Killing the thermistor problem for good...

The title was a bit hopeful on my part but it is something we need to get hammered out for good.

I've switched over to the 100k thermistor and finally got the appropriate capacitor value for C3: 1.5 nF. I also found in the forums that I needed a 4k7 resistor in series with C3. I assume this is R3. Is that right. I've switched out C3 and R3 to the values stated but still don't have this thing functional.

The relevant preference settings are as follows:
Extruder0_Beta: 3960
Extruder0_Rz: 337254
Extruder0_Capacitor: 0.0000000015
Extruder0_ExtrusionTemp: 240

I played around with these values some to see if they were the culprit but with no luck. I tried reducing hm and hb to lower values as per the page here:
[reprap.org]
Got the temperature up from 73 to 107 C. Still stuck at that point though.

My question is, What is the general problem I am encountering with this system? Why can I not go up above a given temperature? Is it a hardcoded value?

Demented

Have you tried just hooking the nichrome up to a variable voltage thingy and see how hot it gets? My thinking is that maybe the amount of voltage that the electronics want to send through the nichrome is just physically not enough.

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Jay

No, it's not that. Other people have gotten it to work but with their own little tweaks and mods and even there own electronics. I'm not that savvy, so I have to wrestle around with it. Others have had similar problems. We just haven't figured out what I am--and a few other people--doing wrong.

Demented

Demented,
That capacitor value looks way too low. See Steve's post here [sdreprap.blogspot.com] he is using 0.22uF.

I don't think the series resistor is necessary for the 100K thermistor. It will add large errors.

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[www.hydraraptor.blogspot.com]

Very strange. With the values you've got, you shouldn't get stuck at 107C. It's almost as if the heater isn't getting hot enough. Any way you can measure the actual temperature? I used a meat thermometer shoved into the heater barrel as a sanity check.

@ Steve

I'm looking into getting a "real" measurement device for a test run. I can sort of tell if the room temperature settings are normal but past that I don't have that sort of measurment capacity.

@ nophead

So I need a larger value, not a smaller one? Can someone check my values at the page I gave above with the given rz and beta values? I'm pretty sure that the calculator said around 2 nF. I can be stupid sometimes, however. ;-) I'll nix the series resistor then. Back to the standard value...

I struggle On.

Demented

The calculator may well give you a smaller one with the bigger thermistor but Steve has done a lot of work on this to get the best resolution at high temperatures so I would go with his value.

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[www.hydraraptor.blogspot.com]

Okay, made those changes and now I'm getting much better results. I started the extruder exerciser and set it to 150 C. The heater turned on and brought it up to 180 C and stayed there solidly. Not even a single degree fluctuation, so that's pretty sweet.

On the down-side, its 30 C off! I turned on the extruder motor and have been letting it poop hot plastic for about 5 minutes now just because it's been so long since it was used at all.

I'm leary about turning it up higher! I don't want it to get out of control

Is the software saying it is 180, or are you measuring it another way?

Have you run the heater profiler as described here: [reprap.org]

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[www.hydraraptor.blogspot.com]

The software says its 180. I haven't been able to run it any other way. Also, I'm getting ooze from the tip as the HDPE melts initially and is pushed out by it's own expansion.

No, I haven't run the heater profiler again since I installed the new cap and resistor. I'm doing that right now as I type this. I'm sure my values are going to be pretty different.

Demented

Yes you do get an initial discharge when warning up. My theory about it is this:

When the molten HDPE cools it shrinks upwards leaving an air gap around the edges.
When you heat it again it sags blocking the nozzle and trapping the air above it. The air expands and blows the HDPE out of the nozzle, sometimes with an explosive crack.

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[www.hydraraptor.blogspot.com]

It should also say that the above effect leaves the extruder with a big deficit of HDPE which needs to be made up before it will extrude again. My warmup routine runs the extruder for a few seconds and then brushes of the excess to give a known staring point. I have no idea what the RepRap s/w does about this.

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[www.hydraraptor.blogspot.com]

It does exactly nothing about it. On the other hand, my extruder doesn't give any kind of explosive crack. It is a gentle oozing of material.

Demented