heated bed

[thereza 0]

I purchased a PCB-based heated bed and was planning on starting my build. I had a couple of questions.

1. why don't people use acrylic for the bottom-side of the heated bed? It doesn't melt till it hits 160C, and the bed only needs to be 70C.

2. can the board that comes with the ultimaker handle a heated bed? I measure 1.3Ohms resistance in mine, which could draw a ton of power quickly. Or should I make a separate implementation using a PID temp controller (like $24 for one these days + cheap thermocouple probe).

3. what kind of power supply is reasonable?

thnx

reza

[chemwhite 0]

I wouldn't use acrylic for the heated only because the thermal conductivity of the acrylic is low, which might make control of the temperature on the top side of the bed a little difficult.

[rewolff 0]

IIRC the heated bed can draw more current than the POWERSUPPLY of the ultimaker can handle. So getting a separate powersupply would be a good idea. Assuming the board has the STP55NF06 as listed on the 1.5.4 board layout, it should be possible to hook up loads up to 7A, or at 19V about 133W. On the other hand, you'll exceed that a bit with 1.3 ohms.

By connecting the external powersupply ground to the ground of the ultimaker, andthen hooking up the heater element between the external powersuppply positive side and the mosfet output of the ultimaker board, you'll be able to control it from the ultimaker. Some electronics experience required I'd say.

[thereza 0]

I like your idea better than what I had in mind, but 7A will not work -- it will need one that can handle >10A. I've got another fet lying around that is rated much higher and can switch off 5v, but it's a SMA part, so it will be an ugly replacement.

[thedudevt 0]

Here are some pictures of my aluminum bed.

I'm still working out how to wire it into the ulticontroller.

Kyle

[thedudevt 0]

I asked my electronics guru buddy how to wire this into the ulticontroller without modifying the board. Here's what he drew. WARNING: This is untested.

I picked up parts today, so I'll try to get it wired up this weekend.

Additional notes:

The grounds from the two supplies need to be tied together.

The N-channel mosfet needs to be able to handle 10A, an should be spec'd to 1.5 times safety factor, so at least 15A.

Kyle

[thereza 0]

I was thinking of doing something like that as well originally, but rewolff has a good point. if it has fet it has on it, then the 19v is always on and the gnd switches on and off. If you want to use it, then you'll have to use a p-channel fet in this configuration (see photo)

the main advantage of replacing the part is that you get the benefits of cooling it from the fan - otherwise you will probably need a heatsink on the fet

[thereza 0]

i just checked and the STP55NF06 part quoted (i'm not sure if it's on my unit or not, i'll check later) goes up to 50A @ 60V with a very low on resistance. If that's the case then nothing needs to be modified, just connect the 12v supply to the heater bed to the ultimaker and share the grounds as said earlier..

[thereza 0]

this is annyoing.. this is what they have listed in the BOM

Part Value Device Package Library mounting side pcb rs-componentsnumber (if available) Supplier Partnumber

Q1 STP55NF06L IRF512 TO220BV transistor-power top 485-7721 STMicroelectronics STP55NF06L

they show both the STP55NF0L and the IRF512. The former is good, the latter is not going to work.

[rewolff 0]

With a separate powersupply you can do something like that. However you're in big trouble when you connect the grounds of both powersupplies together. Hmm. Maybe it won't even work. That "+12" is the "separate powersupply", right? The GROUND you are drawing there is the GND of the separate powersupply and the mosefet output (the switched output) of the board.

It'd be a lot better if you just put an optocoupler in between.

heater-output plus - resistor 2k2 - opto-anode -opto kathode -> heater output minus

(heater output plus is tied directly to the 19V).

On the other side, you have the circuit youve drawn, but the opto-output goes between the +12 and the gate of the mosfet. That's all. Three extra components to do it proper.

[edit] I did the calculations for "acceptable current" with "1W" as the max power. Now these packages can handle a lot more, but you'll have to do something with heatsinks, or know what you're doing and do the math yourself. I like margins. 1W has some margin... :-)

[thereza 0]

i don't know why you wouldn't be able to connect the GNDs together. Especially if they are both switching/isolated supplies. hopefully they have the stp55nf06l mounted and everything becomes quite simple.

[Daid 306]

i don't know why you wouldn't be able to connect the GNDs together. Especially if they are both switching/isolated supplies. hopefully they have the stp55nf06l mounted and everything becomes quite simple.

Indeed, there is no problem in connecting 2 grounds. (except in advanced cases, but you don't have to worry about those)

But as a general rule, don't try stuff you don't understand, unless you are willing to buy new things if it breaks.

[thereza 0]

I checked, and the part is

http://www.st.com/internet/com/TECHNICA ... 002690.pdf

which is a pretty bad-ass component.

Using it in that configuration will burn about 1.6W on the fet, and the fans should adequately cool it.

I also understand this stuff (i design and build medical equipment). I also built an open-source cnc mill controller board and wrote the firmware for it (

http://reza.net/cms/index.php?page=rstep

) -- but thanks for the concern

When I have more time, I'll resume work on it, but the project is kinda in stasis.

Reza

Reza

[rewolff 0]

With a "junction to ambient of 62 degrees/W, that 1.6W of yours will come to 100 degrees. That's 100 degrees above ambient for a total of about 125. 175 is allowed. Now with the fan blowing this number can be much lower. But with the bad fan-casing design (I think so, maybe I'm wrong.) I'm not so sure that the casing + fan will effectively help.

So, feel free to work it that way. It'll get hot. If you check it with your fingers, you'll get burned. If you don't burn yourself, the fan DOES work, and I'm wrong about the effectiveness of the fan+casing... :-)

[thedudevt 0]

Finished my heated bed, the power FET circuit I posted didn't work. It powered the bed fine, but didn't shut it off even with the pull down resistor. The FET also got really hot, even with a heatsink and I didn't trust it unattended. I switched to an automotive relay from RadioShack

http://www.radioshack.com/product/index ... d=12505664

which works great. I recompiled marlin with Daid's BuildMeMarlin to add bed support with a 100K thermistor. The Ulticontroller needed no changes, and lists the bed temp on the watch screen. Cura doesn't (yet) have a check box for bed temp, so I added M190 S62 to the start.gcode and a corresponding M190 S0 to the end.gcode. Cura doesn't recognize M190 during slicing but it's just a warning. Marlin responds correctly and waits for the bed to come up to temp, and shuts it off after completion.

Bed leveling seems to be more critical than with blue tape. I'd seen a few things to mount a dial indicator to the head, so I dug up one of my dials and made a clip.

http://www.thingiverse.com/thing:25692

 

So far so good, i'm currently printing at 200C, with the bed at 62C indicated (60C measured).

This thing makes a great space heater, it'll be nice in the winter.

The one thing I think I've done differently is to add a cable chain for the wiring. I printed

http://www.thingiverse.com/thing:11978

at 75% scale, 100 % fill, 13 links and I think it came out pretty well.

Thank you to all who have blazed the trail.

Kyle

[destroyer2012 0]

Be careful that the PLA objects in close proximity to the bed don't sag! Extended periods at temperatures nearing Tg result in plastics creeping. Those cable guides near the bed might warp over time, and also anything like a fan duct that you have, especially if it's PLA. When I made my heated bed (which is powered by mains voltage running through four power resistors) everything made of PLA anywhere near the bed melted, so I had to reprint all my mods in ABS. Of course I am running my bed much hotter (120C). Still I think even at 60C you will have noticeable effects over time.

[thereza 0]

@rewolff I agree with the enclosure issue, but I plan on modifying it to allow for more air to flow over the FET. Also, that's when it's on full power. I would imagine the power output would drop significantly when the board came to temperature.

@thedudevt I can't tell how you've mounted your PCB and if you have glass over it or print directly onto it?

I've finally made a bit of progress with my build. I was surprised how easy polycarbonate is to work with -- i'm used to acrylic which melts when you try to drill, etc. Anyhow, I made the polycarb holder for the heating element. I overlapped the heater by 1" and followed the contour of the acrylic bed otherwise. Aligning the holes didn't go as well as I would have liked, and I ended up drilling some of the holes much larger and using washers to compensate.

I wasn't sure how the heating PCB was intended to be used. I think i mounted it upside down, but I didn't like all that lame 'keep away from children' crap written on the visible surface. Also, the pads to attach the power cables were way small and I used a knife to scratch off more of the soldermask to allow for a bigger soldering area.

The thermister is attached via katpon tape (not sure how others have done this). There is a short bit where the wires are also attached to the PCB, but I hope they can tolerate the heat (I will find out shortly).

I also have the wires simply running down into a hole. I don't expect them to get caught on anything, though the wire carrier looks good.

My current issue is that now my build surface area is reduced and I need to adjust the location of the limit switches so it doesn't bang into anything.

I've posted pictures here :

https://picasaweb.google.com/1129126825 ... PX36KvqwE#

 

[thedudevt 0]

@Thereza - good question, the pcb is mounted to the aluminum plate with four M3x6 SHC Screws. I removed 2.5mm of the threads under the head of the screws so the pcb can float with expansion. I used a lathe to do this, but a dremel would work fine too. There is a layer of kapton between as an electrical insulator just in case the pcb coating gets scratched, and a few pieces of kapton on the edges to keep the pcb from bowing too much. I haven't tested the max temp, but it should have no problem getting up to ABS temps eventually.

The thermistor is in between the pcb and the bed in a 2.5mm hole drilled in the center of the bed with a 1.3mm x 1.3mm slot running to the back edge for the wires.

I think I found the issue I've been having with the table height - the aluminum bed is not as thick as the original, so when I put it on, the springs were not as compressed. The spring force was therefore lower and the bed was sticking to the threads of the bed mounting screws. The nozzle would push it down, and the springs weren't pushing it all the way back up. I lowered the bed three turns or so and re-leveled it and now my parts are sticking correctly at 200c/60c. So far the tiny planetary set I'm printing

http://www.thingiverse.com/thing:23030

looks good. It's so small though, some of the parts are under the 5mm min retraction distance and have alot of stringing.

@destroyer2012

my PLA fan shroud is sagging, so yes, I'll have to order some ABS soon. Temps at the cable chain are around 34c, which should be find until I get my laser cutter finished and cut some enclosure panels.

Kyle

[thereza 0]

@thedudevt the only thing I don't like about your design is the large thermal mass the aluminum presents. It will take longer to heat and cool. I am thinking of adding a thin piece of aluminium to the top of my build (or glass) -- whichever I can source cheaper really..

I finished wiring the board into the ultimaker - though I've not tested it yet (I will do so in stages), but I'm confident that it should work.

I just added another 4 photos to the same album as my last post (

https://picasaweb.google.com/1129126825 ... PX36KvqwE#

) showing the power supply hookup and thermistor hookup.

[thedudevt 0]

yes, I agree, .25in (6.3mm) aluminum is probably thicker than necessary, but it's a much better thermal conductor than glass, so I shouldn't have any problems with cold corners. It does take a while to heat up, but it's usually hot by the time Cura is finished slicing, so it's not usually a waste of time for me. I also have been pulling parts off the bed as soon as they are done and not waiting for the bed to cool down. You have to be careful not to flex them, they're so soft they can warp easily. Also, I haven't lost any work area, and I haven't made any permanent changes to the frame, so I can swap out the heated bed for the original bed quickly (now that I have a dial indicator holder to speed up leveling).

OTOH, plate glass is way cheaper. Ultimately, I'm not advocating everyone do it this way, if I didn't have access to a mill to make the plate, it would have made more sense to go with glass. There are advantages though, and someday when I get this thing dialed enough to print 200x200 plates full of small ABS parts, I might be glad to have the aluminum on there.

I like how you've mounted your PS under. I wish I could mount all the supplies under. Maybe when I get my laser built I'll design a riser to stuff the power supplies into. That and one of those retractable USB cables they sell in the laptop accessory section, and it would clean up a big portion of the rats nest of cables on my bench. Maybe a drawer for tools too.

Kyle

[jenschr 6]

@thereza I'm curious to hear why you enlarged the soldering points on the PCB?

J