Bed heater arrived today. I ended going with the MK2B heater since cost wise it was $20 vs well over $190 for the 280mm round version becasue of pastage to Australia. The PCB is powered for a 24dvc supply and is attached to a 6mm thick x 320mm diameter aluminium plate which does and excellent job of spreading the heat. The white dots that can be seen in the photo are thermal paste for my test probe. The difference between the centre and outside edge was only 3ºc which is more than acceptable. On top of the aluminium will be a 300mm diameter glass plate that I will pick up tomorrow. The alloy plate is able to expand as the mounting holes are oversize and 3 springs keep the plate at the calibrated height whilst adding some protection should the head crash.
The heater pcb is coated with heatsink paste to fill the air gap between it and the aluminium to aid in the thermal transfer and will be insulated with backup blanket left over from the Heat Treat Furnace I built.
There a new version of the mk2b that includes the alluminium in a octogon patern perfect for a kossel. Might want tcheck it out.
How long to get it to temp using 24VDC? I know the 12VDC versions took forever to heat up.
I’ll do a heat time test tomorrow when the insulation is fitted and post when done
Thanks @Glenn_West . The killer here is postage from overseas but I’ll certainly check if out.
Beware of using mains cable for DC. It can lead to stupid mistakes in the future! (Imagine applying 240v to that hotbed by accident… 
Might be worth ducking into Jaycar to get something red/black.
Changing the mosfet (forgot the number) increased my heat up time but it’s like maybe a couple mins using this heated plate. I had to change it out as the stock one on the Ramps burned out. Putting a heatsink, new fet, and a fan = problem solved.
Heat up times are in and a bit slower than I would have liked. Since I am using glass bed I might change the aluminium plate to 3mm thick to reduce the thermal mass. Altermnatively I could purchase some Kanthal A1 wire and make a spiral element and run it from 240vac
9 min = 60ºc
21min = 85ºc
24 min = 91ºc
30 min = 100ºc
35 min = 105ºc
37min = 110ºc
Also mains cable may not support the current. I jsed unspeced zip cord from diy amd was getting all kind of heating loss.
My beds i got free shipping. Check out aliexpress
Aluminun plate is acting as a heat sink sucking away heat. Why not put a glass plate under the heater, and one on top a a print bread?
@Glenn_West , agreed, the cable is only 0.75mm^2 as all I had on hand. Picking up some 1.5mm^2 today that is equivalent to 16 gauge. Will redo trial after that
Picked up some 2.5mm^2 silicone coated wire. Will have no problems with that given the current draw at 24v is only around 5.5A
I may try glass but the bed I got is machined underneath for the thermistor and wires (flat surface to mount to mk2)
This has got me thinking about heat up times. Given my printer chamber is nearly 1.1m high, waiting for 20 -30 minutes for the bed to warm may not be a problem as the chamber also will need to heat unless I add a dedicated heater for it also. I always preheated the Up printer chamber and bed for 30 minutes then sent the print job. The software even has a pre-heat for 1 hour or 15 minute button.
That’s cool. What about adding a sensor for ambient air?
mine using the 24Volt position is about 10 minutes 15 at most, but if you want fast, do 24volt in the 12volt. I did that on my silicon heater with glass bed, and the glass would crack from the rapid rise.
@Zane_Mitchell I have spare temp inputs on the smoothieboard so sounds like a plan
Well I know why the heat bed is taking too long to heat. The resistance on the 24v leg is 5.0 ohms which means it is only drawing 4.7-4.8A so only 115W. The specs say the resistance should be 3.0-3.4 ohms which would give around 7A draw or 170W. Even the 12v legs resistance is 2.6 ohms where it should be 1.0-1.2. My guess is the copper tracks are thinner than 35um. Looks like a call to the supplier is in order to see if anything can be done, otherwise just put up with the longer heat times for now.
Thanks for your work here Brad. I really need a heated bed due to the temps in my “lab” (15C). I’m also building a heated chamber, but that won’t solve all my issues.
I mounted the heater bed to the printer but only used 2 layers of cardboard lined with to reflect the heat towards the heater pcd and the plate only got to 80ºc becasue there was too much heat loss into the printer base. I will remove and add some decent insulation since when I heated it sitting on oven gloves it got to 110ºc no problem, all be it a little slow.