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extruder capacitor clarification

Posted by ZachHoeken 
extruder capacitor clarification
July 01, 2007 03:07AM
i have a question as to what the capacitor C2 needs to be. it says ceramic / tantalum, but it appears someone (me?) sourced an electrolytic one. i cant seem to find a 1uf ceramic. also, the paragraph on it seems to contradict itself:

Note: The value of C2 (nominally 1uF) depends upon the thermistor that you will use to measure the temperature of the extruder head. The standard thermistor that RepRap uses is a miniature glass-bead type, which is available from RS, part number 484-0149 . This is a 10K thermistor (at 25oC) with a beta value of 3480K and a 0oC resistance of 29K. But the design is capable of dealing with a wide variety of different thermistors - just get one that will go up to about 200oC and use the calculator at this link to work out the value of C2. The best value of C2 for the standard thermistor is 0.1uF.

i'm using a standard 10k thermistor. infact, the one sourced in the reprap bom. bonus points if you use mouser numbers =)

[www.reprap.org]
Re: extruder capacitor clarification
July 01, 2007 04:34AM
Electrolytics generally have more leakage current than ceramic or tantalum which may cause a bit of inaccuracy with high resistance thermistors. With a 10K thermistor I don't think you will notice. They also tend to have worse tolerances but I think the value of the capacitor gets calibrated out.

You can get ceramic up to 20uF but they get pricey and may only be surface mount.

I found this in the wiki:
"The capacitor value is currently assumed to be 1μF and is hard coded. Since ideally this depends on the thermistor used, the capacitor value should be stored in the application preferences. 1μF will not be suitable for all thermistors."

So if it up to date people should be using the recommended thermistor and 1uF caps unless they want to hack the software. I would use tantalum for values above 0.5uF and ceramic below but if you have fitted electrolytic I wouldn't bother changing it.


[www.hydraraptor.blogspot.com]
Re: extruder capacitor clarification
July 01, 2007 02:07PM
okay, so it seems an electrolytic is just fine. i'll put one on my tester board and test it up today. i'm still confused as the exact value. i understand that it can be variable based on what thermistor you use, but i'm intending to put this in the BOM, as the 'official' value, so the thermistor is the standard 10k one.

is it 0.1uf or 1uf? kicad says 1uf, as does the wiki in places, so i'm gonna go with that. hopefully it works =) i should have videos and some pictures hopefully of the extruder working when i get to the lab tonight.
Re: extruder capacitor clarification
July 01, 2007 05:37PM
The capacitor value seems to be a preference, not hardcoded:

[email protected]:~/Reprap$ grep Capacitor lib/reprap.properties.dist
Extruder0_Capacitor(F)=0.0000001
[email protected]:~/Reprap$

This value is 1E-7 F i.e. 0.1uF in the default properties file.
I seem to remember Eric using a 1uF cap per the BOM and having very bad thermistor calibration indeed until he realized this and edited it to 0.000001 to match his extruder hardware.

If we decide 1uF is the default in the BOM, then it should also be the default in the preferences -- there are quite enough hurdles in getting a Reprap working without mismatched defaults like this :-)

Jonathan
Re: extruder capacitor clarification
July 01, 2007 07:14PM
yup, i agree. i tried a 1uf cap today and it didnt like that at all. i'm going to order a fresh set of parts from mouser to test the instructions / bom with. when i do that, i'll run through the instructions on the wiki for determining the proper capacitor and then update all the various places so its consistent and standardized.
Anonymous User
Re: extruder capacitor clarification
July 02, 2007 07:12PM
You have to get measurements of resistance at high and low steady temperatures to calculate the beta and Rz to find the necessary C value. For my thermistor I need 2 microfarad
Re: extruder capacitor clarification
July 02, 2007 10:02PM
i see... i guess when i get the 'official' ones i sourced, i'll do the measurements to double check. thanks =)
Anonymous User
Re: extruder capacitor clarification
July 08, 2007 03:19PM
its the capacitor C3 not C2, or? In the documentation for the UCB C2 is already used (100uF). I think in the documentation for the extruder controller 0,1uF should be soldered to C3 and not to C2, or?
Re: extruder capacitor clarification
July 08, 2007 03:29PM
you're quite right... it is indeed C3.
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