Heated bed won't get hot enough

Hi folks,

My heated bed won't go above 90C. My PSU is 30 amps. Checked the voltage on it and it looks fine. I calibrated the thermistor and measured with an IR thermometer and the temp reading looks accurate. Using heated bed mk2a at 12v.

Is my PSU not high enough power?

Specifically I have this one:
[www.sunsky-online.com]

I take two wires from the +12V side and two wires from the ground side of the PSU and run those to the 11 amp and 5 amp supplies, like so:
[4.bp.blogspot.com]

But the PSU has three screw terminals for +12V, so I supposed one of the rails is going unused. If the rails are divided evenly, it would be 10 amps per rail. Should the 11 amp supply wire be two of the screw terminals jumpered? Or generally will one of the rails pull amps from the other as needed?

Or is there something else going on here?

How hot should the heated bed be able to go?

Thanks!

Those PSUs are, normally, single rail. Extra wires from the PSU to the RAMPS board won't help as much as making sure the connection from the board to the heated bed has low resistance. You could start by measuring this resistance and making sure you have adequate wires, properly soldered on and so on.

However, the problem you're experiencing seems to be quite common. At least I had it, with my first printer (Prusa i3). Best solution is to avoid the heat loss from below the bed. I cut a couple of sheets of corrugated cardboard, covered them with aluminum foil and placed them just under the mk2a. Nothing fancy and it was enough to go past 110ºC.

There are ready made thermal insulators for sale but I don't think they'll be much better than the cheap cardoard option.

foul_owl, did you check the voltage of the power supply WHILE it is powering the bed? I had the same problem with a new power supply. I checked the voltage before connecting it to the printer and it was 12v. I checked the voltage while the bed was heating up and the voltage dropped to 10.2v. NOT ENOUGH to heat the bed. I ended up trashing the brand new power supply and got a xbox 360 203W power supply powering just the bed.

Good call, let me measure that!

Any issues with cardboard insulation catching fire? I looked up the flash point but I'm kinda sketched out by that idea.

Thanks!

About fire risk, see this link (answerbag.com) or this other (tis-gdv.de) one. Ignition point is 427ºC. If something else sets it on fire, it will burn from 258ºC. So, not much of a problem under a bed that will hardly go past 120ºC.

Aluminum foil helps deflect heat upwards but may itself pose another hazard: if connections aren't properly isolated, there is a risk of creating a short-circuit. Kapton tape may help here. As it (kapton) may also be a good solution to fix the isolation material.

I've also seen suggestions from other users to cover the bed with a towel until the desired temperature is reached. Never had to try it myself.

I tried the cardboard and from a cold start it reached 120C easily.

However, after leveling the bed, the temp dropped down to 95C and wouldn't climb back up to 120C.

What do you think the issue could be there? Power supply perhaps?

Thanks!

Bed voltage measures 12.12 VDC when bed is heating.

Also, shouldn't we be able to get real time voltage monitoring on the bed, since the arduino controls the bed voltage?

If the PSU is keeping 12V+ under load, it's probably a very decent unit. Where did you measure the voltage? I mean, was it at the PSU terminals or the output from the controller? Also, did you measure again after leveling the bed when, presumably, hotend heater and motors where also on?

I once had a RAMPS that ended up quite burned (no flames, thankfully). One sign it was failing was that it would intermittently disconnect the bed heating without being commanded.

If you haven't done so already, I would suggest measuring voltage to the bed after leveling, to make sure it is still at or around 12V. If it is, then there is something else draining heat from the bed faster than it can keep up. Without knowing your configuration, I would suspect a fan. Do you, perhaps, have a cooling fan on the x-carriage? If you move the carriage up, away from the bed, does this allow the bed to regain the desired temperature?


Oh, about arduino and voltages... It does not actually measure or know the voltage. All it does is control if and for how long power is provided to the bed...

Edited 1 time(s). Last edit at 04/28/2015 07:10PM by Chavaquiah.

The cardboard did the trick! Thank you!!

The bed never get over 98C, but if I put the glass on the top, it can reach 110C, but very slow.

This is Folger Prusa i3. Voltage (all measured while bed heat on):

11.72V on the bed
11.9V on D8 outlet on the bridge
12.3V on the power unit

Try cardboard with aluminium - doesn't help at all. I can print with a glass, but the glass itself is not hot enough (models get distorted especially if printed closer to the side of the bed).

Should I try changing power unit? Is any way I can adjust D8 voltage? Any other suggestions?

Thank you very much!