SSR for PS

  • I am thinking about adding a SSR to the 120v AC line going to the Power Supply for the heated bed, and control it from my Duet2 using the PS_ON Pin.
    What SSR would you guys recommend?


  • I'd recommend a regular mechanical relay instead if the power supply is of any size, especially on 120VAC. The SSR will have need to dissipate more heat than a relay, and you don't need the frequent or silent switching the SSR offers.

  • None. SSRs are not safety devices and the use of them for safety critical applications is advised against in the manufactures documentation as referenced by the Duet documemtation.

    Get a mechanical relay and add appropriate fuses inline with the relay and coils. Mechanical relays are far more tollerant to transient voltages. While contacts can weld shut on a relay it's not the most common failure mode, on contrast to SSRs fail short.

    The MOSFETs on the duet are already over specced so it is concievable that the trigger for a MOSFET failure (ie lightening strike within a mile, mains spike etc) on the board would also kill the SSR.

  • As an add on to this I'm planning a long overdue safety upgrade to my machines which will include a relay isolated PSU.

    Ideally I'd use and monitor a pair of guided contact relays but the cost of that would exceed the cost of my boards. I'm going to use the following (random supplier shown, not promoting them) with a pair of contacts breaking both live and neutral.

    Added bonus of the board being it is easier to mount and already has flyback/flywheel diode protection on the relay coils.

  • I'll shut up soon! 😄

    Having looked at those specs again for my little printers with 100W mk3 beds and 30 or 40W hotends I could simply feed a DC+ line direct to the relay board and then to the hotend and heater bed.

    This would still be controlled by ps_on and essentially would enable or disable the heaters directly.

  • To be honest, I don't really care if it is a mechanical relay or an SSR... I would just like recommendations on what one to buy.
    I wasn't 'having much luck looking for an SSR the other day. Luck finding something I would say forsure would work.


  • @bluedust

    Are you controlling an AC heat-bed or a DC heat-bed? In low-voltage circuit design, it is generally considered a no-no to mix low-voltage circuits with 'high-voltage' circuits, due to the potential for accidentally suffering lethal electrocution. However that may be, to answer your question, I use AC & DC SSRs from this vendor:

    Re; contactors/relays, consider procuring a sealed unit to prevent (unlikely) pontential of igniting an explosive petro-chemical out-gas product from heated FDM filament.

    Remember to verify that the input 'trigger' voltage range is within the output control voltage of the controller. Many SSRs utilize higher voltage triggers than CMOS controllers provide. In other words a 5 Volt DC input will not work with a 3V3 (3.3Vdc) Volt DC control voltage. An SSR that operates with a range of 3Vdc upto 12Vdc will work with a 3V3.

    Depending upon the amount of the load, how much power is being switched, will determine if the SSR requires a 'heat-sink' to radiate off the heat of the pass element. In my latest application, I custom fabricated a 'drying' chamber for a high-end ceramic tile manufacturing company in Malibu California. It switched 120VAC @30 Amperes, and it required a large fan driven finned heat sink. However, in my 3D printer, as I recall, I switched 12VDC @30Amps (360Watts) for which I used a 100Amp DC SSR.

    I used the switching power-supply case as the heat sink, which works and is adequate. You can see the SSR mounted on the top of the case of the power-supply.

    If switching high-voltage AC, try to route the HV wires away from the low LV wires, use adequate insulators and heat-shrink. Label the HV locations, and be sure to make your documentation for later service reference. Use unique colors for HV circuits different from that of your low voltage wiring. Its your life and those of folks around you, not to mention if you screw up, you could potentially burn down your house. Its your responsibility to fabricate the wiring safely and if you can't, don't do it, or ask some one who is qualified to do the fabrication for you. I apologize for the safety rant.

    Good Luck


  • @3mm said in SSR for PS:

    pontential of igniting an explosive petro-chemical out-gas product from heated FDM filament.

    I'm all for safety, but seriously, what are the odds of that getting to a point where it can actually ignite a air/gas mixtur from the out gassing? I know people were laughing at Peter Griffins volcano insurance, but for all practical reasons the line has to be drawn somewhere.

    If the outgassing is a genuine concern, then any commercial supplier of FDM 3D printers would only supply intrinsically safe parts or parts enclosed in explosion proof enclosures, so either that's fear mongering or the whole industry is in for an rude awakening. Somehow I doubt people will start using supplies like I happened to do, and even it wouldn't pass muster if we assume the print area to be a normally hazardous atmosphere.

  • @bearer I'd second that. If it were a real concern injection molding and extruders (which extrude far more polymer) would use ATEX rated gear, and as far as I'm aware they're not outside specialist applications or environments.

    If there is a growing number of 3D printer accidents the I could see the CE marking of these machines under the IT Equiment guise rather than full machinery directive coming under close scrutiny.

  • @doctrucker said in SSR for PS:

    @bearer I'd second that. If it were a real concern injection molding and extruders (which extrude far more polymer) would use ATEX rated gear, and as far as I'm aware they're not outside specialist applications or environments.

    probably not, due to the temperatures involved the likely solution would be a inert atmosphere, and as such no need for certified equipment.

    Anyways, back to scheduled programming and OP:)

    (👇 I think we can create a new topic if this is something to be discussed in length)

  • Hi All,

    My previous statement does seem a bit ludicrous, but...

    A freind worked with me at RocketDyne, the guys that manufactured the Moon Engines and later the Space Shuttle Engines among other gadgets. Everything there was safety oriented. In his office at work, not a lab, he worked with photo chemicals. A scientist, doing things he had done for over ten years...for high speed photography. Over the duration of a Friday evening, Saturday, Sunday, and Monday morning period, a gas, generally non-explosive, filled the atmosphere of his office, in just the right combination. Monday morning when he flipped his office light switch on, the vapor which had leaked into the non-sealed light-switch, surrounding the switch contacts which closed, bounced a few times (as switch contacts, relay contacts and contactor contacts do), created tiny sparks, the 'room exploded', killing him.

    My point being, there are dangers in these matters, and new, previously unused filament compositions are being introduced almost daily and just because 'something' doesn't happen frequently, doesn't mean that things are any safer. Employ adequate ventillation, and make sure your gadget and environment is actually safe within which to labor.

    Sincerely, 3mm

  • @3mm fair enough. Adequate ventilation is approaching manditory for our machines anyway for Health reasons.

    Some press, media, and individuals do their best to discredit a lot of 'health and safety' but rarely do they say that's bad health and safety risk management. They will just tar the whole lot as irrelevant or unneeded. Essentially bad risk management is adding/advocating excessive control measures or intrusive or awkward guarding. You have raised a concern and now need to back that up, or work out why industral machines that can extrude kg per minute don't have ATEX rated panels.

    As a group largely comprising of hobbyists there is a productive discussion around the area.

    An good level risk assessment (something I will have to tackle soon for my amateur built machines working in business - no CE mark complicates PUWER) will raise and discuss all potential hazards as if they were all uncontrolled or guarded and then discuss the control measures in place, and if any additional actions are needed to further mitigate the risk.

    I thank you for raising a risk that I hadn't considered and I will add to my document. But for your own benefit/piece of mind I would suggest you research the concentrations required for explosion and the quantities of out gassing from typical build lengths with extrusion rates up to super volcano levels.

  • No. Anecdotal evidence of unrelated accident from a once in 10 years practice does not warrant caution for normal operation of a 3d printer. A gas is not generally non-explosive; a gas is flammable or not; whether or not it'll explode depends on air/fuel mixture, confinement and ignition source.

    Yes a 3d printer can emit amongst other things volatile organic compounds as the filament is heated; but calling for explosion proof relays (technically non-sparking) in a 3d printer is nonsense before considering the hotend as a more likely ignition source, and again, if the off gassing warrants precautions during normal operation then its the end of consumer 3d printing as it will turn the print area into a IECEx Zone 0 area (or class 1 division 1 in the US) which is something you don't want unless you have time and money to throw at it. Even with ventilation you'll need to jump through all the hoops to prove and ensure adequate ventilation if you otherwise would have an explosive atmosphere during normal operation. (There are some definitions of the duration and length of emissions, but for all intents and purpose emissions as part of normal operation as opposed to emissions as part of a fault condition lands you in the strictest definition)

    Focus on the health risks associated, but until vendors create filament and printers that needs, and has, warnings and safeguards to prevent explosions lets move on; you'll win the lottery and get struck by lightning while a shark falls out of a plane and kills you simultaneously before the power supply relay will explode your printer.

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