Improving Heat Bed performance

[old man emu]

I recently have been unable to get my heat bed over 98C. I had a number of exchanges with Colin at MakerFarm and these are some of the symptoms and diagnostics.

1. Resistance of the bed's heater coil. Mine measured 1.3 ohms. Colin said that in his experience the resistance value ranged between 1.5 and 3.5 ohms. Lower resistance in this circuit means less heat.

2. Power output of the power supply. According to the label on the ATX computer PSU, it is capable of an output of 18A at 12V. I don't have an ammeter to measure the amperage. The suggested minimum power output for a PSU for these printerss is about 15A or so. So my PSU has the grunt. It was suggested that the power output quoted might not be what was coming out. That's possible as the PSU was salvaged from an old computer.

The original set up of the RAMPS has power from the PSU going to a switch on the board, then out to the bed. This sort of thing can lead to a loss of power, and you can cook your board by running too much power through it, especially if you are running at temperatures above 110C.

The first step in trying to get more grunt into the heater was to stop running current through the board by installing a relay. The relay only uses a little bit of power through the controller board to open and close the circuit from the PSU to the heater.

The next step was to eliminate any power loss in transmission. I found that to run 18A about 30cm from the power supply to the heater, I needed wire that was at least 7AWG which is about 3.6mm diameter and good for 30 amps.

I replaced the power feed from the PSU to the heater with this wire, running via a relay. The relay is switched on and off by connecting it to the terminals of DB8 on the board.

Finally, heat loss had to be eliminated from the bed. I had some thin cork sheet that I had been going to use as underlay for a toy train track. I cut some to size and glued it to the underside of he heated bed, covering the thermister as well.

Came the test run and the bed readily heated to the 100C that I had set. Then I raised the setting to 110C and that was reached in a few minutes. I left the heater on while I ran the extruder to 220C to see if diverting power would affect the control of temperature on the bed. It didn't.

So it seems that the relay, thicker power cable and insulation solved my problem. But ... in conversation with Colin, I learned that he prints at 90C successfully. Grrrrr!

As I've been writing this, it came to me that putting a similar relay in the extruder heater circuit would help save the RAMPS board by diverting a high current flow away from it.

Old Man Emu

[jimc]

thats great you got it working right ome. i print abs only and have never had to go over 95deg. i have printed the same part over and over at all temps up to 120 deg and saw no benefit above 95. actually i think it warped worse the higher it got.

[DrLuigi]

Well ive had 115 in about 5 minutes no problem, It does sometime go back to 110 but goes back at 115 after a few min.

Never had real problems with it like you.

[Kingoddball]

Not sure what heatbed you use, but I have a spare Prusa Heatbed if you want it?
Just an older MK1 Red. Maybe you can just swap it out?

http://reprap.org/wiki/PCB_Heatbed

[old man emu]

I'm happy that the problem has been fixed. I don't think it was the lower than average resistance in the bed, rather that the current getting to the bed was a bit weak. The relay and heavier wire seem to be the fix.

I bought another relay today for the extruder heater circuit. I'm using it mainly as insurance to preserve the controller from heat build up.

On another note, I don't think that the heat bed temperature question has been adequately answered (Which way? Over or under 100C?)

OME

[DrLuigi]

Hey, just wanted to ask how long it takes for you to go to 100 or 110 if you can hit 110 now.


Just wanna compare and look if mine is normal or yours slower etc.

[old man emu]

I will try to get out to my printer today, but I have a lot of domestic chores to do (Wife's "I want ya to" list).

To speed things up, I have been putting a small pile of 4-5 sheets of paper on the glass to act as a blanket. Today I am going to put a sheet of aluminium cooking foil between the pad and the glass to see what effect that has.

Also, I am going to fit a relay in the extruder heater circuit so that high current does not have to pass through the controller board.

OME

[old man emu]

I fitted the relay for the extruder head heater. Typically, I put Tab A in Slot B and I think I've let the smoke out of the circuit. Now I've got a runaway heater element and the only solution is to replace the RAMPS board. I think I also frizzled the USB port the printer was connected to :-(

OME

[KDog]

Crud! Sorry to hear that. I hadn't read this post before but would have chimed in that, without the heated bed, Your setup will use less than 5A and I think most of that goes to the steppers. No real reason to relay out the hot end.

BTW, I caught my first heated bed on fire messing with a relay. Turns out I tried to put an AC version on instead of a DC. Was a beautiful stream of yellow smoke. Stunk up the house something crazy!

[Davo]

OME, we print ABS with the buildplatform at about 60C; I'm sure the glass plate is a little less.

I'm curious - what do you do at 100 that you can't do at 60?