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Bed heating power - why don't they get to temperature quicker.

Posted by MCcarman 
Bed heating power - why don't they get to temperature quicker.
December 30, 2020 08:30AM
Sharing some info.
Maybe some one can spot some improvements.
Theres is often a complaint that beds take a long time to heat up or wont reach 100C.
I have a workaround i have used for some years so don't have an issue with this. But over the break i fitted a volt/ameter to my printer and found the results interesting.
Big surprise to me was that the current for the total system dropped by over 1 Amp during bed heating. I haven't run the hot end long enough to characterise it.
So starting about 18C the system pulled 0.19 amps without heaters. Voltage dropped from 12.1 to 11.8 so i could tweak the PSU a bit. Without my heating fix and with no cover on the bed.
Bed on current was 7.6Amps.
Bed at 50 c was 6.95Amps.
Bed at 76 c was 6.48 Amps
Bed at 97 c was 6.25 Amps. Probably wouldn't get to 100 and had been on for about 35mins by now. Target temperature was actually 110C.
That current drop of about 1.3Amps at 11.8volts is about 15watts. Thats a significant amount given that most kit PSUs are about 120watts.
I thought there must be something over heating but fans on the PSU and the controller had no effect.
Eventually found the only way to get the current up was to cool the bed - obviously defeats the objective.
Checking the coefficient for copper this seems to be all due to the copper bed resistance changing over the 80C temp range. (Probably a biger change at the copper due to heat loss before the thermistor senses it)

One thing to take from this is that PSU performance may not be as critical as first assumed since as the bed "warms" the load on the PSU actually reduces.
I guess the other thing is that you need to start with a low resistance bed as the copper coef will be the same for all so if you can start with a lower resistance the change (drop) will be less.
Another thing i did note was that turning the bed and hot end on together when cold triped my meter. The meter is only rated to 10amps so it was over this. That also may mean the power was limited by the PSU. Often the PSUs are multi channel and rated at about 8Amps on each channel - so use both channels if possible. When the bed had warmed and the current droped then with the hot end on it was under 10amps. So good practice to get the bed warm before turning the hot end on or you may be stressing your PSU (and anything else in the circuit).
Re: Bed heating power - why don't they get to temperature quicker.
December 30, 2020 09:29AM
You're asking your printer to do something it was never intended to do. I know the marketing BS said the printer was "ABS compatible", but if you were trying to sell 3D printers, you'd have to say that too, even if it wasn't true.

There's physics: [jscalc.io]

And there's power input... Unless the bed is very small, 12V power is a lousy way to heat a bed. Most 12V bed heaters are underpowered for the size of bed they are attached to, and most printers that come with 12V bed heaters are not intended to print ABS which requires the higher bed temperature.

If you want to print ABS, you'll need to enclose the printer, probably replace the hot-end on the extruder, and get a higher powered bed heater, then replace the power supply with one that can drive the new heater, then replace the controller when it fries from trying to power the higher powered bed heater. In other words, you'll have to rebuild the machine almost completely, and it is likely to cost more than the printer did in the first place.

Sometimes you get what you pay for, but you absolutely never get what you don't pay for.

Edited 1 time(s). Last edit at 12/30/2020 09:34AM by the_digital_dentist.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Bed heating power - why don't they get to temperature quicker.
January 06, 2021 09:52AM
Had some more thoughts.
24v bed is no better as they are typically 2 x 12v circuits so it doubles the resistance as well as the voltage.
If you push the voltage up at the cold start you draw more current at the start and risk the electonics.
What you actually want is to increase the current as the bed heats.
So im wondering if the best solution is a 24V PSU with one of those step up/down modules thats set to constant 8amp output. That should start at about 12v but push it up as the bed heats (bed hotter = higher resistance = lower current demand [at constant voltage] = lower power). To maintain the current it will increase the voltageBy raising the voltage it will pull the current up and maintain the power.
The real point is as you imply there is a limit to this heating method and for practical purposes its a bit bellow where we would like it to be.

Just looking at my numbers. At 97C it was only pulling 74watts so a120 watt PSU is probably adequate you just need to compensate for the resistance change.

Its probably also relevant to why you need to redo the PID when you change bed temperature. Because the diferent bed temperature effectively means you hve a different bed power output when hot.

I cant remember if i had my chamber open or closed. The bed was not covered. Though i do have cork under the bed.
Re: Bed heating power - why don't they get to temperature quicker.
January 06, 2021 11:08AM
Quote
MCcarman
24v bed is no better as they are typically 2 x 12v circuits so it doubles the resistance as well as the voltage.
If you push the voltage up at the cold start you draw more current at the start and risk the electonics.
What you actually want is to increase the current as the bed heats.

24V is better if you're trying to heat the bed, because it will use 1/2 the current that a 12V heater of the same power rating will use. That means lighter, more flexible cables, less current heating up controller PCB traces, and less current heating up the MOSFET that switches power to the heater. Also, you can get much higher powered heaters for 24V than for 12V. Higher power means faster heat-up and often better regulation of temperature. If your controller is actually designed for 24V operation, the steppers will perform better on 24V, too.

What you really want is for everything- the power supply, the controller, and the heater, to all be working well within their specs instead of right at the limits. The bed heater should be capable of reaching the target value quickly. If it takes 1/2 hour to reach print temperature the heater is underpowered and the controller won't be able to regulate the bed temperature very well. The links provided in my previous post can help decide how much heater power you need. Once you know that, you can start selecting the heater and power supply to drive it. If the heater is rated for 500W at 24V, don't buy a 500W power supply. Buy a 600W+ power supply so you don't have to worry about cooking the supply. Make sure your controller can handle the voltage/current. If it can't handle the current needed, use an SSR to switch power to the heater.

There's absolutely no reason that you have to run the bed heater from a regulated DC power supply. That means you don't have to spend a pile of money on a big, industrial power supply that sounds like a vacuum cleaner. You can power the heater from AC (either low or line voltage) or unregulated DC. I prefer line power switched with an SSR for its performance, simplicity, and low cost. You won't need any big power supplies for that- the rest of the printer will continue to run on low voltage DC (24V is better than 12V for driving steppers) and your controller that's barely spec'd to switch power to a 12V bed heater will happily drive the SSR that switches line power to the bed heater. You can get crazy high power heaters for line voltage operation so you can get the bed up to temperature quickly, even if it is a high temperature for printing ABS or PC, you won't need to insulate the underside of the bed to do it (though you may want to anyway to protect things located under the bed from the heat), and the controller will be able to regulate the bed temperature.

My printer has a 750W line powered heater that heats the uninsulated, 300x300x8mm cast aluminum bed plate to 105C for printing ABS in about 4 minutes. The controller is able to regulate the temperature quite well because it isn't struggling to reach an operating temperature that is at the heater's limit. The rest of the printer is powered by a 24V 200W fanless power supply that has plenty of power to run the controller/motors/hot-end, etc. plus about 20W of LED lighting so I can see and photograph prints.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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