heat bed issues

Hello,

I am just building my first custom sourced printer and am having trouble with my mk3 alu dual heatbed. It takes forever to heat up to and past 90C and then plateaus at 102C. I need 110C for abs. I am using an led power supply 12V 29A that's adjusted to 12.85V but drops to 12V under load. My heatbed resistance is 1.2 Ohm but the terminals on my heatbed only draw 10.5V when heating up. There is no glass on my bed, just kapton tape since it's aluminum and i'm using an inductive sensor. Could anyone kindly help me resolve this issue ?

Thanks,

update 2: re-soldered gnd wire of heatbed and now temp is plateauing at 90C !

This has been my experience with DC based heaters. I have never been able to get them up to the specified max temperatures. All of the ones I have used were rated with wattage values greater than actual. I have given up on DC heaters and now just us AC based ones controlled with a SSR.

If you have 12V at the supply under load and 10.2V at the heatbed terminals you have your answer:

- 12V measured at electronics input: too thin heatbed wires.
- 12V measured at supply terminals: to thin main power wires and maybe too thin bed wires

Update 3: As soon as printer is turned on and plugged in, bed begins to rise on it's own to 90~93C without me setting any temp in pronterface. When I set the temp to 0C to cool down, the temp refuses to budge and stays in the 90~93C interval range. What is happening with my bed ?

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blahhh10
Update 3: As soon as printer is turned on and plugged in, bed begins to rise on it's own to 90~93C without me setting any temp in pronterface. When I set the temp to 0C to cool down, the temp refuses to budge and stays in the 90~93C interval range. What is happening with my bed ?

You may have blown your mosfet.

Probably need to look at your wiring scheme. AFAIK
If you try and draw the bed current through the standard controller you will probably blow it - As ElmoC point out. Typically you don't want to go over 8 amps. With a 1.2ohm bed you will draw 10amps.
If the driver is blown permanently on you will have the bed permanently on.
So you need to drive the heater directly from the PSU. I think these actually are double PSUs so there Is 2 x 12v outputs. So you should be able to use one to the controller and one to the heated bed. You would have to control the heated bed by a separate driver/relay from the control board.
You say dual heat bed! Does that means its a bigger heat bed than the standard 200x200 ? Standard Ally bed need about 120 watts to get to 110C. Your bed is 10Ax12V=120W so OK if its a standard size. If it is double size (200x400) then you need 24v.

Thanks guys. ElmoC, I did some reading and it said that it would be better to use the mosfet of a controller board with pid for the bed instead of a ssr for temperature control. I don't know very much about the advantages or disadvantages as am a newbie with the bed. You said you struggled with dc heaters to get them up to their max. temps but the max temp for my bed is 180 C so I don't know why 110 C should be an issue for these beds ?

MCcarman, when I said dual heated bed I meant it can run on 12V or 24V depending on which soldering pads you choose.

I decided to order a good quality ramps 1.4, since the old was a cheap chinese one. I got this one [www.tindie.com]. I going to try to crank steady voltage up to 13.5 V and use rubber pipe insulation tape and see if it gets to 100/110 C quicker

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blahhh10
You said you struggled with dc heaters to get them up to their max. temps but the max temp for my bed is 180 C so I don't know why 110 C should be an issue for these beds ?

Have you ever gotten your bed up to 180 C? My beds were rated that high too, but I couldn't get them to 90 C.

I'm not sure what you are saying with your mosfet/pid/ssr statement. I control a DC/AC SSR with the mosfet on my controller board (the one rated for a large heated bed) with PID with no problems.

Yeah, I'm struggling as well to break 90C but I was trying to wonder why that with a max temp of 180 should 110 be such a struggle ? I'm using an mk2b with and aluminium sheet on top. What do you suggest I do to get the temp up to 110C ? Also I was reading that although the arduino has a max input voltage of 6-18V, anything over 12V might fry the voltage regulator. If that's the case how come so many people are running their ramps + arduino combo at 14V ? What voltage are you running at ?

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blahhh10
Yeah, I'm struggling as well to break 90C but I was trying to wonder why that with a max temp of 180 should 110 be such a struggle ? I'm using an mk2b with and aluminium sheet on top. What do you suggest I do to get the temp up to 110C ? Also I was reading that although the arduino has a max input voltage of 6-18V, anything over 12V might fry the voltage regulator. If that's the case how come so many people are running their ramps + arduino combo at 14V ? What voltage are you running at ?

To achieve that 180C figure, the bed has to produce a certain wattage. To get that wattage, the board has to have a certain resistance. The Wiki for the MK2B says it should have between 1.0 and 1.2 ohms on the 12V connection, or 3 to 3.4 on the 24V connection. This means it would produce between 120-144W on 12V and 169-192W on 24V (which makes no sense given it is basically two resistors in series or parallel and should produce about the same wattage). On all the boards I have had, the 12V connections has always been close to 2 ohms which produces only 72W. The 24V has been around 5.5 ohms for around 104W. So none of these boards produce close to the proper wattage to achieve the higher temperature. A common trick that is done is to run 24V on the 12V connection. That would produce the needed wattage, but even doing so, I had problems getting them over 100C. So the problem that I was seeing was the quality control on these units is not there. They don't meet the stated specs so that is why they don't generate the temperatures advertised.

I don't run a RAMPS board anymore. I do have a Printrboard (Printrbot) that I run at 12V. On my other printer, I run a Duet 0.8.5 at 24V. On both of these printers I use A/C powered heaters (400W on the Printrbot and 600W on the other). I can get well over 110C with both of these. While there is a danger when using A/C power, if you follow proper safety procedures, it is not an issue. Both of these heaters are controlled with a DC/AC SSR that is controlled with the heated bed connector on the controller.

In my experience, A/C heaters are the only way to go if you want the higher temperatures.

Edited 1 time(s). Last edit at 06/11/2016 11:49AM by ElmoC.