MK2A 300mm Bed Question

I ordered a MK2A 300mm bed off of Aliexpress and came up with a few questions after receiving.

1. They sold both 12 and 24V, I asked for a 24V and I am wondering if I can verify it is actually 24V somehow (there are no markings indicating this). Or did I get scammed and these are 12V and you can run 24V through them although not rated for that?
2. There are also no markings on the solder pads. Does it matter which is + and which is -.
3. If I want to put surface mount LED's on the bed, is there a size/model number that I need?

Similar bed on Ali...

[www.aliexpress.com]

Sorry for the noob questions but there doesn't seem to be much info on this bed as I searched around.

Thank you.

Edited 3 time(s). Last edit at 04/25/2019 08:55PM by heatvent.

if they are really 12v or 24v they have 3 power connections.
But it looks like they make separate pcbs for each. (the only difference is the resistance of the tracks)

polarity for the bed itself doesn't matter,
They uses two leds in opposite polarity so that one of them will turn on no matter which way the bed is plugged in.

I'd try to measure the resistance and calculate what current and wattage will result in connecting it to 24V and see if the PSU will be able to handle it. If not, you can use a PWM maximum limit to limit the used power into the bed.

ie
Current:
I = U/R (where I is current, U is voltage and R is Ohms of the bed)

Power:
P = I^2R
or
P = UI

So if you measure the bed to be 3 ohms:
I = 24/3 = 8A
P = 8*8*3 = 192W
or
P = 24*8 = 192W

If you use a 350W psu, that should be within the ballpark. Other electronics probably use about 2-3A.

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Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Thanks for the information, I will get out the multimeter and give it a try.

Re: the LED's any specific type will work. I assume these are some type of onboard PCB type but not sure of what I am looking for.

Thanks

So my multimeter is reading 2.2 ohms of resistance. So using your formula(s):

(24 * 24)/2 = ~260W

Does this make sense for a 24V bed?

It depends on the accuracy of the meter. Did you check the meter without resistor, ie short the test leads? The resistance measured shorted must be subtracted from the measurement of the bed.

If the resistance is correct then 260W sounds about right for 24V and 2.2 ohm. If you have a ammeter clamp you can meadure the actual current. The calculated current is at room temperature, the current drops when heated. My bed heated at 8A 24V initially at room temperature, about 21 degrees C, then dropped to about 5A at 100 degrees C. Then when the pwm kicks in drops even further.

When you use a temperature sensor with a controller it won't overheat, it will just reach the temperature quicker. Care must be taken in the case of a failure, the temperature can rise dangerously high melting and burning components around it. Make sure proper protections are in place.

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--
Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Thanks, when you say proper protections, I use an MKS board, external mosfet and Marlin (thermal runaway protection). Is there something else that should be added?

Thank you

Yep thatshould do the trick, and maybe a smoke detector?

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--
Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Hi, so this has been sitting on the back burner a while. I finally rebuilt my printer and the bed is struggling to get to 50 C with nothing else running. The bed is connected directly to the power supply with 14AWG wire through a standard external mosfet that you see everyone using (I believe these are rated up to 50V). The power supply is a cheap Chinese one with model number S-400-24. It is rated at 400W and 17A. Looks like the bed is using 260 of those watts and 11 of the amps but I would assume should have enough power unless there is something internal that throttles the power supply. The bed is a MK2a style and is fairly big (328x328 total surface although I think its 300x300 heated).

Any idea why this may be struggling? As noted in my first post there is no marking that this is a 24V bed ... is it possible it is 12V?

I guess I would expect to see the opposite (i.e. smoking and catching on fire) if that were the case but I am somewhat illiterate to all things related to electricity. Also, using the seller specs (below) the 12V version should be ~.5 ohms and the 24V should be ~2.5 ohms so that would point towards this being a 24V bed? I measured my old Anet A8 bed and it measured 1.8 ohms.

If I measure the resistance of my multimeter leads it reads 0.7 ohms. I have seen that you should subtract this from what you read at the bed and that the 24V is 2x the 12V resistance. So if I take 0.7 from 1.8 my Anet bed looks right for a 12V rated bed. If I take 0.7 from 2.2 I get 1.5 ohms / 2 and 0.75 ohms on the 24V bed. That seems low (like this should be able to heat up).

I also measured the voltage at the mosfet.. I get about 23.5V which seems fine to me.

Hopefully this makes sense to anyone knowledgeable of heated beds/electricity. Any further help would be appreciated.

Thanks!

Here are the details of the bed from the seller...

MK2A MK3 300 * 300mm Heatbed Hot Bed 3d Printer Aluminum Heatbed


Heat Size:: 300*300mm

Bed Size:328*328mm

Thickness: 3mm

Material: Aluminum

Voltage 1 : 12V Power 1: 300W
Voltage 1 : 24V Power 1: 230W

​Please tell us which power do you want in the order, thanks! otherwise we will send as default.



Edited 3 time(s). Last edit at 01/26/2021 07:15PM by heatvent.

if it was a 12v and you applied 24v it would a) draw 4 times the current, b) heat up really fast!

Check your voltages on the bed terminals on the bed itself, make sure it actually reads 24v anything less and you have losses in your cables. (ie the cables are warming up not the bed)

Edited 1 time(s). Last edit at 01/26/2021 07:16PM by Dust.

Cables are cold and I am getting 23.5v at the mosfet terminals (power and bed side) and 23.3v where the wires are soldered to the bed. I would assume the small loss in voltage is OK.


I did some more testing and instead of trying to get to 60C I set the bed to 100C and it got to 60, then 70 and so on. I think this has to do with my PID settings. I had turned on PID for the bed and when I went to tune, I think I mistyped and set it to tune at 90C. I don't recall issues getting to 90C but didn't realize I did the wrong temp until later and looked where I copied the command from. I then tried to start doing test prints after doing more setup and the temp was stalling at around 48-50C and Marlin would halt and ask to be reset due to possible runaway / thermistor not working (BTW, according to my infared thermometer the thermistor appears to work correctly).

So I am going to try to get my PID settings again or turn off PID for the bed and will see if that helps. Good to hear I have the 24V bed though.

Edited 2 time(s). Last edit at 01/26/2021 09:03PM by heatvent.

IT'S ALIVE!!!!!!

Thanks for all the help ... ruling out things that were working helped me get to a good place on this. I was starting to think the bed was crap and was thinking if I need to order a different one probably just due to my pessimism on dealing with cheap items off of AliExpress.

The printer is currently printing a test cube and is holding a solid temp on the bed. I manually fluxed the PID settings based on the wiki guidance until it held a steady bed temp (goes between 59.9 and 60.1). Seems odd because the settings seem far off from what autotune calculated.

I actually can't believe this thing is running. It has been partially taken apart for about a year. I was having severe under-extrusion issues that I thought was due to a clogged nozzle or failing heating element, etc. Having the under-extrusion issues with a new hot-end setup clearly pointed to the extruder. The issue turned out to be a mistake I made mounting the titan-style clone extruder as well as my steps / mm were for some reason WAY off.

Thanks again for the help! It's good to see this thing finally working ... now just a bunch of cable management to tidy things up.