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thermal safety

Posted by dissidence 
thermal safety
March 26, 2011 02:38AM
i was wondering if any one was working on adding a thermal safety into the firmware for the ramps board, something like say if it reads 300 it will automatically turn off the heater, and also if it reads 0 for more than say 20 seconds it will turn it off again

Edited 1 time(s). Last edit at 03/26/2011 02:38AM by dissidence.

Re: thermal safety
March 26, 2011 04:47AM
Teacup has such a check in heater.c, but it is disabled by default and I don't know how well tested it is.

There is also a hardware option, which is likely to be more reliable but obviously costs money.
Re: thermal safety
March 27, 2011 01:24PM
The better way that I read from Nophead is to design the tip so that it can't go above 300 C even when driven at maximum voltage (12 V) continuosly.

Turning the heater off when the temperature reading is 0 is a good idea if the temperature reading is reliable since I don't think anyone runs their printer at freezing temperature.
Re: thermal safety
March 27, 2011 01:47PM
i missed that one from Nophead i like that one allot, i wonder how he would go about doing that.?

i have seen a few blogs run into trouble when there thermistor wires came off, the only issue there is i dont know how you would deal with it if the thermistor came off the hot end, it would still be reading the temp but it would only be about 20-30, or even allot higher if you had a heated bed under it

but i do think the idea of shutting it off when reading 0 is a good idea, too

Re: thermal safety
March 28, 2011 10:09PM
I noticed in Teacup firmware that they have this;

check if heater responds to changes in target temperature, disable and spit errors if not
largely untested, please comment in forum if this works, or doesn't work for you!

As it says it is mostly untested but should do basically what you want, If it works.
Re: thermal safety
March 29, 2011 12:42AM
The idea is to choose the resistance for the heating element such that it won't get hot enough to damage anything under full voltage (+12V). The 6.8 ohm resistor was picked for the purpose.
Re: thermal safety
April 05, 2011 11:56PM
I think there is also a need for the Host software to be configured as a back up to the Firmware.

Thus the host then protects the printer from Firmware failure or hardware failure.

Thus the host SW should check curent temp against the target temp for both the bed and Nozzel settings.

The host should then take the appropriate action by sending a new target and re-cheking the temp.
If that fails an error should be flaged and the Firmware should be re-started
If that fails an alarm should sound to tell the user to manualy power off the 3d Printer.

Anothe niggle with the Host SW
The host SW should prevent any extrusion commands issued untill the heated bed and the Nozzel are both up to the correct working tempratur.

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Re: thermal safety
April 06, 2011 12:59PM
If you really want safety the best choice is to had a second thermostat with a beeping sound or something else when it go over wahtever temperature...

I'll advice to never let you printer without supervision... Apparently even the Pololu can take caught fire...
Re: thermal safety
April 07, 2011 04:01PM
i may look into a few of the ideas my self, but part of my thinking was this will be something we has a group should look into more, has more units get out there, i think this issue will become bigger, so i thought a discussion would be good to have sooner than later.

but in order for things to work, i think there needs to be more standardized communication and features, so that we know all the features will work with any firmware and software combinations. that would be the best in the long run.

but for my self i think i might look into the second thermistor idea a little more, i wonder how small and cheap i can make that one work.

Re: thermal safety
April 08, 2011 05:30PM
SW/firmware can fail. Additing additional components like thermostats or thermal fuse increases the probability of failure and reducing reliability. KISS is a good policy in this case. Failsafe design is the best approach. I agree with Nophead. Design the hot end and the heated bed so that it can't possible get hot enough to cause a fire.
Re: thermal safety
April 23, 2011 02:26PM
I think its safe, since its a separate, redundant system.
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