I tried an experiment. I took a handful of resistors, measured each with a multimeter, then attached each to the thermistor connector. I used the RepRap host software in debug mode in eclipse to look at what it thought the resistance was. Here are the results: Real Measured 101 141 6650 6020 26900 24140 46200 41180 99000 87000 What it comes down to is that there's a scale factor plus an offset. The equation come out to something like y=0.88x+52. Now, I can understand the scaling. That could be C3. And I can compensate for that by adjusting the capacitor setting. What I don't get, though, is where the offset comes from. It's acting as if there's a 50ohm resistor in series with the thermistor. Any thoughts on where that might be coming from? https://reprap.org/forum/read.php?4,8768,8768#msg-8768 Sun, 10 May 2026 05:00:16 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?4,8768,8841#msg-8841 https://reprap.org/forum/read.php?4,8768,8841#msg-8841
double resistance = calculateResistance(rawHeat, calibration);

In the source I'm using, that's line 809. Open up the extruder exerciser and wait for the breakpoint. Hovering the cursor over "resistance" should give you the current value. You can then compare that to a reading using a multimeter.]]> degroof Reprappers Wed, 23 Jan 2008 08:21:21 -0500 https://reprap.org/forum/read.php?4,8768,8824#msg-8824 https://reprap.org/forum/read.php?4,8768,8824#msg-8824 Ian Adkins Reprappers Tue, 22 Jan 2008 18:26:38 -0500 https://reprap.org/forum/read.php?4,8768,8811#msg-8811 https://reprap.org/forum/read.php?4,8768,8811#msg-8811
I guess I could compensate for the impedance by adding it to the original resistance measurements and plugging that into the beta/Rz formula. But, yeah, it might be better in the long run to have it as a parameter.

What I'm shooting for is getting reasonably accurate temperature readings at the high end of the scale. Personally, I don't mind if it's off a bit at room temperature but, if it's reading 270C when it's really 250C, that could be a problem.]]> degroof Reprappers Tue, 22 Jan 2008 15:11:59 -0500 https://reprap.org/forum/read.php?4,8768,8805#msg-8805 https://reprap.org/forum/read.php?4,8768,8805#msg-8805
I would guess its output impedance would be a few tens of ohms so you could calibrate that out by doing what you are doing and adding an extra host parameter. It will vary from PIC to PIC and with the temperature (of the PIC!).]]> nophead Reprappers Tue, 22 Jan 2008 13:52:33 -0500 https://reprap.org/forum/read.php?4,8768,8804#msg-8804 https://reprap.org/forum/read.php?4,8768,8804#msg-8804 degroof Reprappers Tue, 22 Jan 2008 13:01:21 -0500 https://reprap.org/forum/read.php?4,8768,8777#msg-8777 https://reprap.org/forum/read.php?4,8768,8777#msg-8777
5V across 100R would draw 50ma which exceeds the drive capability of the PIC. This is why the 10K themistor is no good for high temperatures with the UCB circuit.]]> nophead Reprappers Tue, 22 Jan 2008 04:24:45 -0500 https://reprap.org/forum/read.php?4,8768,8768#msg-8768 https://reprap.org/forum/read.php?4,8768,8768#msg-8768
Real Measured
101 141
6650 6020
26900 24140
46200 41180
99000 87000

What it comes down to is that there's a scale factor plus an offset. The equation come out to something like y=0.88x+52. Now, I can understand the scaling. That could be C3. And I can compensate for that by adjusting the capacitor setting. What I don't get, though, is where the offset comes from. It's acting as if there's a 50ohm resistor in series with the thermistor.

Any thoughts on where that might be coming from?]]> degroof Reprappers Mon, 21 Jan 2008 19:21:52 -0500