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LDO kits and 0.4W/cm²

Yaaj

Member
Rule of thumb = 0.4W/cm²
But also read several times that this rule is obsolete, but was unable to find any answer.

For LDO 0.2 and 2.4 kits, it means max_power: 60%

On my LDO 0.2, I was so tired with the heat soak taking ages that I bumped the power to 80%. No problems so far, the machine is rock steady, 3 months without an issue or any offset tuning (ABS only). I send a file, and don't even verify the first layer.

Finalizing a LDO 2.4/350. The bed also takes ages to heat up...

The LDO beds are thicker (11-12mm vs 8mm). Shouldn't this power calculations be based on bed volume ?
If the rule of thumb is obsolete, what is the new one ?
 
Assuming that you have a perfect PID controller I don't see why you'd need to limit the power of the heater. PID will adjust the heater power based on the difference between the measured and commanded temperature. But is the system "perfect"?

What if there is a delay or lag in the measurement? A too-powerful heater could overrun the PID setpoint before the next measurement cycle. I don't know what the PID control loop cycle time is. or if there is a low pass filter on the measurements. At some point if ther heater is too large the control is unstable. but I doubt we are even close to this point.

Having a thicker plate would slow down the rate of temperature change and allow a larger heater.

35 squared is 1225. 0.6W/mm^2 is 735. I think there are some people using 750W heaters

The next issue is power consumption. It costs real money to run a heater. I think a good solution to heat soak time is double-glazing the panels. and using rigid foam insulation on the not-clear panels. A 350mm printer has a LOT of panel area and the heat goes right on through 3mm PC. Double PC panels would drastically cut down the power used and shorten the heating time.
 
Yes, the LDO heater for the 2.4/350 is 750W.

If I understand correctly, it's about heating slowly so heat gently spreads, avoiding the taco that appears if the top is much cooler than the bottom, and eliminating from the PID algorithm the parasitic delay between heating the bottom, and measuring the top temperature. On Voron kits beds, it seems the thermistor now is screwed into the bed, close to the top, and not sticked to the bottom (old school borosilicate beds or thin alu beds).
This is how I understand it...

BTW, definitely not good at PID tuning ! Tried that a while back, with a Arduino, by hand, for a hotend DIY rework station, and it was a disaster ! Ended with bang bang !

Power consumption is not an issue. Whatever the power is, in the end we pay for the energy ; will remain the same, only duration will vary ; in EU we have 230 to 240V, and any wall outlet can deliver 16 or 20A (2.5mm² wiring, at worst 3700W on a resistive load). [EDIT] Electricity is rather cheap, and during winter the heat is not wasted, it participates to the house heating (it's another story during summer !).

Decided to test... Today, three tests with 0% (ambient temp...), 60% (450W) and 100% (750W). Will do the same at 100°C, but it has to be done from ambient to working temperature : tomorrow ! Takes ages to cool down, and the 6 bed fans (2 on each side + the Nevermore v5) don't help much ! (of course, after PID_CALIBRATE). Tested from ambient (~20°C to 60°C)

The first 3 tests. After some time, 15 to 30mn. The bed is open air (worst case I guess : larger delta T, and we eliminate the bowing gantry). The probe is a Biqu Microprobe. 0.003mm std dev, 3 samples, sample tolerance = 0.01mm, and 16 microsteps.

At ambient temp, after QGL :

bed cold.PNG

At 60°C, stabilized, 60% power, pid_Kp=42.532 pid_Ki=2.923 pid_Kd=154.710 :

bed 60°C 60%.PNG

At 60°C, stabilized after 30mn, 100% power, pid_kp=61.498 pid_ki=7.069 pid_kd=133.758
heats up much quicker !

1707347538545.png

At 60°C, I see no reason not to run the bed heater at 100% . The difference is not siginificant.

Have to let it cool down overnight, and will test at 100°C.
 
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My understanding is that the limiting factor on power is the current through the SSR - max 4A (you can raise tha tif you can deal with the heat incurred). Given that Power = Volts * Amps, then a given bed heater (say 750W) / operating voltage (240 in the UK/EU, 120 in the US) means either a 3A or 6A current draw for the power. For US sockets, that means making sure you de-rate power to not exceed the SSR's margins (120V *4A = 480W. 480/750 = 0.64% max power). The other side is heat loss through the walls, so you need to account for surface area (cm sq) and temperature difference to make the calculation. At some point your thermal loss will exceed max power from your heater, so your chamber temperature will suffer. Bed thermal inertia is mass related (volume and density) and only impacts heat up time, rather than overall losses
 
Makes sense...

Didn't think of this because the SSR is bolted to a huge alu plate (300x350mm), as well as all the electronics (in a drawer, as a ground plane and a heatsink)... As a result, 20A max, and that's it (large heat sink, low thermal resistance, everything stays cool)
But it is interesting...
The SSR that comes with the LDO kit is a Omron G3NB-220B.
According to the datasheet, it is rated for 4A free air up to 40°C ("approved safety standard without heatsink" ; weird, it is 4A from -30°C to +40°C, don't understand why this plateau). But there's some room for more watts with 230V... Not that much, but some, if the fans do their job at maintaining a reasonable temperature. Luckily, 230V here !
Also according to the datasheet, it has to be derated to 3A @ 50 to 60°C if no heatsink, 2A @80°C. Again, happy to have 230V. (even a bit more, close to the grid transformers)

A real problem for north americans... But read somewhere you can get 240 between 2 phases for your ovens and for space heaters ? 2-phase is very rare in Europe, but exists in some places. Do you take advantage of it for printers ? (here, 230V 1-phase+N or 400V 3-phase+N)

For now the panels are not on the printer... I installed several thermistors to see what happens in the chamber, in various places, using or not the bed fans. Planning this since the very beginning. But for now it is open air, under testing and tuning, extruding PLA.

I plan to reuse the insulation materials I've been using at some point for the bed slinger that printed the Vorons. Even had to add a polar blanket in order to reach 60-65°C last summer. (ghetto rig) Polar blankets are *very* efficient, they gave the 10 missing degrees (with 30°C ambient at this time + active heating I will not describe, I read the rules !)
Also plan isolation for the deck : fearing some deformation.

For now, will trust Omron. But didn't trust the knockoff SSR when I built the bed slinger controller : I pushed the required amps into the SSR using the lab PSU (making a short), sticked a thermocouple to its back, and verified it wasn't overheating. And BTW relearned that a SSR stays ON with DC ; at first thought I fried it 🤣
 

The LDO V2.4R2 350 "Rev C" kit I'm assembling now actually came with a 10A-rated Omron G3NB-210B-1...

That is interesting, as the "Rev C" wiring assembly guide photos show the 20A G3NB-220B-1.
The "Rev A/B" assembly photos show the 10A G3NB-220B-1.

Of course the LDO instructions are universal for the 300 and 350 sizes... but if they used different SSRs for the different bed sizes, I would have expected the larger SSR with the 350 kit.
I can't see that the specific SSR part number has ever been listed in the LDO BOM, only that it is an Omron brand SSR.
 
It is in the Voron sourcing guide : https://vorondesign.com/sourcing_guide

For the 2.4 :

Omron G3NA-210B-DC5 (10A)
Crydom D2425 (25A)
Crydom 84134000 (10A)
Panasonic AQA211VL (15A)

And for the 1.6, 1.8, 2.2 and Trident :

Omron G3NA-210B-DC5 (10A)
Crydom D2425 (25A)
Crydom 84134000 (10A) SSR

No idea why they recommand these, and the Panasonic is in the 2.4 BOM only. Maybe they only list the parts that were used and tested good by team members while building the prototypes ?

LDO probably are buying whatever batch they can get from the brokers at a good price at the moment they prepare the kits...
G3NA vs G3NB : the differences are explained in the G3NB datasheet, page 6 (surge tolerance)
 
Tests at 100°C, power at 60%, then 100% ; QUAD_GANTRY_LEVELING and BED_MESH_CALIBRATE after 30mn, open air, starting from 20°C :

Ambient :

bed cold.PNG
60%
bed 100°C 60%.PNG
100%
bed 100°C 100%.PNG

No measurable difference between 60% and 100%, 60°C or 100°C. (I don't consider a few hundredths as significant)
Will have to test again after 100 or 200 hours, as this was done with brand new magnetic mat and adhesives. No idea how they will behave over time (maybe some micro bubbling)

[EDIT] forgot to sllighly loosen three of the four screws that secure the bed on the extrusions ! Will redo all the tests with loosen screws.
 
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