How to assemble a Heated Build Platform

None of the PLA lifted away from the print bed during printing

Prototype Raygun printed without warping on heated bed

When my first prints of my raygun prototype warped up from the print bed, I knew I was going to have to bite the bullet and build the heated build platform. Here is the result. RESULT!

I’ll write about what I did, what I’d do differently next time, and how to hook it all together.

The shiny grip of the raygun has stayed firmly stuck to the printbed

Isn't clear PLA beautiful. And now it stays down

Top view of the barrel of the raygun, showing the gap caused by warping as the PLA cools

The early print, without the heated bed

Immediate Benefits!

If you compare the new photos in this post with the one from the previous post, you immediately see that the warping has gone.

The PLA has stayed firmly stuck to the blue tape on the build platform, with no lifting at all. In fact, even after everything had cooled down, this print was more difficult to remove from the bed than any earlier ones.

(Btw, I use a small stainless steel artist’s painting knife to remove prints from the bed. The edges are thin and slightly flexible without being sharp. So usually I get away without damaging the blue tape.)

The heated bed components

The MakerGear Mendel Prusa kit came with a big red printed circuit board, called the “Heated Bed PCB”. It was designed (so it says on the silkscreen) by Joseph Prusa, who designed the printer variant as well – it is named after him. He uses a different system now, but he proudly claims his personal printer is never the same two weeks running. He is always modifying and experimenting.

One side of the pcb has a big ‘Heated Surface – do not touch’ warning. The other side has the tracks that do the heating, and space to solder on two surface-mount LEDs, a surface mount resistor, and pads for connecting the leads to the RAMPS controller board. The LEDs and resistor are optional. There is also a thermistor for temperature regulation, but no obvious place to mount it.

The online instructions for this are, um, challenging – if you can find them at all. The pages on the MakerGear website are for earlier, different, heatbeds and for the Mosaic which is different again. The ‘Differences from standard Prusa‘ file covers this a little, and I recommend you read it (especially for the photos), but I could have used a bit more help.

Board orientation and cable location

You’d think that you would want the warning side up, so you could read it. But that puts the LEDs underneath, where you can’t see them. And no, the PCB is not translucent at that point, so they don’t glow through from underneath.

Most importantly, you need to fix the thermistor somewhere, so the RAMPS can tell when the bed is at its correct temperature. The print bed is in constant motion, so you need some form of strain-relief for the wire that leave the printbed and connect to the RAMPS. Surface mounting wires onto PCBs is fragile, and won’t last long if the soldered joint flexes at all.

The kit comes with two lengths of red wire with crimps at each end for fitting into pushon connectors. I only found one unused connector, that fitted the T1 thermistor pins on the RAMPS board. I plugged the crimps into that connector, and twisted the wires together so I could tell how much length was available. This helped me decide which way round to mount the PCB.

After much pondering, I decided I needed to mount the heatbed PCB so the surface mount components and the connecting wires were nearest the RAMPS board, which means they are coming off the side of the printer. The photo below shows the terminal strip that the thermistor and heater cables connect to. It also shows that the wires from the terminal strip to the RAMPS go underneath, between the printbed and the y-carriage, and out the back. I’ve taped them to the underside of the print bed. That way there is no flexing of the joints going into the screw terminal, and nothing sticking out to catch on the RAMPS board as the y-carriage whizzes backwards and forwards.

I chose to mount the PCB with the ‘hot surface’ warning on top. Although I’m covering it with blue tape, I wanted to have the heater tracks underneath the pcb in case of massive nozzle crashing damage. If I get the print height wrong and scrape the nozzle along the surface, it’ll only scrape off some warning, not cut my heater tracks.

Soldering the cables and the tiny surface-mount thingies

You have to cut the single piece of green-insulated high temperature wire into two pieces, for the connection to the heater tracks. Strip and tin both ends of both pieces.

Solder the wires to the tiny square tabs, as the ‘differences’ file shows. I carefully and gently scraped the squares with the tip of a scalpel until the copper was nice and shiny. Then I tinned them with a 25w soldering iron, because my 10w fine-point couldn’t heat the pcb up fast enough. This was my first attempt at smt soldering, so you don’t get photos from me. Please read a how-to on smt soldering if you haven’t done this before.

You don’t have to attach the LEDs and resistor, you won’t be able to see the LEDs anyway if you mount the board the way I do. Life would have been simpler for me if I had known this. I cracked one of the LEDs by applying too much downward pressure while trying to get it to seat against too large a blob of solder.

If you do decide to use the LEDs and resistor, use your multimeter to check that you haven’t got a solder bridge under the resistor. The resistance from wire to wire should be around [Update: 1.6 ohms]  not zero!

Testing the heater

You can test the heatbed at this point if you wish.

  1. Put the ends of the green wires into the white screw-terminal block.
  2. Put one end of the black twin-core power wire into the matching holes in the terminal block.
  3. Put the other end of the twin-core into the D8 connectors on the RAMPS board. The + goes to the + sign (Dur!) and the – goes to D8.
  4. Ensure all screws are done up firmly
  5. Turn on your computer and the printer power supply.
  6. In Pronterface, connect to the printer. Set the Bed temperature (in the bottom left of the window) to 60. The setting doesn’t matter because you don’t have a thermistor attached yet anyway, so RAMPS will just run the heater at maximum.
  7. If you installed the LEDs and resistor, one of the LEDs should light, but not both of them, unless you oriented them wrong.
  8. Listen to the powersupply strain, and notice the bed getting warm.
  9. Reset the printer, and turn the printer’s power supply off.

Carving holes in your nice flat print bed

Having the heater tracks on the underneath of the PCB means the LEDs, resistor and connections are also underneath. We don’t want them pressing up against the underside of the PCB, because that would make the bed uneven, and cause all sorts of problems. I carefully cut hollows and grooves in the top ply of the print bed, so the components would lie beneath the bottom of the PCB.

The connections to the heated bed PCB and thermistor are via a 4-wire screw terminal block

Thermistor and bed heater connections via screw-terminal block

You can see the grooves for the wires in the photo at left. You can also see I was not careful enough, because a couple of wiring grooves are unused – I put them in the wrong place.

You can also see that the green wires don’t connect to the terminal block. This is a mistake! Don’t do it the way I did, do it the way I wished I had. See the next section, on the Screw Terminal Block, where I explain.

Because I was too lazy to remove the printbed (and have to re-level the whole thing? Nooo!), I had to be careful not to use too much downward pressure while drilling and carving. I started the hollows with an electric drill, just enough to break the surface. Then I used some very sharp lino-cutting tools to cut the grooves and lines to shape without going below the first glue layer of the plywood. I was afraid if I cut too deep, I might change the flatness of the build platform.

The screw-terminal block

All along I’d been wondering how to connect the wires so they don’t flex, and don’t catch on other parts of the machine while the bed is moving. In the end I decided to use another piece of the screw terminal strip. I think I’m going to buy some more to keep in my toolbox – it is so useful.

By the time I thought of the terminal strip I had already soldered the black cord to the red plugs, as written in the ‘Differences’ file. I’m now wishing I’d just taken the ends of the high-temperature wires from the heatbed straight into the terminal strip, next to the thermistor wires. Then plug the ends of the thick black wire into the other side of the terminal strip, and off to the RAMPS. That would have saved me some iffy soldering – I don’t like soldering wires onto connectors without some sort of hole to loop the wire through, so I’m not happy with the red plug and socket.

Steps for thermistor installation

This is how I did it. I’m sure there are better ways, because this was no fun, and I prefer elegant solutions. I couldn’t find a writeup anywhere, so at least you can laugh at mine, while you do whatever the obvious better technique requires. I still can’t think of an obvious better technique, so if you can, comment!

  1. Ensure you’ve already mounted the LEDs, resistor and heater element wires, and cut the hollows and grooves for them.
  2. Lay the thermistor on the build platform with the legs hanging far enough off the side that you’ll be able to put them into the terminal block (see previous photo for guidance). That shows you where you need a hollow for the thermistor and a groove for the leads.
  3. Draw around the thermistor and leads in pencil. Put the thermistor somewhere safe – it is tiny and almost invisible, don’t drop it on shag pile carpet.
  4. Carve the hollow and grooves. Don’t go below the first ply.
  5. Lay the thermistor down on the sticky side of a 1″ wide piece of kapton tape, with the flatter side of the thermistor up, where it will contact the PCB. Ensure the insulation on the wires is firmly up against the glass body of the thermistor.
  6. Lay the tape-and-thermistor on the plywood build platform in the hollow and groove, sticky side and thermistor on top. Push the thermistor down into the hollow, so the tape bunches around it a little. This is hard, because if you touch the tape, it won’t want to stay in place.
  7. Put the heated bed PCB back in place on the build platform. Get the mounting holes aligned on the side opposite the thermistor, before gently lowering the pcb onto the thermistor and kapton tape. Press the PCB down to ensure the tape has stuck to the underside.
  8. Gently lift the PCB up, and lie it thermistor side up on a flat surface. Burnish the tape down onto the PCB, starting around the thermistor so the tape presses it against the underside of the PCB. Avoid bubbles as much as you can.
  9. Put the PCB back in place again. Take special care that all the wires are in their grooves, and that the PCB is laying flat against the build platform. If all is well, bolt it down with the mounting bolts and washers. (Mounting it this way is sub-optimal, in that as the board heats, it expands, and because the corners are fixed, the only way to expand is to bow upwards. People have tried all sorts of ways to fix this, but mostly its not too crucial.)
  10. Connect the clear-covered thermistor wires to the screw terminal strip. Connect the free ends of the red wires you prepared earlier to the corresponding holes on the terminal strip. Yes, it’s another waste of perfectly good crimps.
  11. Connect the green-covered heat bed wires to the screw terminal strip. Connect one end of the black twin-core to the corresponding holes on the terminal strip. I don’t know why polarity should matter here, but get it right anyway.
  12. If you haven’t already done so, screw the terminal block to the side (or underneath, if you are really clever) of the printbed.
  13. Run the black twin-core and the red twisted wires between the print bed and the y-carriage, and tape them to the underside of the printbed as they exit at the back. Ensure no wires stick out to the side where they could catch on the RAMPS or the RAMPS wiring.
  14. Plug the connector end of the red wires into the T1 thermistor pins next to your hot-end thermistor on the RAMPS board.
  15. Plug the bare end of the black twin-core into the D8 connector on the RAMPS board. + goes to +, and – goes to D8.
Heated bed PCB connections to RAMPS

Heated bed PCB connections to RAMPS

All Done

I think that’s it.

Here’s a photo showing the twisted red wires heading off towards the ‘T1’ thermistor connector.

The two heavy black wires directly under it are the heatbed wires, with – on the left and + on the right.

The medium-weight red and green wires on the left are the hot-end nozzle connections. The unused D9 connector in between is most often used to run a fan, but I haven’t done that yet.

Please Comment

I hope that’s enough (along with the ‘Differences’ file) to get people going. Please comment on any confusion I may have caused, any better ways of doing things, any warnings you may have.

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11 Responses to How to assemble a Heated Build Platform

  1. Gary says:

    Sure glad you got into RepRap before I did.
    Thanks for another time and $$ saver.

    • BrazenArtifice says:

      Thanks Gary
      As a newcomer to the world of blogging, I’m astonished at the pleasure I get when someone shows they read something I wrote! Some sort of primal primate thing I guess.
      Hope your build goes well.

  2. Brock Tice says:

    Wow, you did a much better job of this than me (the original author of the ‘differences’ document as I was finding my way, much has been added/changed since). That said, mine all works fine without the terminal block, I just make sure there’s enough slack that strain isn’t an issue. Nice work, please feel free to add it to the doc!

    • BrazenArtifice says:

      We’re all just finding our way! I’d have been lost without the differences file you instigated. And as you can see, I too have joined the tradition of the “don’t do what I did, do what I wish I had done” photo.

      I don’t know how to add material to the doc. I’m burning out (its 33 Celsius in here today) on just writing up printer stuff on the blog, which I understand. Feel free to add it yourself. Maybe if I can find some step-by-step instructions? 😉

  3. Mine works fine without any strain relief. My heavy cable is so thick that the solder joints don’t seem to care much about the movement, and my thermistor wires are zip-tied to the heavy cable so they take advantage from the cable’s strength too. It’s still just as you see it in

    On an unrelated note, if your resistors light up, something is very wrong! That’s a privilege usually reserved for the LEDs 😉

    ps: the LEDs and resistor are totally optional, the heater works fine without them.

    • BrazenArtifice says:

      Glad to know your wires haven’t come loose. I guess the sparks could be exciting at 16 amps or so! I learnt decades ago to never rely on solder for mechanical strenth, so I’m nervous about just tacking a bit of wire on with a drop of solder.

      Thanks for the catch about the resistor. I’ll update it pronto. I’ll blame my brainfade on trialling PLA without active cooling – I’m dripping with sweat here 😦

    • BrazenArtifice says:

      Ok, I’ve just looked at the photo you linked to, and I can see why you have more confidence in your solder joints than I did in mine.

      On the PCB provided by MakerGear, the un-solder-masked squares for soldering each wire are less than 2mm square. That’s not a lot of surface area. You’ve probably got ten times that surface area in each of your joints. That’s why I wanted the high-temp wire connected to something that wasn’t going to flex the joint.

  4. Thank-you so much for providing much needed information about this “kit”.

    One question, it looks like you’re following the RAMPS 1.4 wiring diagram from the RepRap site. I’m going this route as well but I was told that connecting the heated build platform to the board in this was (as opposed to directly to the power supply as described in the “differences” doc) will “fry” my RAMPS board. Apparently this is not the case? 🙂

    I ask because I’m having trouble getting the heater to engage connected in this way, but I’m guessing now that it might be a software thing (I’m using the MG supplied Sprinter firmware and RepSnapper, and the board doesn’t turn on when I manually try to engage it from Repsnapper).

  5. BrazenArtifice says:

    Hi Jason,
    I’m afraid I don’t know what the reprap site’s diagram looks like from memory, and with my satellite broadband dead, I don’t have much bandwidth to play with in researching this. So I’m going from memory here.

    You can’t connect the heater pcb directly to the atx power supply because the RAMPS board would have no way of controlling the power to the heater. Switching high currents (the heated bed, the nozzle heater and a fan if you have one) is what the mosfet switches on D8, D9, and D10 are for, so it won’t fry your RAMPS.

    Is your heated bed thermistor returning meaningful numbers? It should give you roughly the same ambient reading as your unheated nozzle’s thermistor. Don’t try to run the heated bed til the thermistor works.

    You have two different pairs of wires coming from the ATX power supply. Both carry roughly 12 volts, though the exact value varies with the power supply and the loads.

    1. Check with a voltmeter that the voltage going into the RAMPS on the 11A connector is roughly 12V.

    2. Check that when you turn on the heated bed that the voltage at D11 shows up on the pins on D8.

    The voltage at the 11A socket should drop a bit when you turn the heated bed on. Then when the bed gets up to the temperature reading you set in repsnapper or pronterface, the voltage to the heated bed gets turned off till the temperature drops a bit.

    If your thermistor works, you’ve got 12V at the right places, and you are able to communicate with Rick’s version of Sprinter, the only other thing I can think of is that the connections to your heated bed pcb aren’t working (open circuit, dry joint, something like that). Unless repsnapper thinks the heated bed is somewhere other than the D8 pins. I use pronterface, so I can’t help there.

    I hope that helps. Sorry for the delay in answering, I won’t have much web access til my new satellite system gets installed. Could be weeks.. 😦

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