Quote Originally Posted by Boyan Silyavski View Post

Please correct me bellow if i am wrong before i burn sth:

1.Now as i understand it the wire resists temperature 200C. Obviously if use 3 meters of wire at 33 ohm per meter i will achieve the desired result.

2.They say :"The product can withstand the power of power: 25W/M" but thats for floor heating, so i assume if wire is connected to a temperature regulating device which does not permit it to go above 130C /+ the thermal and current fuses/ there will be no any problem????

3. I understand that also i have to push fit the wire in the channel/ 3mm/ so there is not any air gap between plate and wire which could cause overheating due to incorrect temp measurement.

I plan of: 6/8/mm plate=+ channel+tight fit wire+thermo conductive silicone flush with bottom surface. So no air there art all

Apart from that i will use some insulator sheet from bellow, still not clear but most possibly Teflon, Silicone, Mica ???

What do you think?

I know i can buy 40 euro 500 W ready heater from China for 40-50 euro, stick it to the aluminum bed from below and thats it. But that is expensive compared to what i propose, Chinese will not care to make trace separation properly and so on. I mean its expensive and i dont find it more reliable than sth i could make. Plus machining all will cost only around 10 euros with the plate, which is 1.3 kg

Calculation shows that 3000mm cable/400mm bed =7.5 which is roughly 7 channels on the long side. So ~200mm/7=~28mm separation between channels which seems quite right to me.
Your figures are broadly correct, but there's a bunch of information missing from the cable spec - max current rating (you can guess at 0.8A given the 25W/M - but that's likely derated given the intended installation buried in a concrete scree) - to be honest, I'd just try it and see. Yes, you'll get a bit over 500W heating which should give a respectable amount of heat energy for the 1kg block of ali that you're using. Temperature control you've mentioned, and a thermal fuse (you mention) a necessity in this system; zero-crossing SSR also a good idea as well as a current protection device (fuse). All good stuff, but very little in the data on the cable to give support to your design simply because you're using it for a different purpose. Try it and see - my thoughts are that it should be workable.

My only reservations are: No indication of the thermal expansion of the cable - if you bed this too tightly in the ali block you could sheer the cable strands - I'd look to a loose fit and infill with thermal paste when you look to productionise.

Experiment, is the name of the game - you've nothing to lose (provided you keep a fire-extinguisher to hand :) ) In case the cable fails catastrophically I'd certainly use a test bed well away from anything flammable and be able to isolate the supply easily. Once you have your thermal control in place then test worst-case performance with it shoved in a cardboard box crammed full of thermal insulation (cloth sheets) to make sure that the control system still works.

The one area I'd be concerned about with this cable is making the ends off correctly - looks like you are supplied with copper ferrules to crimp - that to me is the area that you're likely to see failure if not done properly.


I'm not sure I'd agree that this is a cheap solution, though - in the UK I can source heated beds off ebay for £6 each (214mm x 214mm) and these will be being etched in their hundreds or thousands in china with the precision from decades of investment in PCB manufacture. Only 120W, but two in parallel for the 400mm bed and you're not a million miles from your solution.