10 - Power Resistor heat bed (12/27/14)

Post date: Dec 27, 2014 6:45:25 AM

Phase I

In a prior post, the steps taken to build a heated print surface were chronicled. In that post, the heating element identified was a 12" x 12" silicone heating pad purchased from QU-BD. I have now experienced problems with two of their heating pads. The first stopped heating above 83 C. They replaced it then the replacement became intermittent. I think it's related to where the supply wire meets the heat element. As the Y axis moves, over time, the copper fractures internally at the junction point from the constant flexing. My attempts to get support from QU-BD a second time on the device have gone ignored (re-opening the old ticket and submitting a new ticket). Update: I received a message from a support person at QU-BD who basically confirmed it was the I mounted it with continuous stress at the junction point and recommended a 90 degree mount for better strain relief. This "hobby" is very much DIY but I still think they would do better to offer some sort of installation guidelines with the product. They are reviewing whether they are going to ship another replacement. I will update with any news.

I decided to experiment with an alternate approach so ordered some Aluminum clad, 1 Ohm, 25 Watt power resistors from Mouser and another aluminum plate from Speedy Metals. After drilling countersunk holes and mounting the resistors to the aluminum with heat sink compound, securing with thread lock, and wiring together I connected it up for some tests.

Top surface view of the hole pattern:

The resistor network, re-cycled thermistor, and silicone insulated supply wire from the QU-BD heating device:

Here is is Installed, the resistors are slightly visible nestled in the insulating foam:

The results:

It took 8 minutess to get to 60c

It wouldn't get to 110c

It took about 30 minutess to get to about 105

The highest I could get it was about 107 but the test prints I did at 106 the ABS stuck just fine.

There are a couple reasons (that I could surmise) as to why it wouldn't get to 110:

• The tolerance of the resistors. I had calculated the ideal resistance in the network configuration at .75 ohms. At 12V that would mean 16 Amps. The expected V drop was 4V across each and the expected current was 4A through each. This would mean the power dissipation on these 25W devices would safely be at 16W. Once they were wired up it measured more like around 1.2 and when I measure the current it was 12A. The power dissipation is only 9W.

• The resistor derating curve, and to a much lesser extent the temperature coefficient. Once this model device reaches around 100c it's properties change somewhat...

component details:

• Resistors

Welwyn WH25-1R0JI (1 Ohm 25 Watt @ $2.46ea x 12) Note: revision 2 below uses better resistors.

• Aluminum alloy

6061-T6 (<$10 @ 12" x 13")

• Mounting Hardware (not stainless)

M3 x 12mm flathead machine screws, lock washers, nuts (x24)

• Heat sink compound

the cheap stuff 276-1372 Radio Shack

• Thread Lock

Permatex medium strength (blue) PX #24200

• Drill bits

1/8" for hole

1/4" for countersink

• Wire

20AWG solid copper hookup wire between resistors

Connector wire to heater salvaged from QU-BD 12x12 heat pad (good silicone insulation)

• Thermistor

Salvaged from QU-BD heat pad (Marlin #11, 100k beta 3950 1% 4.7k pullup) (good heat resistant wire and thermistor housing tube.

There was enough irregularity on the bottoms of the resistors that a flat file helped ensure good contact with the aluminum plate. Also it was important to use a fine sanding block on the AL plate to give it the brushed surface and then clean it with acetone. I also added a couple extra stand-offs to give more leveling adjustment.

Phase II

The goals for the next phase of the resistor based heat bed were to heat faster and get to 110 degrees C. The idea was to "simply" replace the 1 ohm resistors with .82 ohm. The power dissipation would be up to 20-ish Watts per device but still within safe limits using the same network configuration.

Left-over high temp insulated wire from the E3D-V6 hot end was used this time to interconnect the resistors.

It now gets to 60 deg C in about 6 minutes and does get to (and hold) 110 C but takes a little over 20 minutes. I'm finding fewer reasons to print with ABS these day anyway so the longer time is fine. I haven't measured the current yet but not concerned because the power supply I'm using has a 65 Amp 12 Volt rail. Update: 12/30/14 The current measures right at 15 Amps with the new configuration. Additional note: the power supply is now a Corsair RM650 (I had to use the power supply from the first heated print bed blog post to replace a Corsair 650 in a household PC that had died. Corsair replace it under warranty with the modular version, very nice.

component details:

• Resistors

TE Connectivity / CGS - THS25R82J (.82 Ohm 25 Watt @ $2.80 each x 12)

Rather than start over with a new aluminum plate I reused the one from the 1 ohm resistors. This meant breaking the thread lock, which resulted in a few stripped Phillips heads as well as a tough job cleaning and re-using the M3 hardware. I tried soaking the hardware in Acetone and Mineral Spirits (separately) because I could not find a solvent listed by Permatex for the hardened thread lock. I resorted to wire brushing it off. If the hardware store wasn't closed when I was doing the re-work, I would have probably just purchased all new machine screws.

The crusty hardware:

A modification to consider in the future:

Change the resistor layout pattern, moving them closer to the edges. The thermal pattern is more intense in the middle and fades a couple degrees at the edges. Need to compensate for the center concentration.