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Heat bed Mk3

Posted by jsluke 
Heat bed Mk3
November 22, 2013 11:06AM
I have a PCB Heatbed Mk3 (photos below):

Am I correct in saying that I don't need a glass plate on top of the PCB Heatbed and that I print straight onto the Heatbed?

Also, which way up should the Heatbed be mounted?



Edited 1 time(s). Last edit at 11/22/2013 11:14AM by jsluke.
Re: Heat bed Mk3
November 22, 2013 12:36PM
Side 1 goes down.

Like many things, there appear to be multiple things called a MK3. The one you have is not the aluminum core version of the board. It appears to be made like a very normal MK1 or MK2. If it was a aluminum version, your side 2 picture would simply be a piece of aluminum sheet.

Do not print directly on that board, use glass with it.
Re: Heat bed Mk3
November 30, 2013 06:11AM
Thanks Uncle Bob

I'm now trying to figure out exactly what I have bought (it was an early purchase in my 3D printer journey so please excuse my innocence). The URL on the board says [www.reprap.org] but that page no longer appears to exist on the wiki.

Examining the board more closely raises a number of questions =>

Any help / advice would be greatly appreciated.

Many thanks.

Re: Heat bed Mk3
November 30, 2013 09:50AM
There's not a lot of control over what things get called. That obviously is a "next generation" version of the standard MK1 and MK2 boards.

I'd grab the ohm meter and see what things measure. If the magic beast on the board ohms out to 100K it's probably a thermistor. If it goes down in resistance as you heat it up (hair dryer ...) it's a thermistor. The gotcha is - which thermistor with what curve? The only people who would know are the ones who made the board.

I see no real value to LED's on heated beds. They face down so you can't see them blink. I'd just ignore it and move on. If you want to use it, hook up a 5V supply and see which way it lights up. You won't hurt anything with that low a voltage.

I would ohm out the heating element. It should be around an ohm. If it's close, hook up a 12V supply and amp meter to see if it really can pull 10 to 12A. Hooking it up for a minute or so will not hurt anything. There have been reports of people getting 2 ohm heated beds. That may be (but probably isn't) ok for 24V. It's not going to work for 12V.

The board does not look like a 12 / 24V configurable board, but that could be just the pictures. If it is configurable, you will have a heater trace with two segments. They go in parallel for 12V and in series for 24V. They would measure 4 ohms on the 24V "taps" and 1 ohm on the 12V taps.
Re: Heat bed Mk3
December 02, 2013 07:20AM
Thanks Uncle Bob.

I put an ohm meter on the small device and it measured 111 ohms - I then heated it with a hair dryer and reading dropped to 35 ohms.

I also put an ohm meter on the two pads that I believe are the power connection for the element and the reading was 0.8 ohms. That was without any resistor on the device.

Before I hook up to a 12V supply should I solder the resistor across the R2 contacts?

I also noticed an email address on the board (which I should have seen earlier) so I've requested a wiring diagram.


Re: Heat bed Mk3
December 02, 2013 08:44PM
If the thermistor on the bed really is 100 ohms (and not 100K ohms) I'd just ignore it. Mount a glass bead and move on. The center hole may be for 5 point bed leveling. No real need to put the thermistor in the hole.
Re: Heat bed Mk3
December 03, 2013 08:34AM
Sorry Uncle Bob - I mis-typed - it is 100 KOhms.

Would that change which side you would mount upwards? You previously advised that side 1 (the side with the surface mounted thermistor) should go down .... isn't it better to have the thermistor in contact with the glass print plate?


Re: Heat bed Mk3
December 03, 2013 08:56PM
The glass plate is flat, the heated bed is also flat. Keeping the flat surfaces together is what gives you good heat transfer. Any air gap will degrade the heating quite a bit.

Even with the thermistor being 100K, you still need to know it's beta and which table it goes with. Without that data ( or measurements to create that data) it's not worth messing with.
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