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  1. #11
    Technician
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    Quote Originally Posted by Mjolinor View Post
    Wow, I didn't realise they used 12 volt for the head heater, why does it not use mains (230 in my case) or 110. I can see no advantage for using such a low voltage for the head heaters at all.
    That is a very good question. However I doubt they wanted to introduce 120V!!!! on the ramps or similar board. Once you have a heated bed on 120v it would be more complex in controling the heat. For example ABS from what im gathering needs between 50-65C to adhere well. Other plastics need less. I am aware you could put in a separate controller on the 120v heated bed to get the proper temp then its another step before printing, when its all in one, theoretically once you have your gcode generated you put it on your sd card and let her rip. Once the bed is hot enough it will start printing. 12v makes for a more user friendly environment vs the potential of killing someone who is trying to enter the world of electronics for the first time.

  2. #12
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    Both the head and the table should be properly controlled with a PID PWM system. I assumed they were but as I say, I know nowt about Repraps. You only need one transistor to PWM the heat no matter what the voltage and the advantage of using either AC or DC 120 or 230 matters not, no need for a regulated supply, it just doesn't matter how crap the supply is if it is PID PWM controlled then it will perform correctly. Better to use a seperate board as you say but a small PCB with a PIC that is just todl what temperature you want and leave it alone is by far the best way or you could just buy a PID controller for a few quid more.

  3. #13
    Engineer-in-Training nka's Avatar
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    There's a lot of relay/module to dim light using PWM and Arduino. Could still use the "all in one" solution, just running the DC to the AC relay/module.

  4. #14
    Staff Engineer printbus's Avatar
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    Ignoring the safety concerns, you'd of course need a heat bed designed to operate off the mains voltage. A 120V or 230V heat bed would be tougher to fabricate than one that operates on 12V.

    We can calculate the required heater resistance using R = (V * V)/P, where V is the voltage applied and P is the desired power dissipated as heat. For a nominal 130 watt 12V heat bed, this comes up with R equal to about 1.1 ohms. Such a heat bed is readily achievable in a circuit board layout by applying the input voltage across a trace designed with a particular copper thickness, trace width, and trace length.

    For 230V mains and the same 130W heat dissipation, the same formula shows R would be around 400 ohms. This would be extremely hard to achieve with just a circuit board trace, meaning the bed would have to be formed from a resistive material or multiple resistive components.

    Those interested can play with the necessary trace characteristics using any of several online circuit trace resistance calculators.
    Last edited by printbus; 06-11-2014 at 10:20 AM.

  5. #15
    Engineer-in-Training nka's Avatar
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    I might be missing a point (and I'm not an expert in electronic), but for a 130W Heatbed, you'll be running 230V@0.56A with a resistance of 400 ohms or 12V@10A with 1.1 ohms.

    IMO, it's much more a problem to have 10A than 0.56A? You have much more current directly into the cicruit. Also, isnt the resistance the heatbed directly?

    I will personally use this : http://www.aliexpress.com/item/Keeno...881899879.html (Custom order for my size if not available) with an SSR.

  6. #16
    Staff Engineer printbus's Avatar
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    You are right that for the same power the current draw will be far less at 230V than it is at 12V. I was unaware you could get heat beds that ran on 120V or 240V. The ones that run on 120V or 240V likely use resistance wire; suggested by the wirewound reference in the resistance description. From a thermal standpoint it should work. Most of us would just prefer to deal with low voltage at a higher current than mains voltage at a lower current.

  7. #17
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    Again, to me I see value in having a 12v heated bed. Simply put I like being able to control the heatbed via firmware. To me having a central control point is worth wasting power. At least at this point for me. I plan on integrating a insulated and heated build chamber to my build, and that too will use a 100-200w ceramic 12 heater, and hopefully I can get the firmware to control that one too.

    I have to figure out proper fusing for my build as well.

  8. #18
    Engineer-in-Training nka's Avatar
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    I think I found the perfect PSU. I may order it when I'll upgrade to a heat bed.

    SilverStone ST45SF. Very small (125 mm (W) × 63.5 mm(H) × 100 mm(D)), provide 36A to the 12V and 22A to the 5V of MAX (not peak).

    It's an ATX PSU, so it will need a "load" to power it. 0.6A isnt that much, so a fan could do the job. ATX PSU are really easy to modify.

  9. #19
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    Looks to me like a nice PSU. I have heard the latest corei7 cpu's pull more from 12v than any other rail, this is in effort to have proper power stabilization, which means that psu manufacturers have adapted to. It certainly looks like a nice piece of kit. Let us know of the outcome. My own personal update on the power matter seems to be that due to stable clean power the arduino is booting up much faster and no longer suffers from disconnecting from the serial connection as some have had issues with. It seems that the regulators on the ramps/arduino are not very capable of compensating for large power swings.

  10. #20
    A friend of mine gave me his old (basically new) gaming psu. It's way over the top for just running one printer as it has 4x12v rails capable of 18amps each plus 30 amps at 5v. 12 volt signal is clean and stable without a 5v load and it's almost silent. It will be handy for when I want to run two or three printers at once.

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