How do I wire an AC SSR with RAMPs 1.4?

  • I was advised that it would be possible to use an AC SSR to wire up a Mains Based heat bed. Any idea how one would do this with a RAMPs 1.4 board; Also, what do I hook a heated bed of this sort into?

  • If you take a kettle lead with a wall plug and cut off the other end, it will expose 3 wires: earth, live and neutral. These are color coded depending on your country, usually earth is green (possibly with yellow stripes), neutral is blue, black, grey or white and live is brown, red or black.

    The SSR should have 4 terminals: 2 terminals for the switched load (which are interchangeable) and 2 terminals that connect to your electronics, which should be marked negative and positive. These terminals should connect to the negative/positive heated bed output of your electronics. The terminals of the SSR should be clearly marked, and you can verify which terminal is what from the SSR's datasheet.

    Typically, your heated bed will have two wires for power (which are interchangeable). One of the wires should be connected to neutral. The other wire should go to one of the load terminals of the SSR, while the other load terminal of the SSR should connect to the live wire. It is also acceptable to do it the other way around (neutral to the SSR and live directly to the bed) but this is slightly less neat.

    Finally, and this step is extremely important, the earth wire of the plug has a protective function: should something fail, metal parts of your printer may become electrified, and shock you when you touch them. To prevent it, you should electrically connect exposed metal parts of your printer (such as the frame, heated bed plate, power supply case, etc...) to earth. This provides a path for the current to flow (and trip the protective RCD breaker) should something go wrong.

    I would further recommend that you protect your heated bed (particularly if it is a high power model) with a bimetallic thermal switch. These are available in a variety of ratings, and will switch the power to your heated bed off when it goes over a certain temperature. This switch should be wired between the relay and the heated bed, and be mounted on the heated bed so it makes good thermal contact. If you plan to print with the heated bed at 110C, you might get a 120C or 130C thermal switch.

    As mains voltage can be deadly, you should take appropriate precautions: never work on the printer while it is plugged in, cover any connections (in particular, make sure you buy or print a cover for the SSR's terminals and wrap solder joints in heat shrink or electrical tape) and always treat wires coming out of the SSR as live (even if it is switched off, some current can still flow).

  • A link to the bed you have/are buying would be very helpful. AC heated beds exist but are fairly uncommon.

    As one of the answers in the question you linked notes, the heat bed is probably not actually intended to use the 110 VAC/220 VAC directly from the wall and instead needs either 12 VDC or 24 VDC.
    If the current it needs is less than 11 A, then it can be connected directly to port D8 on the RAMPS 1.4, where it will take power from a suitably powerful 12/24 VDC supply connected to the 11 A input on the board.
    A more powerful heater will need an external relay/MOSFET/etc. to control.

    If you do, in fact, have an AC bed, you can connect a SSR like this to either D8 or D9 on the RAMPS (D10 could be used, but it would need a supply connected to the 11 A input) making sure the polarity is correct, as long as your RAMPS DC supply voltage is within the input range of the SSR.
    The linked one is 3–32 VDC, so almost any supply voltage compatible with the RAMPS will work.
    The AC outputs of the SSR should be connected in series between one side of the bed and the AC hot wire, with the AC neutral wire connected to the other side of the bed so most of the circuit isn't carrying high voltage when off.
    This video has some useful hints.

    However, while an AC bed can get you faster heating, will let you use thinner wires, and avoids the need for having a big DC power supply, I would recommend against using a mains voltage bed if you don't know what you're doing—a great deal more care must be taken and there is a serious possibility of injury.

    The question very clearly states "mains based heat bed". I don't see why you would presume that it would be a 12V/24V DC one.

    @TomvanderZanden Because the one in the linked question said "mains based" and was actually not.

    Oh wow, I just checked the product description linked in the linked question. It says "12V or 24V - uses mains voltage". What a mess that listing is. However, I think the OP is quite clear about that the linked product is not a mains bed, as in the commments it's mentioned that it's not *and* in the comments it's also suggested that "How to hook up a mains bed would be a good question" so I guess that's the purpose of this question.

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Content dated before 7/24/2021 11:53 AM

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