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Mintemp - what is it for??  

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01Pad
(@01pad)
Trusted Member
Mintemp - what is it for??

Hi there,

I live in the UK, and currently it is 10 degrees C outside. (having risen from -2)  In the bedroom where I do my printing the temp is 12 degrees, having risen from 8.

When I switch my Prusa i3 Mk3S on, it shows a Mintemp error for both the bed and the hotend. As a result it won't let me do anything except move the nozzle up and down the Z axis.

I can't do any printing until I wave a 2kw heater around my printer for a couple of minutes to bring the temperature of the nozzle and the bed up above 15 degrees. Then it tells me to do a reset and everything is fine - I can print and all is fine and dandy..

But what is the point?

If no mintemp  error had occurred, I would start a print, and the temperature of the bed and nozzle would rise (with the expenditure of about 150 watts) above 15 degrees and onwards  to 215/60 and then it would start a print.

It seems like an environmentally unfriendly measure to be wasting the 2kw of heat just to meet what appears to be an arbitrary target.

Can anybody please explain why this is a useful feature?

And if not, is there a way to prevent it from happening?

Many thanks

Pad

 

Posted : 11/01/2021 6:30 pm
bobstro
(@bobstro)
Illustrious Member
RE: Mintemp - what is it for??

It is a safety feature. If the thermistor is reading a low temperature while the controller is pumping current into the hotend, it means something is broken. With no checks, a disconnected thermistor (which gives a low reading) would result in uncontrolled heating of the heater cartridge. The cartridge can generate temperatures that can melt aluminum and certainly cause a fire. Unfortunately, thermistors are only reliable within a set range, so the safety check is at a temperature (MINTEMP) within its reliable range. That means a given thermistor may not reliably detect temperatures below that range.

Have you considered placing a hot cup of tea near the hotend on cold mornings? This or a few moments with a heat gun are usually sufficient to warm the thermistor to operating range.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 11/01/2021 6:39 pm
Neophyl
(@neophyl)
Famed Member
RE: Mintemp - what is it for??

It part of the safety software. As the thermistor changes temperature it’s resistance changes , I believe below a certain point and the board can’t easily tell if it’s connected or not.  So it expects a minimum temp for a connected thermistor. Think yourself lucky that the mintemp was lowered to what it is now. It used to be much higher. 
Use the Joan method when it’s cold, make yourself a cup of tea or coffee and stick it on the heat bed (take the print sheet off first). That should warm it up enough to get going on a cold morning. No kw heater needed, just a civilised drink 😀

Posted : 11/01/2021 6:41 pm
01Pad liked
01Pad
(@01pad)
Trusted Member
Topic starter answered:
RE: Mintemp - what is it for??

Both answers gave me pleasure, thank you.

I get the first, if there's a low temperature reading and loads of power going in, then there should be an error to prevent fire.

But in my case, no power being sent, just a cold room.

And then it turns out that the resistance /capacitance/whatever a thermistor works on may be so low that it the board doesn't even know it's there.

In any case, for as Brit, what could be better than a nice cup of tea in the morning? Sounds like a highly satisfactory answer.

Many thanks.

Pad

Posted : 11/01/2021 6:54 pm
bobstro
(@bobstro)
Illustrious Member
RE: Mintemp - what is it for??

Not 100% sure, but it makes sense that MINTEMP would cause an error even if the nozzle is not being heated. Better not to start heating if unsure of whether the thermistor is intact.

Now that you've identified a cuppa as the logical solution, you'll have to determine whether the milk goes in first or not. I'm not sure we can help you there!

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 11/01/2021 8:58 pm
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