If you choose to use mains voltage for your heated bed, as I did, it's important to understand one thing. Handle it improperly and IT WILL KILL YOUR ASS and/or BURN DOWN YOUR HOUSE. Proceed with caution. If you're not familiar with mains voltage wiring and components, stick to 12-24VDC for your heated bed. It's much less likely to KILL YOUR ASS or BURN DOWN YOUR HOUSE.
Unfortunately, many people jump into building or modifying a 3d printer with no prior knowledge when it comes to the electrical side of things. While "Ignorance and Enthusiasm" would be an apt title for a book written about my life, this is not an area where you want to "wing" it.
I am not a licensed electrician and the content provided here is for informational purposes only. I take no responsibility for any harm resulting from the use or misuse of the following information. Improper wiring can lead to serious injury and loss of life and/or property and should only be attempted by qualified individuals. Be smart and be safe.
Now that we got that out of the way, let's learn how to build some cool shit.
Solid State Relays
For controlling a mains-voltage heated bed, Solid State Relays are preferred because relays with moving internal parts often can't handle the current required. However, when an SSR fails, it will tend to fail shorted. That is, in the "on" position. Your controller will be unable to cut the power from the device and your device will continue to heat up until it catches on fire. As such, it's important to use high-quality SSRs (Opto-22, Omron, Crydom), in line fuses, and thermal cut offs mechanically clamped to your device.
So much was done incorrectly here but I'll focus on what caused the failure. Instead of using a fork terminal, this person just clamped the bare wire. This improper connection loosened over time causing higher resistance which generated heat. Always terminate stranded wire with some kind of crimp terminal.
Properly crimped terminals and appropriately colored wires. This is what controls the bed heater on the Wildbot.
Solid State Relays suitable for controlling a heater for a 3d printer heated bed tend to run $30-$50 USD. However, many online retailers of list several SSRs in the $8-10 range. These often carry counterfeit UL logos so be vigilant. This is not the kind of component on which you want to go cheap. Underwriter's Laboratories put out a press release on the subject with instructions on how to spot a fake.
Fuses and Thermal Cutoffs
In any AC circuit, an in-line fuse should be used to protect against ground faults. Otherwise, a shorted hot wire would pull more and more amps until a breaker tripped which may or may not a happen before your wiring starts a fire.
I used a 120V 200W silicone heater on the Wildbot. We use the formula (P/I)*1.25 (see above) to calculate expected amp draw then use the next highest rated fuse, which makes the appropriate fuse rating 3 Amp. So if the circuit pulls more than 3 amps, the fuse will blow and kill the power to the components inside our 120VAC panel.
A thermal cut off is similar to a fuse but instead of limiting amperage, it 'blows' if heated to a certain temperature. For the Wildbot, I used a 115C thermal cutoff wrapped in a layer of Kapton tape (which provides electrical but not thermal insulation) and clamped mechanically to the build plate. So if the build plate temperature gets above 115C, the TCO will blow and power will no longer be supplied to the silicone heater. This protects against a failed/stuck/shorted Solid State Relay.
Keeping 'em separated
When building the Wildbot, I felt it was a good idea to keep the DC components (such as the Duet Wifi) and the AC components (such as the SSR for the heated bed) in completely separate panels. This makes it safer for me and removes the risk of a problem with the AC circuit damaging my controller.
All the AC wiring, the ATX power supply and breakout board, and the SSR are all located in a large metal cabinet attached to the back of the table the printer sits on. For AC wiring, it's important to use a metal enclosure so it can be properly grounded.
The 12VDC components (Duet Wifi, power distribution for the LEDs, etc) are located in a plastic enclosure mounted to the side of the printer. Part of my reasoning here was that I often have to interact with the controller and didn't want to stick my hand in a panel with live AC voltage.
Why path to ground is important for AC components
When dealing with AC (mains) voltage, proper grounding is very important. You want to make sure every part of your machine that can conduct electricity has a very good path to ground so that if a "hot" wire somehow becomes to shorted to that part of your machine and you touch that part of your machine, you don't become the shortest path to ground. My advice is just put ground wires everywhere. There's no such thing as "over-grounding". The Wildbot has green wires everywhere. Also, remember, green is always ground, ground is always green.
On belt-driven systems, your belts will generate some static electricity which will arc over the windings of the stepper motor. This can lead to driver chips failing prematurely. Because of this, it is recommended to ground the casings of your stepper motors.
When using crimp terminals, don't just smash 'em together with your Channellocks like a flunkie maintenance man. Use proper crimpers. I use Klein crimpers and I cannot say enough good things about them. These will be the last ones you ever need to buy. But you can buy cheaper ones and they will probably work fine. I'm just a bit of a Klein fanboy.
You always want to use ferrules to terminate stranded wire. This avoids loose strands that can short out with nearby connections. It also helps get a tight clamp with screw terminals since the strands can't just fan and flatten out. Always use the smallest ferrule that will fit over your wire. I'll be honest, I use my regular Klein crimpers for these because
I'm a flunkie maintenance man I find crimpers that only crimp bootlace ferrules too specific but I'm sure someone will come along and tell me how wrong I am.
Ground wires should always be ring terminals as opposed to fork terminals or, worse, bare wire wrapped around a screw (bad maintenance man). Always use proper crimpers as stated above. These are sized by color so always use the correct size. It's also important to note that crimp terminals should only be used on stranded wire, never on solid wire. On something like a 3d printer, you'll probably only be using stranded wire since you'll have moving components but I felt it worth mentioning anyway.
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