Temperatures

K8200 3D Printer K8200 - Wiring and electronics
1

The bed temperatures started reading around 98 degrees some time ago so I couldn’t heat the bed, but that didn’t matter as my prints work just fine on a cold bed. I replaced the bed and the thermistor, but no luck.

Now, the extruder reports 91.5 degrees. I have tried different wiring and a different thermistor, but no change.

Is there anything I can do before I buy a new main board? I think I’ve tried everything - voltage is steady at 15.5 volts too so it’s not that. I’d just like any further tips before I shell out a lot of cash for a new board.

Thanks.

Colin.

2

Unplug Therm1 (Extruder) and Therm2 (Bed)
In the software both temps should drop to 0C
If they do check the resistance on the cable not the board.
They should both be around 100K at room temp.

3

They drop by about a half a degree each when I do that, but stay high in the 90’s I’m afraid.

Colin.

4

This is starting to sound like the control board may have an issue

5

My view too. Any other tests I can do on the board itself before I shell out for a new one?

And if I do, any suggestions for the best price for a new one?

Colin.

6

I am in thermistor hell.

I replaced the 8200 controller, and cannot get correct temperatures.

I started with the original parts, had correct temperatures, then I installed the 8203 direct drive extruder, struggled with temperature at the outset, then the PEET barrel melted. Obviously, I lost temperature control.

Then I mechanically damaged the 8200 controller, and replaced it. Since then, I can’t get normal temperatures. I’ve tried thermistor codes 1 and 5 on both the extruder thermistor and on the heated bed thermistor. Nothing produces the 20 degree approximation of room temperature. The reading on the bed hovers near 5 degrees, and is responsive to change. The extruder reading sits at 40 degrees and is unresponsive to temperature changes.

I have several thermistors, some are direct replacements for the Velleman extruder thermistor, some came with new extruders. They are typically reading 120K in my cool room. They are responsive.

I tried hooking up potentiometers to the controller board, to substitute known/measurable resistance for the thermistor.
I am finding that, for the bed, 77K reads 20 degrees, 19.8K reads 60 degrees, 907K reads 15 degrees.
For the extruder, 97K reads 41.30 degrees, .49K reads 100 degrees, and 58 k reads 199 degrees, 0 ohms reads 350 degrees.

I have seen correct temperatures read from the bed thermistor, with my original controller. With the new controller, the temperatures are inaccurate.

How can I demonstrate whether the controller is reading temperatures correctly? What thermistor codes should I use?

Thanks in advance.

7

Hi all,

Try disconnecting the sensor from the problematic channel. using a variable resistor / fixed resistors. Connect the resistor across the controller’s sensor terminals, record the voltage and record the resistance used. You should get a clean exponential curve, if you dont that may indicate that the microcontroller input is faulty.

To prove this replicate the sensor input circuit: 5v -> 4k7 resistor -> sensor terminal 1, sensor terminal 2 -> ground. Connect the test resistors across the sensor terminals and record resistance and voltage now compare results.

Geoff

8

Here are measurements that I took, substituting a potentiometer for the thermistors:
controller therm1a
-> 27K -> 1K variable -> 100K variable
-> therm1b

Ohms Therm1 Therm2
open 39.00 0.00
123.9K 39.9 7.00
123.0 K 40.0 7.9
101.4K 40.1 12.9
100.7K 40.2 12.90
89.5K 40.2 16.4
88.6K 40.2 16.4
68.8K 40.5 24.20
67.9K 40.6 24.5
49.8K 41.0 34.1
48.8K 41.0 34.5
33.6K 41.7 45.2
27.7K 42.2 50.8
14.3K 44.5 69.8
7.72K 48.5 88.6
2.66K 61.0 124.7
1.78K 67.9 139.1
.849K 84.2 170.0
.501K 99.3 195.3
.200K 135.3 249.7
.098K 170.7 325.80
.029K 240.0 713.0
.012K 346.8 738.8
short 738.8 738.8

I have a thermistor that I purchased from my supplier, here is how it reads:
standard ohms degrees
ice 300K 0.0
room 112K 20.0
body 57.5K 37.0
steam 65K 100.0
I believe my replacement thermistor is similar to the other ones on my other extruders. They all read 112K for room temperature.

I believe I have Thermistor code 5 set in the firmware, although I have also tried code 1.

9

Viewing / Plotting your data Resistance against temperature shows that you seem to have a plateau of around 40degC between resistances 123.9kohm - 67.9kohm. I would suggest that something other than the thermistor is sinking current through R8 or its equivalent for Therm1- i.e the microcontroller input pin is drawing significant current indicating that the chip is NFG.

This is also highlighted by the fact that when no sensor is connected, you observe a temperature reading of 39degC.

I think this is yet another board failure probably due to poor controller input circuit design!

10

Thanks for your insight. I agree that therm1 looks faulty.

Therm2 has a nice curve, but with the previous controller and the same thermistor on the bed, the reading was 20 degrees at room temp, reliably.

I have not found a thermistor table in the firmware that matches the Therm2 curve, although clearly the default tables were useful with the original controller.

I am wondering if there are incorrect resistors in the therm circuits, and different incorrect values at that.

11

Hi ,

What is your 5V rail like when measured with a voltmeter?

12

Sorry, Geoff 104, but I am enjoying a generous swap test of my controller with my supplier, Vetco in Bellevue, WA (Seattle area). They lent me the controller from their demo K8200, and I am getting some printing done at last!

A key to my success, was raising the reference voltage to 0.6v on the extruder stepper, which should increase the current to 1.5A. The chip doesn’t get injuriously hot, and my filament feeding trouble is over.

Also, don’t put the nozzle too close to the bed :wink: or there’s no room for extrusion. At least, I can play with other variables now that my filament feeds correctly.

13

Hope you have a heatsink on that chip. Pololu rates those A4988 chips at 1A without active cooling. Although, I’d be happy to help you with your issues, since I live and work in Bellevue, WA.

14

Thanks, Lynk. I have checked the chip temperature with the fingertip method. It doesn’t burn my finger, but I can’t leave my finger on it, either. Borderline.

I do have the higher current driver on hand, that and a heatsink would be belt+suspenders. OTOH, I’m headed for a RAMPS controller for the extra channel. The supplier graciously sent heat sinks for each driver with my RAMPS kit.

School starts tomorrow (music at Shoreline), so my 3-d progress will slow a little. My next plan is to become an expert on temperature management, then work on calibration printing.

I can’t remember what I wanted to build, but I am getting the adventure I hoped for, and am loving it.

15

Hooray! Go, Expert.