Another Noob Heatbed temperature issue

Hi everybody, I just set up my prusa i3 kit from Folger Tech, which comes with the RepRap MK2B heated bed. With my initial testing, I found that I couldn't get the board hotter than 90 degrees or so. In trying to fix this, I went through the relavant wiki pages, quite a few forum posts here, and tried many steps of my own before posting. Here's my findings.

I'm, running marlin 1.0.2, cloned from github 2 days ago, and configured with the proper settings for this printer. (folger supplies an older version of marlin, I used a diff program between the 2 config files to find the right settings). I have confirmed that my thermistor tables are right through the code and with an IR thermometer. It's table 1 for a 100kR thermistor.

My solder joints are good, and I am using 14 awg super high strand count silicone shielded wire. I used a piece of it to bridge pads 2/3 together.

My room temperature is 22 degrees, no moving air.

The PSU I was using is the 30A one that came with the kit. During testing, bypassed the RAMPS and the PSU of questionable origin, and attached a server power supply directly to the bed. It's rated for 65.8A constant output on it's single 12v rail. I've powered two 350w subwoofer amplifiers and 4x 10a DC air compressors before, and the PSU handled fine. Measuring the voltage during heating, and I get 12.92v.

Unfortunately, my multimeter is not incredibly accurate for measuring resistance. It only measures down to the whole ohm. It reads 2 ohms, but it could be reading that for anything greater than 1 ohm, even if it is 1.2 or 1.3 ohms, which I hear is the resistance of a properly working heatbed.

Is there anything I have missed in testing this? could something be defective? Is it worth using the server power supply for the entire machine over the other one? What about going to 24v? I have a few of these power supplies, and it is possible with a few modifications to put them in series.

Thanks for your help!

This is unfortunately a common problem. There are a few things you could measure:

1. With the heated bed connected to the printer electronics and heating up, what voltage do you measure between the heated bed terminals? How does that compare with the voltage at the input to the printer electronics?

2. With the heated bed connected to the server PSU, what temperature does it reach eventually?

The results will indicate whether the problem is lack of voltage when the bed is powered from the printer, or lack of heating power in the bed itself. There are a few potential remedies, depending on which one it is.

---

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].

thanks for the quick reply!

1. Using the supplied PSU, the voltage during heatup is 12.14v at the bed, 12.15v at the RAMPS power connector. For the Server PSU, it's 12.91v / 12.92v.

2. With the Server PSU I am reaching 96 degrees. This is with the thermistor taped directly to the PCB(underside) of the bed about 2mm from the center hole.

So the bed is getting enough voltage, but not producing enough power. To increase the temperature, you need to improve the insulation and/or increase the voltage a little.

Put a sheet of thermally insulating material under the bed if you haven't already. A sheet of corrugated cardboard or cork will do. While you are heating the bed you can put an insulating mat on top of the bed to speed up heating.

To increase the voltage, check whether your PSU has a voltage adjustment potentiometer. The cheap Chinese LED power supplies included in some kits have one, close to the low voltage end of the terminals. But beware of the voltage regulator on the Arduino overheating when you turn the voltage up if you are powering an LCD display or anything else from the 5V rail.

---

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].

Following your advice, I put some insulation under the bed, switched to the server PSU and cranked the voltage up to 14v or so (no load), and put an old halogen lamp approximately 40 cm above the bed, for lighting and keeping the ambient temps warm. Works like a charm! thanks for your help!