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400W 250mm heat bed

Posted by Pippy 
400W 250mm heat bed
February 18, 2019 03:03PM
Hi

We're building our first 3D printer and thought someone out there might like to see a test setup we've just done with the bed.

The PSU is a russian (I think) KRV300, it alone will power the bed via a 210A external fet (3 milli-ohm). We set the PSU to 16V output, with a bed resistance of 0.6 ohm we had around 27A running through it, so around 430W. The PSU is beefy and fan-less and was very happy at that power, it just sat there nice and quiet and cold. The panel was warming the room nicely though.

Took about 10 mins to reach 170C, left it there for probably another 10mins, all is well it seems. We will be fitting some 10A 190C thermal fuses in line with each row of resistors (5 rows, 3 ohms each row across the PSU) thermally attached to the base of the aluminium bed just as an added safety measure.

The bed plate is 250mm * 250mm * 4mm with 3 point leveling. We'll have a 1mm SRBP sheet on the top for the prints to stick well too and a lite 1mm heat shield between the top bed heated plate and the lower runner plate under neath.

I guess this is an average amount of time to take to reach that temperature ?

Anyway, so far so good, it seems.

Chose to use cladded WW resistors because we heard that the silicon heaters can sometimes fail after a while .. is that true ? .. the resistors we've used should last a life time at the normal PLA/ABS running temperatures.
Attachments:
open | download - 250mm x 250mm 3 point bed assy 2.jpg (220.5 KB)
open | download - KRV300 13.8V PSU.jpg (250.4 KB)
open | download - 250mm x 250mm 3 point bed assy.jpg (245.4 KB)
Re: 400W 250mm heat bed
February 24, 2019 10:45AM
Interesting; currently looking for a custom solution to add thermal fuses as it seems most readily available solutions lack this. I was leaning towards a custom silicone pad, mostly to avoid holes in the heater plate.

Are you able to get a thermal image of the top of the bed to see how even the heat is distributed?
Re: 400W 250mm heat bed
February 24, 2019 12:41PM
Hello

We don't have a thermal imager or IR heat sensor but we could run a thermister over the top of it (with some thermal compound) to gets a mesh like temperature profile. Will post the result afterwards.

If we were to start again, we'd glue the resistors onto the under side of the aluminium plate using thermal silicon glue, then place a blank fiberglass PCB under the resistors to sandwitch them to the aluminium plate rather than drill all those holes again. It would be much easier that way. Might still do so, will see.

We are going over all the screw heads on the top side with thermal glue to create a nice smooth finish. Will post some more pictures of it as we move along. Just waiting on various other parts at the moment.

A silicon heat pad would certainly be the easiest way !

Edited 1 time(s). Last edit at 02/24/2019 12:44PM by Pippy.
Re: 400W 250mm heat bed
February 24, 2019 12:49PM
Quote

Took about 10 mins to reach 170C, left it there for probably another 10mins, all is well it seems.
...
Chose to use cladded WW resistors because we heard that the silicon heaters can sometimes fail after a while .. is that true ?

What are you printing that needs a bed temperature of 170C?

Silicone heaters are reliable at the more common bed temperatures of 50C to 120C. I've no idea how reliable they are at 170C. The rated temperature is typically 250C to 300C.

Edited 1 time(s). Last edit at 02/24/2019 12:50PM by dc42.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: 400W 250mm heat bed
February 24, 2019 01:49PM
Quote
dc42
What are you printing that needs a bed temperature of 170C?

Silicone heaters are reliable at the more common bed temperatures of 50C to 120C. I've no idea how reliable they are at 170C. The rated temperature is typically 250C to 300C.

We don't need 170C, it was just a first switch on test to see how it heated up is all. At least we know we have spare capacity to easily maintain a more normal temperature range, but the reason for wanting more than average heating power was mainly for quick heat up times .. not unlike pushing over 2kW into our kettles just for that much needed cuppa winking smiley
Re: 400W 250mm heat bed
February 24, 2019 03:34PM
Quote
Pippy
A silicon heat pad would certainly be the easiest way !

Will see about that when I get the parts and set about to try to make one! Still a few open questions, but I'll make my own thread when I get the parts and start making a mess smiling smiley

Looking forward to seeing how this turns out!
Re: 400W 250mm heat bed
March 12, 2019 12:03PM
Just did a quick first PID auto tune of the bed. Looks as though it's just right really.

16V'ish bed voltage, Marlin PWM'ing the external 210Amp FET switching the bed on/off.

Have embedded 4 * 100k B3950 thermistors into the base of the bed, silicon thermal rubber glued into holes. The 4 thermistors are equally spaced to ensure a good average temperature reading of the 4mm aluminium bed plate.

The tuned PID produces a slight over-shoot as it approaches final temperature, but that's normal for PID. Increasing the 'D' part will calm that down (as per animated GIF).

Edited 1 time(s). Last edit at 03/12/2019 12:44PM by Pippy.
Attachments:
open | download - First_Bed_PID_AutoTune_120C.jpg (75.1 KB)
open | download - PID_Compensation_Animated.gif (401.4 KB)
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