eBox Lite Filament Dryer - NTC Thermistor with much too high resistance

[ra-ma]

Hello, I do have an eBox Lite Filament dryer. It works fine (display, buttons, fan) except that it's not heating. I experimented with replacing the NTC Thermistor with normal resistors to find out if the rest of the circuit is working. It turned out to be all ok except that the input value from the NTC seems to be much too high. And therefore it never starts heating.

The NTC is reacting to temperature changes, it starts with some 3 MOhm for 15 degrees, in the 60 degrees it's around 600 kOhm and for 100 degrees at some 300 kOhm.

With an adjustable resistor I found the values where the heater shuts off in the respective levels. The manual states the target temperatures of the hot plate which should be 61, 63 and 75 degrees celsius in the levels 1, 2, 3.

Level 1 (61 degrees): 15,47 kOhm
Level 2 (63 degrees): 12,56 kOhm
Level 3 (75 degrees): 8,98 kOhm

It starts to heat with an value slightly below 1MOhm, with an resistance of 1013 kOhm it didn't start to heat at all.

My question: How can I find a NTC to replace the existing one? I'm not sure where/how to search for it and checking some tables for 10K NTC's it doesn't match the values I need.

Looking forward to any suggestions!

[wraper]

Looks like circuit is tuned to 68k NTC with β of 4185 K. Chose 68k NTC with closest β you can find. Although hysteresis is under question, it may actually shut down at temp a bit higher than set and turn off after cooling down to temperature lower than set. In which case you probably need slightly different β.

[ra-ma]

Hi wraper, thank you very much! I found NTCS0603E3683FHT from Vishay with 68k and Beta of 3985. It should be pretty close to what I need. Can you tell me/link me how to get from the data I had to matching NTC values?

[wraper]

Hi wraper, thank you very much! I found NTCS0603E3683FHT from Vishay with 68k and Beta of 3985. It should be pretty close to what I need. Can you tell me/link me how to get from the data I had to matching NTC values?

It's quite far actually. Here is with β of 4190 https://www.tme.eu/en/details/640-68k/tht-measurement-ntc-thermistors/vishay/ntcle100e3683jb0/
I just googled some online NTC calculator and fiddled with numbers for a few minutes. Butt IIRC I've seen calculators where you can just input two points and it will get β automatically. EDIT, I just tried one of those and got nonsensical results. I guess your measurements are not precise enough for that. Also I re-checked numbers for NTC I suggested and for level 2 resistance would be right for 66^o instead of 63^o. Although it's makes little sense to make 2^oC difference between level  1 and two as I really doubt NTC and overall device is precise enough to make use of that. So part I suggested still should be good enough.

[wraper]

Hmm... I just googled eBox Lite dryer and it says it has 40^o, 50^o and 55^o levels, not what you wrote. And even if it's hot plate and not actual temperature as you wrote, numbers still do not correlate.

[ra-ma]

Yes, thanks. I agree with the 61 and 63 degrees. But in the handbook they actually provide a table with the "inside temp" and the "hot plate temp". And the difference between level 1 and 2 is quite small... I think at the end it'll be anyhow not very accurate with all the effects around like ambient temperature etc. Therefore it should be fine to somehow match it with a lower and a higher level. And the higher level should make sure it's not melting... 

I'll go for the NTC you proposed and let you know the results.

Thanks for the support! 

[wraper]

If it was like 70^oC instead of 63 for level 2, it would make sense. But it's 61^oC plate->40^oC actual, 63->50, 75->55 which is total nonsense, as 2^oC difference in heater temperature cannot make 10^oC difference in chamber.

[ra-ma]

Just to give an update: ordered the part as suggested, soldered it in: And it's working again. As written the inside temperature is not very accurate, but for sure accurate enough to dry the filament. Thanks for the help, learned a lot again.