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PostPosted: Sat Aug 24, 2013 11:31 am 
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Hello,

It takes a long time to heat up the bed to 60°, and 120° for ABS will be much longer later.

I have a 12 V, 12.5 A power supply here, is it possible to heat the bed with this power supply but still control the temperature with the original board and the repetier software?

Thanks!

Flo


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PostPosted: Sat Aug 24, 2013 3:47 pm 
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Location: Lint Belgium
At the moment i have 24v on my bed, switched with a 12v relais, connected on the original board. Reach 100 degrees now. Forget 12v, not enough, velleman ps is already 15v.


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PostPosted: Sat Aug 24, 2013 4:20 pm 
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Thanks for your answer.

How do you switch the relais? Directly with the original power cables for the bed? How high is the current at 24V?


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PostPosted: Sat Aug 24, 2013 4:58 pm 
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The bed has a resistance (R) about 6 Ohm.
The power supply has a output-voltage (U) of 14.7 V
Following Ohm's law (U = R x I --> I = U / R) there is a current (I) of 2.4A
So with 24V you have to expect 4A
No idea if the pcb-tracks are designed for this current ...


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PostPosted: Sat Aug 24, 2013 6:56 pm 
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Location: Lint Belgium
At the moment, i just use the output of the board to drive the relais. (Only for the bed!) so i can regulate the temperature like before.
And the 24v is connected through the relais to the bed. Works fine with a glass plate and kapton tape.
If i kill the bed die to overheat, i already have a mk2a 12v 1.5ohms ready.


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PostPosted: Sat Aug 24, 2013 8:23 pm 
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Thanks at all!

@RudydG: I think I will do the same :)


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PostPosted: Sun Aug 25, 2013 11:35 am 
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Hi,

It is not clear for me how to connect the heatbed (maybe mk2 or something like that). I think a normal relais is not the best solution. Its better to use a mosfet.
But I don't have any experience with mosfets.

I think I need something like a IRFP460.

Anybody who knows how to connect that?

Thanks

Flo


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PostPosted: Mon Aug 26, 2013 4:34 pm 
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I would also like to have bit more reserves, in particular when needed cooling with the fan. Connection with a MOSFET could be done as follow. I dont know if the IRFP460 is appropriate, since it has a resistance of 0.29Ohm, so at 5A current it has to handle 7.25W. I would prefer the IRLR8743PbF with 3.1mOhm.

But before I apply more power I would like to know from Velleman if the PCB heat bed can handle 24V.

Image


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PostPosted: Mon Aug 26, 2013 5:19 pm 
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hoow did you make such an electronics schematics?


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PostPosted: Mon Aug 26, 2013 5:56 pm 
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Hi Edirol,

Thanks so much, exactly what I need :)

I ordered a mk2a heatbed now, because I have the 12V 12.5A power supply here. The mk2a has a resistance of 1 - 1.2 Ohm.

Are the resistor values and the IRLR8743PbF suitable for that too?

Thank you!


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PostPosted: Mon Aug 26, 2013 6:19 pm 
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Based on the original head bed (assuming 15V/2.4A, did not measure) has 36W, with 12V 1Ohm 12A you would have 144W (!). The IRLR8743 with 3.1mOhm would handle 12A*12A*3.1mOhm = ~0.5W sould not be a problem. I would
though use heatsink.

I use Eagle to draw schematic: http://www.cadsoft.de


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PostPosted: Mon Aug 26, 2013 6:58 pm 
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Quote:
But before I apply more power I would like to know from Velleman if the PCB heat bed can handle 24V.


Well: What could happen?

The bed is a resistor, designed to produce heat. If driven with a larger voltage, it gets hotter.
Given 15V/2.4A it has a resistance of 6.25 Ω.

When 24V are applied, the resistance results in a current of 3.84A, which in turn results in roughly 92W, about 150% more than the original design.

Since there's a PTC sitting right in the middle of the business, any damage to the hotbed could only happen if the power input to the hotbet would heat it so fast that the controller would be unabled to turn off the power fast enough to prevent overheating.

I severely doubt that. To overheat an area of 400 cm² within a few seconds, you'd need much more power.

It might be advisable to replace the heatbed wiring from four parallel ribbon cable conductors to something more substantial, but otherwise only the ribbon cable would get warm.

With just 4A, not that very much.


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PostPosted: Tue Aug 27, 2013 8:59 pm 
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Quote:
Well: What could happen?
The bed is a resistor, designed to produce heat


Acually your are right. So I build a prototype of a 24V MOSFET controller, but to completely decouple the extra voltage from the controller board, I changed the electronic design and use now a optocoupler. Now I can keep the bed at 60° even with 2 fans at 100% speed ;-). 3.9A@24V=94W

Image
(With 12V input will you have to change R2 and R3 to 1.2k !)

Edit: The MOS-FET can also be the IRLU8743PbF (I-Pak version).

Edit2: Note that schematic signs and hardware pinouts generally do not correspond! This may be a trap for less experienced users. If you have built this circuit and you get a "alway on" output, check wiring and pin numbering of the optocoupler.


Last edited by edirol on Tue Apr 01, 2014 8:41 pm, edited 4 times in total.

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PostPosted: Tue Aug 27, 2013 9:13 pm 
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The next question is: How would the controller fare if connected to 24V instead of 15V?

Let's look at the schematics: http://reprap.org/wiki/File:Schema_base.jpg

The linear voltage regulator (7805), responsible for generating the 5V needed by the AVR microcontroller would get slightly warmer, but still is within specs.

The stepper drivers can handle voltages up to 35V, so they should be within specs.

What remains are the power mosfets for both heaters and the fan -- which by the way already is outside specs by the original design, supplying it with 15V instead of rated 12V.

Did I miss something crucial?


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PostPosted: Tue Aug 27, 2013 9:22 pm 
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clm wrote:
Did I miss something crucial?


Yes you did. The extra power comes from a additional power supply. This way, the k8200 power supply will get relieved from heating the bed.


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