ATX Power Supply On and Off
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SSR would not be trusted in a safety interlock circuit as far as I'm aware. Solid state devices are more sensitive to very temporary voltage spikes. Simple contact overload is preventable by fuse, as you've mentioned elsewhere mosfets are a great protection device for fuses. They certainly weren't trusted by the contractors developing the the safeties on MCPs/MTT/Renishaw's selective laser melting machine while I was involved or service people I recently contracted to swap out some mercury contactors on a polymer chip drier.
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This is straight from the Duet wiki page. That page is also what gave me the impression the M81 on heater fault was already available.
"If your power supply supports the PS_ON pin, then the firmware can use it to shut down the whole 12/24V circuit, leaving only the low-power 5V circuit. If you have an ATX power supply, then this is simply a matter of connecting the appropriate pin."
"The manufacturers of these parts warn that they can fail as a short, that is, supplying full power to the attached device regardless of the input signal, and they advise against building devices whose only off switch is an SSR in case this occurs. For a heated bed powered this way, it may be a good idea to add either an appropriate thermal cutoff to the bed (see below) or an electromechanical relay attached to PS_ON (so that the MCU can shut down the heater)."
This is direct from Omron's pdf that Duet's wiki refers too:
"Therefore, for fail-safe operation of control circuits that use SSRs, do not use circuits that shut OFF the load power supply only with an SSR, but rather also use circuits with a contactor or breaker that shuts off the load when the SSR fails. In particular, it may not be possible to ensure safety if the SSRs are used outside the rated ranges. Therefore, always use the SSRs within the ratings. When using an SSR, always design the system to ensure safety and prevent human accidents, fires, and social harm in the event of SSR failure."
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There is a lot to consider I see…
Waht do we think about this:
http://3dmodularsystems.com/en/upgrades/282-pson-kit-auto-power-off-your-reprap-3d-printers.html
Seems a cheap solution, but I would want to add 1 or 2 temp fuses, under the bed and perhaps on the HE. Would I use normally closed fuses between the PS_ON signal and the input side if the relay? -
This is straight from the Duet wiki page. That page is also what gave me the impression the M81 on heater fault was already available.
"If your power supply supports the PS_ON pin, then the firmware can use it to shut down the whole 12/24V circuit, leaving only the low-power 5V circuit. If you have an ATX power supply, then this is simply a matter of connecting the appropriate pin."
"The manufacturers of these parts warn that they can fail as a short, that is, supplying full power to the attached device regardless of the input signal, and they advise against building devices whose only off switch is an SSR in case this occurs. For a heated bed powered this way, it may be a good idea to add either an appropriate thermal cutoff to the bed (see below) or an electromechanical relay attached to PS_ON (so that the MCU can shut down the heater)."
This is direct from Omron's pdf that Duet's wiki refers too:
"Therefore, for fail-safe operation of control circuits that use SSRs, do not use circuits that shut OFF the load power supply only with an SSR, but rather also use circuits with a contactor or breaker that shuts off the load when the SSR fails. In particular, it may not be possible to ensure safety if the SSRs are used outside the rated ranges. Therefore, always use the SSRs within the ratings. When using an SSR, always design the system to ensure safety and prevent human accidents, fires, and social harm in the event of SSR failure."
If you are looking for a fail-safe solution, you shouldn't rely on the Duet to implement it at all. You should use a simpler system, such as enclosing the entire printer in a fire-resistant box and using a non-resetting thermal cutout at the top of the box to cut the mains supply. Perhaps also a smoke detector that cuts the mains supply.
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There is a lot to consider I see…
Waht do we think about this:
http://3dmodularsystems.com/en/upgrades/282-pson-kit-auto-power-off-your-reprap-3d-printers.html
Seems a cheap solution, but I would want to add 1 or 2 temp fuses, under the bed and perhaps on the HE. I suppose I can use NO thermal fuses parallel to the emergency button?That's the same arrangement that I have previously (a year or two ago) suggested for providing an auto power off solution.
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There is a lot to consider I see…
Waht do we think about this:
http://3dmodularsystems.com/en/upgrades/282-pson-kit-auto-power-off-your-reprap-3d-printers.html
Seems a cheap solution, but I would want to add 1 or 2 temp fuses, under the bed and perhaps on the HE. I suppose I can use NO thermal fuses parallel to the emergency button?That's the same arrangement that I have previously (a year or two ago) suggested for providing an auto power off solution.
Thanks David, You have replied before I edited. Looking back to the schematic I think it is not wise to do anything on the mains side. If I want to add thermal fuses, I think I should do that on the 5V input side, between the PS_On signal and the relay, right?
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If you are looking for a fail-safe solution, you shouldn't rely on the Duet to implement it at all. You should use a simpler system, such as enclosing the entire printer in a fire-resistant box and using a non-resetting thermal cutout at the top of the box to cut the mains supply. Perhaps also a smoke detector that cuts the mains supply.
Agreed you can't rely on the duet to make the system safe. Safety systems (particually programable ones) are typically seperate from the main control. What I was trying to implement on the back of the wiki page advice is giving the duet the chance to say "help I'm out of control" or to consider the atx-on as a "I'm ok signal" until the signal is no longer to ground.
Your other points are also read from same wiki page. Smoke alarms are after the event and a good last line of defence, particularly if the machine is built into a fire box. I've seached for thermal cutouts but not found much suitable for the hot end. I welcome any supplier advice on that! In addition the thermal fuses and snap switches are can fail badly if they fall from the zone they are monitoring. I considered electrically grounding the whole frame to 0V but aluminium is very quick to oxidise and could easily create a new failure point it the circuit raising more false trips.
The exisiting fault detection for the heater is a great way of detecting failure before it happens, and this is easily tested with something as little as a wrench on the hot end or a block or aluminium on the hotend to push the response of the system out of limits. I've seen it trip a few times, but been able to link it to something daft that I've done. Testing thermal fuses isn't ideal as the snap point isn't exact and would naturally be a safety magin above normal operating temperatures. You either end up adding an external heat source (not necessarily similar to the machine out of control) or trying to push the heaters beyond their intended range just to hit the trip.
I've set up a 24VDC interlock loop to kill mains power to psu for steppers and heaters. At the moment this is just grounded by the 5VDC relay and broken by a big red e-stop button. The intention is to expand it to include other safety measures that are not likely to nusance trip. The interlock loop is based on force guided contact relays whose status will be verified by the user through unused normally closed and open contacts. The relay contacts are quick blow fuse protected to well within there operating range.
SSRs are great to replace electromechanical where high switch rates are required. Other than that the electromechanical are often cheaper with the flyback diode and very robust once the contacts are fuse protected. I originally wanted to move away from relays on my trucks loom, but the additional work needed in transient voltage suppression pushed it well into the relms of too much work.
