Bertha Mark 3 heated bed

double-posting to catch up.  For Mark 3, I added a heated bed, an LCD and an SD card, and I hid all of the electronics underneath the heated bed.  Heated bed info is a bit weak, so I figured it deserves its own post.

For my heated bed, I settled on power resistors.  I used a 12->24V upconverter as well, which I do not recommend - too many cumulative losses.

This is the initial test.  Resistors taped to an eighth inch aluminum plate, on top of some plumber's heat cloth - Carbon wool.  Good insulator, relatively cheap, not as nasty as fiberglass.  You can see the fan running on my converter - it uses two transistors, both of which get quite hot.

I used 6 5ohm resistors, in parallel pairs giving (5+5)/3 = 3.333 ohms resistance.

V=IR gives us 24V/3.3333ohm = 7.2 amps, total wattage = 7.2x24 = 172 watts, which is plenty for this tiny plate.

I was able to reach 120C on this plate.  Pro tip: a cheap IR thermometer won't read aluminum, too much bounces off.  Put a piece of tape on it and read that instead.

The converter I used was an Ebay special, similar to this guy: http://www.ebay.com/itm/10-32V-to-35-60V-DC-Converter-Power-Voltage-Regulator-/260985897805?pt=LH_DefaultDomain_3&hash=item3cc3f8c34d

it's a 10~32V to 35~60V DC Converter Power Voltage Regulator.  Note that it maxes out at 5 amps output - giving its max rated wattage of 120 watts at 24v (24*5=120).  So I fiddled with the pot on the back, upping the output voltage a bit.

I also made a nice box to slot it in.  This is what RepRaps are for!  Designed in about half an hour, to fit the exact converter and fan I had on hand.  Design is up here: https://github.com/paenian/rostock-redux

It fits!  And this was on the first try, too.  The connectors on the other end were tight, but it worked well enough.

The bed, mounted before I attached heater resistors to it.  It's a large piece of 1/4" aluminum plate, cut with a hand-held circular saw.  Use a guide and a blade with lots of teeth, and go slow - like cutting frozen butter with a warm knife.

Such a nice bed.  Good thermal stability.  High environmental losses, though.

The bed's mounted on sprung screws driven down into the aluminum reinforcements.

I mounted the resistors to the bed with small screws, and covered it with carbon wool.  It works up to about 80C, as long as the fans are running that is.  The aluminum dumps too much heat into the air to get any hotter.

My next heated bed adventure will be using glass with Nichrome wire - it's the same principle, really, just with longer, smaller resistors.