Posts made by BetterHalf

@mrehorstdmd I agree completely, everything should be in a fail safe condition. And after seeing how strongly everyone feels about adding a thermal cutoff, I see no reason not to add one in addition to the relay.

There will be at least 3 layers of independent protection.

• If the SSR fails closed the controller should open the relay and cut power above 120C it may cycle on and off in slow intervals at worst case scenario not exceeding 120C by much.
• If the controller fails in an ON state due to hardware or software the thermal cutoff can disconnect power, regardless of self reset or one-shot worst case result means power cycles in slow cool down and heat up intervals at ~140C (depending on the thermal cutoff)
• input power to the heater will be fused close to 115% as feasible (probably 8 amp fast blow) as its a 750w heater. In the event of a short circuit any electrical failure power should be cut. And the main plug will be fused at 10amps
• Kenevo stated the heater will only reach ~200C in open ambient air at full power. I may test this to confirm, but that would only be enough to deform some surrounding support brackets (PETG) and not cause a fire.

I'm still debating on what thermal cutoff to use. If I use a standard thermal fuse similar to the one in the picture Danal posted, I should just use high temp RTV and stick it to the bottom of the heater? I'm also considering insulating the space between the bottom of the bed heater and the frame using a glass wool blanket material used in industrial heaters rated for 1200F. Could this cause any issues?

@Phaedrux I did read the fire safety section on the wiki before my original post. That was part of my justification for using a relay cutoff. I wanted to keep the option of shutting down the dc power supply and I was looking at using a 24v relay (which I already have several) vs a 5v relay. The 24v relays I have available can be mounted in a relay base and have screw terminals. I am mainly looking at ease of use and parts on hand.

@dc42 Thank you for the confirmation. Setting a limit and disabling the output above that limit seems like a simpler approach to the same goal.

@Danal I did find some additional options for thermal cutoffs. I was initially only searching "thermal fuse" which gave me only oneshot devices. After searching for "thermal cutoff" or "thermal switch" I found some self resetting options, but most of them seem to go from 90-100c to 200c with very little in between. If I find something in the 110-120c I'll definitely add one.

Thanks again to everyone who gave input, I'll take it all into consideration during the build.

@Danal the specified issue of "if the controller loses its mind" (@mrehorstdmd ) was interpreted as a software issue NOT a physical issue. I assumed that was fairly clear, my mistake.

But, yes I realize the any semiconductor switch can fail closed, that was the entire point of adding a mechanical relay. Also watchdog timers are pretty standard, I realize this. That is also why the idea of a duet failing ON in the event of a software fault was concerning. I understand the hardware basics, I realize electronic things fail, I'm trying to explore every possible safety option.

The fire extinguisher ball you use seems fairly similar to the charged nitrogen and bi-sodium carbonate fire suppression cannons used in industrial "dust collectors." I'm a controls engineer by trade and I have to look at remote possibilities for worst case scenario every day, so its become a bit of a habit.

The bed heater I ordered is from kenevo, I wasn't aware that they offered a self resetting overtemp device. All of the Thermal fuse style devices I have been able to locate are oneshot. If you have any suggestions on a supplier for self resetting thermal cutoffs that can be attached to a silicone heater such as digikey or mouser, I would appreciate your input (a link or partnumber).

I do appreciate everyone's input on using thermal cutoffs, but no one has answered the original question. I would still like to know if controlling a cutoff relay with bed heater enable is possible/feasible.

@mrehorstdmd I agree that I could use a thermal fuse and simply sticking it to the bottom of the silicone heater would work.

On the other hand I could feed a 24v relay directly off the power supply through an E-stop contact and then the thermal fuse, avoiding potentially exposed AC voltage in the event of adhesive or silicone breakdown. This would be preferable.

But, the idea of redundant, not active, disconnects is widely used and commonly accepted in industrial equipment. I may end up using both a relay driven from a fan output and a thermal fuse. I'm a bit concerned at the idea of the controller holding an output ON in the event of a fault. This should generally be one of the first potential problems addressed in development of a controller. If you're suggesting that this is remotely possible I may look at a different controller.

I did some failure testing on a few ramps 1.4 cheap clones and a MKS S-base and tried to lock up the processor and could never get any heater to fail on. Example: running from SD and removing the SD or running from USB and unplugging the cable mid heatup on both bed and extruder and during a print. Heaters all shutdown.

If this is actually a concern I hope to see some input from developers.

I'm currently compiling a list and starting to order parts for a modified Hypercube Evolution build.
The bed will be a 310x310 120v silicone heater switched by a SSR. From work experience I know SSRs can fail shorted or closed. This is obviously a huge concern with a large heater running mains voltage and I have plans for some large 30+ hr prints that cannot be monitored 100% of the time.

Some light searching in the forums tells me the fan outputs can be remapped and handle way more than needed for a 530mw 24vdc coil on a spst mechanical relay (with flyback diode).

Now software configuration is not as 2nd nature as wiring to me so I want to double check. Can the fan pin be set to ON/OFF configuration and be automatically enabled whenever the bed is enabled and automatically turned off in an overtemp scenario? Assuming an overtemp alarm would also disable to the heater this should all work if the fan output can be configured to turn on any time the bed is called to a temp greater than ambient.

Basically I want a safety cutout of the 120v AC feeding the SSR in case of any abnormal operation.

The 2nd extruder could also be used if necessary as I only intend to use a single print head.

I have looked into thermal fuses but I would prefer to avoid this as additional milling of the the cast aluminum plate is a tad expensive and any failure would require me to remove the bed heater that is fixed to the plate. Replacing a $15 dollar SSR is much easier.

Example mechanical relay: https://www.mouser.com/ProductDetail/Omron-Electronics/G2R-1A-DC24?qs=sGAEpiMZZMtSzCF3XBhmW%252bLX8z0ndwwuQxuJGoSlrl4%3D