Installing safety system on my printer

Hello all,

I have been reading through some of the posts regarding fire hazards, and I got quite shocked at what hobby-grade 3d printers are capable of.
I was thinking of installing an Arduino-based safety system by adding two DC-DC relays to the inputs coming from the power supply.
The Arduino would be connected to a smoke detector near the printer, and shut off as soon as smoke is detected.

I would also like to make an encasing for my Megatronics board, also housing a 12V fan (for additional cooling of my stepsticks), while adding a thermal fuse behind the relays to prevent the electronics from burning up.
The casing would be 3D printed, thus I would like to add a thermal fuse to detect high temperatures.

I am well aware that the hotend may fail due to the thermistor falling out. I have a E3DV6 hotend, reinforced by 4 wraps of Kapton tape to keep the thermistor firmly in place, while it's inserted into the temp probing hole.

Now, regarding the relays, I have ordered two of these: [www.ebay.co.uk]
I was wondering if my following assumption was correct for the heat bed:

If my heat bed works on 12V, and has a resistance of 1.3 to 2 ohms, I should be able to use these relays, as the amperage will likely not exceed 10A.

Am I correct, and are there any other suggestions (other than a metal casing and webcam)?

Been awhile since I paid attention to my e3d, but isn't the thermistor held in place with a screw?

Also, I believe a 12V heat bed can occasionally pull up to 15A.

I'm thinking of implementing a secondary safety arduino that is programmed as simply as possible to shut down as much as it can (extruder heater, bed heater, stepper motors) under the following kinds of conditions:

- thermistor goes open circuit (e.g. wire break)
- extruder heater continuously on for more than X seconds (have to experiment - depends on PID values, etc)
- don't receive a heartbeat signal from main printer controller (requires change to printer firmware)
- shutdown signal - button or from the main printer controller?
- smoke detection signal

There's a lot of dangerous failure points in the circuits in our 3D printers - I've had a few happen while watching the printer - hate to think what would have happened had I not been there.

Quote
powool
I'm thinking of implementing a secondary safety arduino that is programmed as simply as possible to shut down as much as it can (extruder heater, bed heater, stepper motors) under the following kinds of conditions:

- thermistor goes open circuit (e.g. wire break)
- extruder heater continuously on for more than X seconds (have to experiment - depends on PID values, etc)
- don't receive a heartbeat signal from main printer controller (requires change to printer firmware)
- shutdown signal - button or from the main printer controller?
- smoke detection signal

There's a lot of dangerous failure points in the circuits in our 3D printers - I've had a few happen while watching the printer - hate to think what would have happened had I not been there.

I work in safety-critical software for a living. The first 4 items in your list can be implemented in the printer firmware (the 3rd one is done using one or more watchdog timers), and already are in the best 3D printer firmwares. So IMO a good firmware with those protection features, plus a smoke-triggered backup device to cut the power to the entire printer, would be sufficient.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

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

With DC42's help I put together an electrical device to help prevent fire and have written it up here [forums.reprap.org]

Edited 1 time(s). Last edit at 11/02/2015 04:40AM by DjDemonD.

A big thing one can do for reliability is use thermocouples instead of thermistors, they can be bolted right onto the hot end, no tape needed. the controller has detection of disconnection built in, and if you hit em with a hammer they just become a better thermocouple rather than shattering. Not to mention you can always trust the temperature is the 'true' one since they give absolute readings and don't have to be adjusted with a resistor specific tempco curve.

Marlin at least supports having both a thermocouple and a thermistor, it will compare them and if they get out of sync it will throw an error, giving the best of both worlds.