems easier to seal up. what theory do I need to apply to heat up say
4 feet of wire not just the resistor?


Four feet of wire with a resistance... g OK, try this for size:

Go to a hardware store and get an eight foot gutter/roof eave heater.
Get the fixed length kind (the really cheap ones), that has just
simple resistance wire that forms a total loop length of about 16 feet
(don't get the kind that can be cut to length, it works on a different
principal!)

Now, we have 16 feet at 120 volts, so if we use only 12 volts (a good
safe voltage) we need a tenth of the length, or about 1.6 feet. If 1.6
feet is not sufficiently long, use two 1.6 foot lengths wired in
parallel.

Personally, I have no confidence you had make such a setup water
tight. I'd make sure the connections are outside the water if at all
possible. Also make sure that any silicone you use is rated for high
temperatures, and allow the silicone to cure for at least a week to
prevent out-gassing (the stuff gives off some nasty gases while
curing!)


I am with you, fortunately this cable is not going in the water, the
link I provided on my post though, they do submerge it. If I needed it
to go under water I would just buck up and buy one. This just sits
atop or just under the soil. And my inhabitants, frogs and hermit crabs
probably wont chew it, but I am going with a lower voltage like 12V.

Maybe you can answer another related question, sometimes I ask a
question that is clear to me, but gibberish to others... i looked for
roof heaters here(Washington State) and the hardware store told me they
didn't sell them here.

Ok a 12 volt 1.2 amp wall transformer for example. What is the formula
to determine the amount of resistance I need to not burn up the
transformer? Because I did hook a transformer up to some resistive wire
and toasted a few transformers.

BTW, thank all of you for your help.

Called OHM's law... g

Try http://www.the12volt.com/ohm/ohmslaw.asp as an example, or just
Google 'ohms law' (no quotes) and you'll get several sites.

Quickly:

1 volt, 1 ohm, 1 amp, 1 watt

so

12 volts, 12 ohms, 1 amp, 12 watts

(divide voltage in volts by resistance in ohms)

To compute the other way (say you want 15 watts at 12 volts):

(volts * volts) / watts

that's volts squared divided by watts, or

(12 * 12) / 15

which is

(144) / 15 = 9.6

which is 9.6 ohms!

How much current?

Well there are several ways to compute it, but the easiest is just
watts / volts, or

15 / 12 = 1.25 amps

I'd use a trasformer rated at about 2 amps myself.

All the above are simple DC computations. They work well if the load
is purely resistive, and the frequency is low enough (which AC line
frequency is).

BTW, 15 watts, if misused (concentrated in too small an area) will
absolutely present a fire hazard... My pencil soldering iron is about
12 watts, and it gets hot enough to melt solder... Use caution, and
spread the heat out over as large an area as possible.