Jimustanguitar wrote:What's actually inside of a silicone heating pad? For $250 I'd try burning the house down by making my own first
I deal with prototypes all the time, and $250 for 3 is really not a bad price at all. Look at it this way: that is $83.33 for one. The Onyx runs for $69 for one.
Once you start getting actual quantities, prices drop. As a system, an AC bed heater does not need any huge DC current supply which will make it overall a lot cheaper.
Yes it will require a little module that has a Triac, an opto-coupled triac driver, and maybe a bit of a heatsink. (a triac burns about 1.6v off as heat. 1.6V * 4A is 6.4W for a 500W bed.)
It could be a problem if you want to UL list it [the triac module], because then you would have to run through the approvals. Might be why a lot of low volume manufactures use DC instead.
There would be AC SSR's available that would do the job, too. They seem to be about $6 for an infamous FOTEK brand one
http://www.amazon.com/Bessky-TM-White-C ... ywords=ssr
I have a sample of one of these heaters here on my desk. It is 1mm thick, 6x6 inch square, flexible, 245W at 12V. It has a 1.5x1.5 inch area that is bumped up to 5mm thick for the wires.
This one has an adhesive backing, I don't think we would want that (At least not to affix it to the glass, maybe the snowflake). It would conform to the flat glass very well.
Etched Foil Elements
Beginning with a Durex engineered photo tool, Durex photo develops and then
chemically etches a highly precision, resistive foil circuit, to be vulcanized
internally into a silicone heating element. Durex’s silicone etched foil elements can
have internal element coverage of greater than 50% of the entire heating element
area. This coverage lowers the internal heating element temperature providing
longer heating element life. Etched foil elements are superior for high watt density and
higher temperature operation. In addition, because the circuit trace width can be varied in the design and manufacturing
process, etched foil elements are ideal for distributed watt densities within the same heating element. Durex’s
efficient production methods result in etched foil being the better choice for low cost, high volume runs of smaller size
heating elements
Then they bond it into fiber reinforced silicone.
Oh, he says it can run at up to 200c, but he doesn't like them run that hot contineously. The 110-130c for a HOT bed would be no problem. And it would reach that temp in minutes.
I don't think I would want to mess with a walmart heating pad. I don't think your going to find any that have 500w output for a square foot or so...
