Large Aluminum heat bed can't get up to 110c

Hey Everyone,

I recently snagged a 200x300 aluminum heat bed from my friends up at folgertech in NH (You may know them from their I3 kit, I have been buying all my supplies in person from them for over a year now and they are great people)

I am replacing a 200x300 pcb heat bed that I got from them a few months ago because I want to use an inductive sensor for auto bed leveling. To get the original PCB bed up to temp I replaced the fuse and mosfet with similar higher rated counterparts. For the record, my PSU is a repurposed Corsair 750W computer PSU which is rated to 60A on a single 12v rail.

Originally I was using 2x 18 AWG wires to connect the bed to my ramps board, using this method I got to around 82c in about 20 minutes which is the longest I'm willing to wait. After an hour it made it to 92c (because science)

I decided I needed insulation, so I got some aluminum topped foam pipe insulating tape from lowes and applied it to the bottom of the bed, helped, but not good enough. now I hit 92c in 20 minutes, 100 after an hour.

Next I decided to use thicker wire so I switched from 2x 18 AWG to 1x 14 AWG wire that I grabbed from a generic PC power cord. Little change, capped at 102c after an hour. I measured 11.6v @ the bed connectors

I then decided to see if I was losing power/voltage by going through the RAMPS board. Some people say to hook your bed right up to the PSU using a relay to control it. I decided to try science before buying a relay and hooked my bed directly to 3x 18AWG yellow 12v wires from the PSU and turned it on, after 20 minutes I got to 88c... got frustrated and didnt wait an hour. 11.82v @ bed connectors

Not sure what to try next. 60A + thick ass wire shoud = tons of heat but I just cant seem to get there. Help?

The easiest solution is to use a 12V LED power supply instead of the ATX power supply, and turn the voltage up to 14V. But be careful if you are using Arduino/RAMPS and it is also driving an LCD, because the voltage regulator on the Arduino will already be getting hot, and increasing the supply voltage will make it run hotter.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

is the printer enclosed? or at least shielded by breezes? You may think ambient temp is nothing, but it's all relative. If it's 80f in your room....and the bed is 80c (200F)....if you walk by and create a slight breeze...you're moving air more than half it's temp across it. Cooling it a lot. Even just a quick breath over it u can see a temp drop.

My bed was taking too long to heat to 80c....until i made a 4 sided enclosure (not top or bottom) to cut down on any slight air movement across the bed. I bought 2'x3' foam craft board from walmart.. $.88 a each. Taped them together and now my bed heats very quickly. I've even gotten it to 90, which it wouldn't do before.

I'm no expert...maybe it is something electrical. Just offering my experience.

Yes the printer is enclosed. I build a dedicated cabinet to house both the printer and the filaments. This is my setup:
[www.thingiverse.com]

Ehhh.....
Few things:
First, you didn't mention whether you updated the firmware when you went to the new thermocouple. A mismatch in temperature sensor and firmware can lead to weirdness. That's probably a given, but I figured it should be mentioned...

You need to check the resistance of the bed. Your voltage drop seems wrong. After you changed to 14 AWG wire, you noted that the voltage drop was 11.7 volts. I'm assuming you're running 12 volts through that, or else your voltage drop is way higher then I'm thinking and you should disregard this.

Volts = Amps * Resistance (in ohms.)

Comparing a bit...
11.7 volts / 12 volts ~ 0.975, which is very small for a heating element that behaves allot like a purely resistive element (most heated beds are resistive circuits.).

If you're bed isn't resistive enough, it's not going to produce heat, since the formation of heat in a resistive circuit equals ohms times amps squared.

P(heat) = Ohms * (Amps squared).

If the bed isn't resistive enough, then increasing the amperage load may not work because the circuit can't convert enough of the amps into heat. Check the resistance of the bed. I think it's supposed to be around 64 ohms, but I might not be correct on the inherent resistance. Someone else can and probably will correct me if i am mistaken.

Finally, a thin glass build plate would still increase the rate at which the bed heated up, simply because glass is a very poor thermal conductor. Every little bit helps.
Also... if your printer has dual fans on the dual extruder in that picture, you're probably going to need to vent those up and away from the bed. That could cause problems even in an enclosure.

Edited 1 time(s). Last edit at 05/31/2015 11:09PM by Qcks_.

Firstly, I am using the same themistor I was using before and have also checked the temperatures with a infared thermometer and temperatures are accurate (good thought though)

I measured the resistance of the bed and got 1.5 ohms. In quickly measuring my old bed, which was able to hit 110c easily (though it is PCB not aluminum) I get .9 ohms. I havent been able to find good information on the resistance should be on the 300x200 bed

For these tests I am still using my glass plate because I dont have the inductive bed leveling setup yet.

Thanks for the help!

EDIT: Yes im running 12v

Edited 1 time(s). Last edit at 06/01/2015 08:00AM by McSquid.

Looks like the resistance is the problem. Using info I found here:
[forums.reprap.org]
Then here:
[rapidtables.com]
It looks like my aluminum 1.5 ohm bed can only pull 8 amps for 96 watts.
For comparison my .9 ohm bed could pull 13.333 amps for 160 watts. Which sounds correct to me since it blew the 12 amp fuse that I then replaced with a 15 amp.
Looks like I'm gonna hit up the folgertech boys for another bed with lower resistance.

...or look for another 12V PSU? Does the heated bed have different solder pads for 12/24V?
You already use a relais, so you could put another 12V PSU in series for the heatbed only and still use your old PSU for the RAMPS ( and the heatbed )
-Olaf

@o_lampe:
The MK printed PCB heated bed have solder pads that, using some extra bits of wire, can be easily configured to run either 12 volts or 24 volts.
The instructions for changing between one configuration or the other is printed on the board.
I'm not sure about the Folgertech stuff, but I imagine it's similar.

Unfortunately, The one I have has only 2 solder pads. I have seen others that have the 24 volt extra pad but no such luck on this particular model. I'll post a picture soon, I'm going back to my old bed for the time being. Got some stuff that cant wait.