Heating pad temp control help

[PatHastings]

so I have 2 cats stuck on my porch that atr not allowed into my house for the time being. some times it gets on the colder side out there so I decided I was going to make their beds a little warmer for them. so I have some 30 AWG Teflon wire and an old computer power supply. I figure I'd put about 10ft of wire in each bed so that gives it 200ish BTU per bed not sure exactly how hot that's gonna turn out so I figure I'd add something like a 50 ohm linear pot to adjust the temp... would it be that easy???
Thanks
-Pat

 

[shrtrnd]

I'm thinking your power supply is going to get hotter than your 30 gauge wire, right before
you burn it out from the short circuit.
If you're doing this because you want to create your own project, you need to come-up
with some type of heating coil system. 30AWG Teflon wire is regular wire, with Teflon
insulation, and that's all. The pot would serve as a resistor for the current to the wire, and
that's all.
Maybe check with pet supply companies, and see what they market. There are plenty
of people-type heating pads available, but I don't know how well they'd work in an outdoor
situation, or what would happen if the cats kept clawing them.
My thoughts. Maybe somebody else has better ideas.

 

[PatHastings]

haha good thing I asked then. I found the idea on some site where the guy was making heated clothes for his motorcycle that would hook up to the 12v battery of the bike. ill have to post it when I get the chance to see what you guys think. yeah thought that was the idea, increase the resistance to lower the temp, but guess not haha

 

[davelectronic]

Power resistors on aluminum sheet raised and fairly thick, play about with load and size of sheeting, cover with a none combustible material, home made heated pad.

Resistors could be aluminum clad power resistors, ratings need some tinkering to get temp and power levels safe. Just an idea.

 

[Resqueline]

There's nothing wrong with using copper wire as a heating element, but I would be concerned with the cats possibly chewing it into a non-functional state.
A long time ago I had this strap-on electric car seat heater. It plugged into the cig lighter and worked like a charm. I think it consumed 4A (50W) in total.
Eventually it failed, and when I ripped it up I found closely spaced slings of cheap thin PVC insulated copper wire neatly stiched to the fabric.
A 30 AWG has a current carrying area of 0.051mm2 and a resistance of 0.34Ω/m.
A current of 0.5A could be considered safe under all conditions (unless it's rolled into a ball, where 0.2A would be safe), and 3A could lead to a wire temperature of 60 C.
So let's say you want to run a relatively safe current of 2.5A (at 5V). That's 12.5W and a resistance of 2Ω, and thus a length of 6m is needed.
So in my view your estimate of 10' in each bed is spot on, if you connect them in series. That gives only 6.25W per bed, which is almost nothing.
You probably meant to connect them in parallel, producing 50W per bed. I suspect this could be too much, and it would require a very expensive (& hot) pot.
The BTU must be one of the most cumbersome modern units made btw.. 200 Btu = 58.6 Watt-hours, not a very good measure of power..
Now, the big question is how large are those beds, and of course if the cats will be comfortable with the power density (W/m2) thus produced.
If you were to halve the current (1/4 of the power) you'd need to add a 2Ω 3W resistor. A 12.5W 5Ω pot for manual adjustment would be more than adequate.