These are available commercially, but are quite costly, so I got around to considering building some. I am in the UK on 240 volt mains and need five pads to go in outside wooden kennels in separate dog runs. The concept is two metal plates in a sandwich about 4 feet long by 3 feet wide. Within the sandwich is a heating element.
My first thought is run them on 240 volt AC, maybe through an isolation transformer, rather than dropping down to say 12 V AC. For a heating element what about several metal clad power resistors screwd under the top plate? I envisage about 25 Watts of heat energy warmth being enough versus running costs. The plates will of course be well grounded using armored cable. How many resistors to give a decent heat spread through say 3mm thick alloy plate for the top of the sandwich construction, with thinner closing the bottom? If I went the safety route how inefficient would it become on say 12 Volts using the transformer from a big automotive jump / charger I have redundant?
Any ideas for other types of heating element welcome and just general thoughts? The kennels are quite far apart, and mains power is available very close to two of the runs with two kennels in one run and a single kennel in the other run, which are directly alongside two sides of a big steel building. The other run with two kennels in it is separately located and about 35 feet from the steel building. The steel building has mains power. It's quite possible to easily bury armored cable from the steel building to the separate run, as the ground between is earthen.
Thanks for reading!