With all this talk of Heated Beds, I decided i'd jump on the Band waggon cos i like a nice warm bed !
The design brief is simple: easy and low cost.
1 reel resistance wire from Maplin: £6.49
1 Plumbers heat risistant Matt from plubing Shop: £2.00
1 Bit of 4 mm Plate galss 200mm X 200mm from the glass shop: £1.00
This is not yet tested in real life but is working (The Glass gets hot)
Maplin resistance wire runs at about 1 ohm every 200 mm.
Draw around your glass amd measure 4 mm in from 1 side.
Then mark 8mm Intervals and put lines 8 mm apart. use a light coloured Pencil.
You should end up with 24 or so lines and about 4 mm at the other end.
Now draw another line 6 mm from the other side.
Cut 8 X 800 mm lengths of resistance wire.
With a needle sew the wires onto the surface of the Heat Matt.
Each wire makes 3 passes over the mat along the lines you marked.
To connect this up:
Solder all the ends along 1 side to a bit of a bit of 22 swg copper wire so that the glass plate buts up aginst it.
(You can solder Constantine wire but NOT Nichrome)
Along the other edge you need about 2 mm space for the glass to expand. I used my Ruler.
Solder the other ends in 2 sets of 4.
That will give you about 1.5 Ohm resistance or about 100 w at 12 V
I then covered one side of the glass in kapton tape so that if it cracked it won't shatter everywhere.
It's drawing 7.7 amps at 12 volts.
That's 92 watts.