Using the PS_ON_PIN as safety feature

Hi,

I came across a potential problem during the redesign of my printer. One of the problems of my printer was the low bed temperature. I solved this by buying a 500W heater than runs on 230V. This very powerful heater will be placed in a more or less enclosed space. This is a huge candidate for burning my house down. I need some extra safety features.

The heater will be switched by a solid state relay. There is a possibility in which the SSR stays opened due to a massive failure of the SSR. In this case my heater will burn my house down. So I came up with the idea of using a extra normal relay which is operated by the PS_ON pin of the board. I ran a test in which I triggered a heater failure (I asked the printer to heat up the nozzle with the nozzle heater disconnected). The printer halted as it should, but didn't turn of the PS_ON pin.

Is there a way to modify this? Or is there a better way to do this?

I'm using Marlin 1.1.0-RC6

Edited 1 time(s). Last edit at 10/12/2016 10:10AM by JanJans.

Have the power for the heater run through a thermal fuse, such that it gets too hot the fuse will trip and kill the power.

Why didn't I think of that! That one is too obvious.
Thanks for your advise and my apologies for my stupid question.

I would like to reopen this topic.

I added a thermal fuse as suggested. The thermal fuse will trip at 150 degrees Celsius. I couldn't use the lower (120 degrees Celsius) because I was able to trigger the fuse in really hot applications. I never tried to trip the 150 degrees fuse. That is really high for a bed heater. The heater itself will probably be close to 180 or 200 degrees running at full power before setting of the fuse! Although I mounted the fuse as close as I could to the heater, there will still be a substantial leg between the temperature of the fuse and the heater. Especially because a 500W heater heats up so fast.

I sort of happy with this solution. I every thing fails the fuse will stop the heater. However, I would still like to ad an normal relay so that the firmware can stop the heater before the thermal fuse kicks in. The thermal fuse should be the last line of defence before melting down.

I need to modify Marlin to operate the relay. The relay should be activated at start up and should be turned of if kill is activated. I was looking for the start up and kill procedure but couldn't find it. My admittedly limited programming experience isn't helping with this. Any suggestion on how to do this would be greatly appreciated.