The general idea: a thermal switch detects an overtemperature condition and shuts off the power supply for the heaters and the extruder motor.
A sketch that should accomplish that goal:
The -Power On pin is 14 in the 20-pin Motherboard connector and 16 in the default ATX 20+4-pin connector.
The thermal switches in my heap have their case connected to one lead, so it makes more sense to put ’em on the low side of the circuit, with the case grounded. That way you can attach ’em directly to, say, the Thermal Core… or at least not worry about inadvertent shorts.
How it works
When you turn the ATX supply’s AC power switch on, the +5VSB supply goes active and the AC On LED lights up. Nothing else happens: the relay remains open and the Thing-O-Matic appears to be dead. This is how it should work!
The +5VSB supply provides initial power to this circuit; all other ATX power outputs are disabled and all Thing-O-Matic boards are inactive. The AC On LED indicates that the standby supply is active.
The Test/Fault LED is lit in this condition to indicate that the relay is inactive; there seems to be no way to unambiguously indicate a thermal fault without at least one more relay. On the upside, you know the LED works.
Pressing the Power On pushbutton closes the DPST relay, assuming the Normally Closed (NC) 65 °C Thermal Switch is closed and the Estop button is not pressed. I happen to have such a thermal switch in my Parts Heap, but a higher temperature may be desirable; they’re stock items at Digikey / Mouser. See below for a discussion of the sensor location.
You can use a DPDT relay, which may be easier to find.
One pole of the relay bridges the Power On pushbutton switch, holding the relay active when you release the button. The other pole pulls the ATX -Power On input low to enable the ATX supply voltages to the rest of the Thing-O-Matic, which lights up and begins working as usual.
The +Power Good signal eventually goes high and turns on the Power OK LED.
If the temperature exceeds 50 °C, the Normally Open (NO) 50 °C Thermal Switch closes and the Heat Alert LED goes on. That temperature may be too low.
If the temperature exceeds 65 °C, the Normally Closed (NC) 65 °C Thermal Switch opens and releases the relay. That releases the -Power On input and immediately turns off all the ATX supply voltages: the Thing-O-Matic hardstops. Because the other relay pole no longer bridges the Power On switch, the relay remains off.
As suggested there, putting a high-temperature thermal fuse directly on the Thermal Core insulation blanket would be a Good Thing. That fuse goes directly in series with the 65 °C Thermal Switch as a deadman cutout: when it blows, you cannot turn the Thing-O-Matic on until you replace it.
The Test/Fault LED goes on when the 65 °C Thermal Switch cools off enough to close. That’s suboptimal, although the 50 °C Heat Alert LED will be on while it’s cooling.
Pressing the Estop pushbutton switch also releases the relay. That switch is Normally Closed (NC) because most wiring faults involve a wire not making contact. A Normally Open (NO) switch wouldn’t give any indication of the wiring problem, while an NC circuit simply won’t allow the TOM to power up.
As nearly as I can tell, the Motherboard ignores its own Estop switch input.
Important: don’t get clever by replacing the relay with a semiconductor. You want something dead simple that won’t suffer from static discharge damage or software failures. Relays, buttons, and switches tend to be very simple, easy to test, easy to verify, and not prone to weird failures.
Thermal Switch Locations
The ideal location has a physical attachment to the Thermal Core, rather than an air gap. The top of the Thermal Riser Tube seems ideal, but I measured the heatsink temperature as exceeding 90 °C with the Core at 220 °C.
What’s worrisome about that: the glass transition temperature for acrylic plastic is in the 85-165 °C range. The top of that tube is much hotter than it really should be, given the stresses imposed on the Filament Drive frame.
I’m thinking of adding a heatsink across the top, extending out of the openings under the top of the support structure. That should cool the Tube, make the acrylic happier, and provide a location where the temperature remains below 50 °C in normal operation. When the Core exceeds, say, 300 °C, the switches would trip.
I think that’s messy, but putting higher temperature switches on the existing Tube doesn’t avoid that problem. Obviously, running the TOM in a hot room will produce different results; there’s a tradeoff between false trips and burning the house down.
More study is needed…
ATX Power Supply Signals
These must be hijacked from the ATX power connector at the Thing-O-Matic Motherboard, maybe using square pins & heatshrink to fake a connector. You could repurpose the 4-pin +12 V / Ground CPU power connector, but then you’d be forced to come up with a mating cable connector.
Or just solder a cable to the Motherboard, cut the -Power On, splice in the wires, and be done with it.
The +5VSB (9 Purple) supply is active with the AC line switch turned on. This supplies power to bootstrap the protection circuit, much as it does in an ordinary PC.
The -Power On (14/16 Green) signal has an internal (and unspecified) pullup resistor. The Motherboard pulls that line to ground, so the new connection must be spliced into the middle of the existing ATX wire. The pin number is 14 in the 20-pin block and 16 in the 24-pin block.
The +Power Good (8 Gray) signal remains low until all output voltages get within the tolerance limits, then it goes high. There’s no spec for output current, but we assume it can drive an LED. Presumably this goes low when any voltage goes out of spec, so a glitch catcher should be instructive.
A Common or Ground (7 Black) line provides the circuit ground. The Motherboard has very low current requirements, so stealing any of the Ground wires will be OK.