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Thermistor value for heated bed.

Posted by seatroutfisher 
Thermistor value for heated bed.
February 07, 2016 09:43AM
Hi All,

Does anyone know the specifications of the thermistor used in the fisher delta hot end?

I have made a hotbed for the fisher using a Mini Kossel kapton heater that came with a MF58104F3950 thermistor. I need to know what figures I need to put into my config file so that the temperature is read correctly.

My heated bed temperature starts at 6 degrees and increases when the bed is heated (the hotend is reading 20.1 at the same time as the bed gives 6.0)


Any help would be greatly appreciated.

Seatroutfisher

Edited 1 time(s). Last edit at 02/07/2016 09:43AM by seatroutfisher.
Re: Thermistor value for heated bed.
February 07, 2016 12:50PM
Quote
seatroutfisher
Hi All,

Does anyone know the specifications of the thermistor used in the fisher delta hot end?

I have made a hotbed for the fisher using a Mini Kossel kapton heater that came with a MF58104F3950 thermistor. I need to know what figures I need to put into my config file so that the temperature is read correctly.

My heated bed temperature starts at 6 degrees and increases when the bed is heated (the hotend is reading 20.1 at the same time as the bed gives 6.0)


Any help would be greatly appreciated.

Seatroutfisher

With that thermistor I would use a B value of 4008 for it to read correctly at 100 degrees

You need to increase the H parameter to adjust the low end to be roughly the same as the hotend

Edited 1 time(s). Last edit at 02/07/2016 12:51PM by dougal1957.
Re: Thermistor value for heated bed.
February 07, 2016 01:10PM
I would be very interested to see some pics of your heated bed and maybe the source of the parts?


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
February 07, 2016 01:26PM
Thanks for you help.

I will post some images when I get the chance.

Thanks again
Re: Thermistor value for heated bed.
February 07, 2016 05:10PM
For that thermistor, you will also need to use the T100000 parameter in the M305 P0 command; but I suspect you have already done that.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Thermistor value for heated bed.
March 06, 2016 09:41AM
In trying to setup one of these heat beds I found that the L parameter doesnt appear to have any affect.

With the hot end reading 17 the bed reads 60 so I thought setting L to -40 would have the affect of making the bed temp read about 20c.

M305 P0 T100000 B3950 L-40 H0 X0


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 06, 2016 10:02AM
The L paremeter affects the reading when the thermistor is hot. It is not really relevant to the heated bed calibration because the bed does not get hot enough. It is the H parameter that affects the reading at low temperatures.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Thermistor value for heated bed.
March 06, 2016 11:31AM
Thanks DC42. I have set that to M305 P0 R1000 T100000 B4008 H-240 and it now correctly reads low temps. Is there a way to increase the sample rate of the temperature as it seems to over shoot the active temperature by about 15c and takes a while to cool back down.


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 06, 2016 11:34AM
Something is wrong, because H-240 is much too large a correction. What reading do you get if you use H0? Are you sure you have a 100K thermistor, and a Duet with 1K series resistors?



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Thermistor value for heated bed.
March 06, 2016 11:45AM
Not sure which resistor is in the Duet 0.6 but the hot end is set at R4700 so I set the same on the bed and instantly get much more sane results, I will just need to wait for everything too cool down to be sure.

M305 P0 R4700 T100000 B4008 - although think3d recommend 3950

This is the heater I'm using. [www.think3dprint3d.com]


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 07, 2016 02:52AM
Just to follow up. Setting R to 4700 makes a big difference. However I still need to apply H-50 to get accurate low temperatures.

M305 P0 R4700 T100000 B3950 H-50 ;

Also changing to B4008 doesnt seem to have any cold reading effect.


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 07, 2016 12:47PM
The closer the temperature is to 25C, the less effect the B value will have.

H-50 is within the normal range, which is about -60 to +60.

All Duets use the same value series resistor for all thermistors. Duet 0.6 boards from RepRapPro and T3P3 originally had 1K resistors, but boards shipped in about the last 18 months have 4.7K resistors. Boards from Replikeo still have 1K resistors. Duet 0.8.5 boards haver 4.7K resistors. DueX4 expansion boards mostly have 1K resistors, but very recent ones may have 4.7K resistors.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Thermistor value for heated bed.
March 07, 2016 01:05PM
Quote
DADIY
Just to follow up. Setting R to 4700 makes a big difference. However I still need to apply H-50 to get accurate low temperatures.

M305 P0 R4700 T100000 B3950 H-50 ;

Also changing to B4008 doesnt seem to have any cold reading effect.

No it wont have much effect at the cold end but is around the correct value for 100 degrees (the quoted B of 3950 is at 50 Degrees according to the Spec Sheet I got from Keenovo)
Re: Thermistor value for heated bed.
March 17, 2016 12:56PM
So with the heat bed set as

M305 P0 R4700 T100000 B4008 H-50 ;

And the bed temperature set to 70c the heat bed didnt stop heating and went to 150c and didnt get turned off is there a setting Im missing to have the duet control the temperature?


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 17, 2016 04:03PM
Sounds odd. Did the bed temperature display correctly in the web interface? Did the bed remain cold until you set the bed temperature, or did it start heating as soon as you applied power?



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Thermistor value for heated bed.
March 18, 2016 03:31AM
I got to the bottom of it, the cheap SSR25d relay I purchased from China didn't turn off when the duet turned off the heat bed, the led turns off but it doesn't actually cut the supply. A google search seems to suggest that there are a few fakes out there that do this.


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 18, 2016 04:34PM
Why are you using an SSR? The bed of a Fisher is small, so the current taken by the heater should be well within the capacity of the Duet.

I've lost count of how many times I've said this. SSR-25DD relays are no good for driving heated beds because they have a high voltage drop (1.6V at full current AFAIR). If you must use an SSR, choose one with a much lower voltage drop. What probably happened is that your SSR overheated the first time you used it, and fused short circuit.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Thermistor value for heated bed.
March 21, 2016 05:15AM
I was using an SSR as the Fisher is currently running at 19v and the only heat bed I could find that would fit was 12v.


DC42 Kossel 330mm x 2meters
My Thingiverse Creations
Re: Thermistor value for heated bed.
March 21, 2016 05:33PM
It's possible to use a Duet running on 19V to drive a 12V bed heater. Just common the 12V and 19V PSU negative outputs, and connect one side of the heater to +12V instead of to the Duet bed heater + terminal.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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