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