120V heater pad
This page explored 120V ac heaters. Natural choices are heater pads, but these are expensive and custom items. So we made our own nichrome heaters, which have the advantage of complete flexibility for power and size. They are time-consuming to wind without a winding jig.
The nichrome DIY heater element works, but the connection from nichrome to wire is difficult as it goes from copper to steel and there is galvanic corrosion. Further, we run the heater at high temp for fast heat - so we have had the nichrome elements burn out regularly. Solutions are to run the nichrome at about 600 C max, well under its 1000C rating. Another great solution - which turns out to heat faster because it doesn't require insolation over the heater element - is a halogen light bulb.
Halogen bulbs get hot - several hundred C - and radiate about 90% of their energy as heat. Thus, they are fast, effective heaters, and becuase no insulation is used over them, they can heat the 3D printer heat bed rapidly.
- Use resistors instead - 
Halogen As Heater
- Halogens are about 85% heaters.
- For 3D printers, use a 240V bulb, run it at 1/4 power with 120V (P~I^2R) so it doesn't burn out with on-off cycling - by running it only at a fraction of its rated power.
- Use a holder - 4.69 or 4-11/16" holder, for a 240V T3 bulb. 
- R7S is the style of holder -
- 500W 240V - This bulb is T3 R7S, 118 mm long - 
- 1500/375W 240/120v - . Overpowered but may be just perfect - $3/each - advantage that only 1 needs to be used.
- Ideally - want 1000W 240V T3 R7S 118 mm for 250W with single bulb?
Use 2 120V 500W in series, more even bed heating, using 2 bulb holders
- Bulb source: 
- Full description of build is posted under the YouTube video - go to YouTube to see it
To build the halogen heater, take 2 118 mm halogen holders, and solder them in series. SOlder them to the 200C wire that would otherwise attach to the nichrome heater element. Since the wiring is no longer hot, solder will not melt at the connection - making the halogen heater actually much more robust and easier to build.
After you are done soldering insert the heaters inside the heat bed, facing away from each other with the wires inside, so that the heaters are facing the 1/2" conduit sides of the heater bed. This way, the wires are not being heated.
The halogen holders should be parallel to the 5-1/4" lenght of the conduit.
Use the carbon fiber blanket on the bottom for insulation.
Screw the print surface on top of hte conduit. The glass of halogen lights should be pointing to the side, not up - that's the only they will fit without the glass of the halogen bulb touching the top plate.
Note that one of the ends of conduit is taped, see picture below. The inside of the conduit is a convenient place to put the thermistor. One end is taped up, so there are no drafts inside the conduit, which would make the thermistor temperature reading inaccurate.
- Note that for the 6" bed D3D Universal 2, 118mm (about 4.69") is the longest that fits in the bed. Pay attention to not using longer than 118 mm for a 6" bed - but longer bulbs can be used for larger beds.
- We have had a heater element under testing 24/7 for the last 2 months - there is no indication of forthcoming failure, and we expect the lifetime of the heater to be well over the 2000 hour typical  lifetime of a halogen bulb if we are using 240V bulbs and running them at 1/4 power by using 120V.
- JET - Tried new 1" bars on heated bed and these caused smoking once heated to around 50C. I believe this is caused by the coating on these particular pieces of metal.