Back from winter holiday, I found some nice blue 0805 smd LEDs in the mailbox, along with some SSRs, fuse holders, RAMPs boards and some other stuff. So yes, it's finally time to test drive a heated bed from the first batch.

Soldering

On this first board, I'll only solder the LED circuit and bridge all fuses to test the PCBs thermal stability only. Sorry for the messed up macro shots.

The 0805 LEDs go to the top side..

..while the current limiting resistor..

..goes to the bottom, next to the bridged fuse. Note that there are pads for solder bridging, but I didn't use since I want to install the picofuse lateron.

Wiring

Next step ist preparing the wiring, mostly attaching and insulating cable lugs and inserting the fuse holder.

Also, we need a little bridging wire to bridge the pad from the fuse to the phase pad.

The picture below shows the neutral wire, it goes from the neutral pad directly to the neutral connector of the mains line:

And the next picture shows the phase wire with the fuse and the fork cable lug for the SSR, the ring cable lug connects to the phase pad on the PCB.

For the 230V~ wiring, we need 4 M3x5mm screws, 8 M3 washers and some heatshrink tube, the center pad remains unconnected but of course needs to be insulated:

After the screws are tightened, the tube is shrinked over the connections to insulate them safely. High quality heatshrink tube is vital here. Since having the board house mill out 5 fingers from the board is basically free, this is actually a flush and economic alternative to clunky screw terminals.

Another important step is insulating the foldback clamps that are traditionally used to clamp borosilicate glass plates or PEI (or whatever) to the heated bed. This is absolutely necessary, since the sharp edges of the foldback clamps could scratch of the soldermask and then conduct dangerous mains voltage. I'm using pieces of large diameter heatshrink tube here to insulate standard foldback clamps. I hope I'll find a better solution to this in the future.

The wiring of the SSR will be documented in the future. If you don't know how to do it, you probably shouldn't do it. It's something I believe can only be done in one way and everybody who is actually supposed to do it knows how to do it :) I also had trouble fitting all the cables in one frame. However, since the SSR has mains voltage on the connectors, of course it needs to be sealed away safely. I used a standard IP54 distribution box:

I love those IP54 boxes. Did you know that an Arduino UNO fits into it just perfectly? They're also cheap and available in many colors :D

First power up

Since we're not dealing with nice 3.3V logic levels here, bit of faith is required for this step. So I hooked everything up, attached a thermistor to the center of the heated bed using kapton tape and thermal compound, installed the heated bed and powered up the printer. Reading reasonable temperature values of the controller, I then carefully set the heated bed to 30 °C. For a few seconds, both the SSR and the blue led on the heated bed flashed.

I went on with 40 °C. Still none of the apocalyptic events I was prepared for seemed to happen.

Ok, that made me confident, I started the preheating program to bring the bed to 110 °C.

Whooo, the temperature ramps up. Still no apocalypse. But the heatup time is much shorter than expected. It's only a few seconds. The insulated clamps also work nicely on the new heated bed:

Testing

With this up and running, I will now start testing the thermal stability of the PCBs. I'll run this first board at 110 °C for 30 days continuously to see if the FR4, copper traces, solder mask or silkscreen degenerate in any way. More testing setups for testing heatup/cooldown cycles and extrem temperatures will follow the next days.

Further thoughts

Even though it looks cool with the blue LEDs and gets hot quick, this is still very much unfinished. I.e. the polycarbonate cover for the LED circuit is still missing. The LEDs are too bright, too. I think I'll try a higher value than 1 kΩ for the current limiting resistor next time. The wiring with the cable lugs worked better than expected. Prepping all the wires is tedious though, I guess I need to have them made professionally. I noticed I need to find a better solution for the foldback clamps. The fuses also need to be tested, soon, both their reliability in deflecting the apocalypse and their ability to operate reliable close to the apocalypse. But well, so far that looks really, really good. And look, I can now find a new purpose for that 24 V~ 500W transformer that used to power my old MK2B heated bed: