Longboat Prusa heated bed

I'm building a Longboat Prusa Mendel from the RepRap Kit Store. Unfortunately their instructions are incomplete, and have not been updated for over a month. Also, they now seem to be selling a slightly different version so I fear they may never provide complete instructions. Brian's build blog has also not been updated for weeks.

So, has anyone here built one successfully? I'm at the point of assembling the heated bed, for which there are two different sets of wiring instructions. Do the "alternative" instructions work, and can anyone provide more detail?

I updated the instructions on the longboat wiki page and they are working fine for me. the official instructions seem to miswire the FET so it will work but get too hot. I still haven't received the promised corrected thermistor so don't know if that will help too.

I hope the instructions get updated or that other builders join in updating the wiki. It's frustrating that there are snippets on a whole load of blogs instead of everyone pitching in to the shared wiki.

Mine is working to a point but not quite right. Biggest problem is I can't do overhangs like nopheads meter key as they sag too much but otherwise prints are ok, the wiki should describe how mine was built.

Has anyone heard anything more from the reprapkitstore? I have still never had an email reply and not seen updates on the mystical thermistor fix...

Nigel

Thanks for your reply. Can you clarify a few things?

1. The unswitched +12V - is it reasonable to take this from one of the IDE disk connectors on the power supply?
2. When you say "pulldown to source", does this mean connect gate and source with the supplied 1M resistor?
3. If the source and the resistors are connected direct to the power supply, there will only be one wire from the electronics?

I've attached a corresponding circuit diagram, can you confirm it is correct?

I received two replacement thermistors from the RepRap Kit store, their room-temperature resistance seems to be about 130k. How do you advise attaching the printbed thermistor? There's nothing about it in the build instructions.

Will answer what I can...

> [*] The unswitched +12V - is it reasonable to take
> this from one of the IDE disk connectors on the
> power supply?

Yes. Not sure which connector, but I used 3 x yellow and 3 x black from the power supply for the bed in order to ensure the wires can handle the current.

> [*] If the source and the resistors are connected
> direct to the power supply, there will only be one
> wire from the electronics?

Yes

>
> I received two replacement thermistors from the
> RepRap Kit store, their room-temperature
> resistance seems to be about 130k. How do you
> advise attaching the printbed thermistor? There's
> nothing about it in the build instructions.

I drilled a small notch under the bed and set the thermistor into it with silicoset 158 from Rs components. you could just use kapton tape to fix it but I think high temperature silicone is supposed to give better thermal connection. If you use tape, make sure you tape it well!

Sorry I didn't answer your question about the circuit diagram. Not my area of expertise so would rather not comment!

ps Having been doing this for a couple of months now, I've done a couple of mods to improve the bed, you may be interested:
1. I swapped the supplied bed resistors for 4 x 1ohm resistors (2 in series x 2 parallel) to give effective resistance of 1ohm. My bed used to take 15 mins to get up to 65 degrees C but now takes less than 5 mins.

2. Having battled for hours levelling the printbed with 4 bed mounts, I switched to 3 mounts and bed levelling is now a piece of cake. I kept the 2 holes on the x axis motor side and drilled a hole in the middle of the x axis idler side in the bed. This makes bed levelling easy because the bed only pivots on 3 points.

You might want to come back to mods like that if you are just getting started though!

ebcdic Wrote:
-------------------------------------------------------
> Thanks for your reply. Can you clarify a few
> things?

I think 'Another' has probably clarified all you need, but didn't mention the FET wiring.

> [*] The unswitched +12V - is it reasonable to take
> this from one of the IDE disk connectors on the
> power supply?

That's what I did. Look at the supply, choose a supply rail that can handle the resistor load (I don't have my printer to hand, but I think I used the 12V supply). I 'salvaged' a power connector from an old hard drive so it could be unplugged and can replace the PSU easily if required.

> [*] When you say "pulldown to source", does this
> mean connect gate and source with the supplied 1M
> resistor?

Yes, just like your schematic. Could you attach it to the wiki instructions please as it's much clearer than my word instructions. Thanks for taking the time to put your other updates on there.

> [*] If the source and the resistors are connected
> direct to the power supply, there will only be one
> wire from the electronics?

You could do, I wired both signal and ground. It shouldn't actually matter in this case but from an EMC side I always prefer something closer to mesh grounding so every signal has a low impedance return path. Do whichever is easiest

> I received two replacement thermistors from the
> RepRap Kit store

Lucky you, I'm still waiting, ever more impatiently...

> , their room-temperature
> resistance seems to be about 130k. How do you
> advise attaching the printbed thermistor? There's
> nothing about it in the build instructions.

I put some thermal compound on the base, squashed in the thermistor, and covered with kapton tape. I'm still using the originally supplied thermistors with my bodge to the electronics until the final part of my printer arrives, hopefully soon, over 2 months after the rest of it!

I located it on the underside, a couple of cm out from the centre (you could think of it being on the circumference of a circle whose diameter is half that of the 'circle' of resistors), thinking that would be a better measure of mean temperature, but it probably doesn't matter in practice. The footprint of the print probably has more effect than the position of the thermistor.

> I 'salvaged' a power connector
> from an old hard drive so it could be unplugged
> and can replace the PSU easily if required.

I found that already had a suitable cable - many PC motherboards come with adaptor cables for powering SATA drives. Just cut off the SATA connector.

> Could you attach [the circuit diagram] to the wiki instructions please

Already done.

> I put some thermal compound on the base, squashed
> in the thermistor, and covered with kapton tape.

I ended up drilling a (very) small indentation in the bottom of the board and using kapton tape without thermal compound. It seems to work well: using table 1 in the Sprinter firmware, it registers within a degree of a thermometer
on top of the bed.