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29Jun/10Off

Guest Article: My heated bed modification (updated)

I'm really pleased to announce, my first customer article ever!


László's Heated Bed Installation

I ordered László's heated bed and received it -very well packaged, into a big banana's carton. Here is a short description of the installation I made, not always following László's howto...

The bed is very well machined, very flat (more flat than BfB acrylic bed), with locations allowing to quicly set the magnets.

I decided to connect (serial connection) 9 resistors, 5 x 22 ohms and 4 x 18 ohm, directly to 220 volts. We get more or less 265 W of heating power like that: 220*220/182.

Direct plug into 220 volt can be very dangerous, so it is not advised to do that without good electrical skills. Using a transformer is a non dangerous alternative.

This picture shows the resistors installation:

I used thermal grease between resistors and plate. Wires between resistors are soldered, and insulated with glass-fibre braided sleeving. On resistors, the two 220 volts arrivals are insulated with kapton tape, the other resistors weldings are not isolated because the tension here is only 24 volts more or less (voltage divider: 220/9).

The plate is plugged to the ground, which I verified, and there is a ground fault circuit interrupter into my electrical installation, that I verified too... So, normal precautions are taken against 220 volts dangerousness.

In the picture you can see the wires going to the thermistor: I am using an Arduino based temperature control system, the thermistor is inserted into a little hole I made into the plate and fixed with kapton tape.

A picture of the first test:

My Arduino's system (RBBB) is into a CD box. I zipped the schematic and .pde sketch here.

And the heated bed installed:

After some days of utilization, I found that László's z-switch holder was not strong enouth and didn't guarantee a constant z home position, so I made a little hack:

I cutted the bed a little and placed the z-switch into the same x & y positions as with acrylic bed. The z-switch is firmly fixed with wood. There is an error hole that you can see in the picture :-) ))

And now the heat isolation. I screwed some wood under the plate on 3 sides:

and added removable wood sheets (so one can reach the magnets):

and finally, removable wood parts on the plate, that I arrange during the beginning of the print:

This is not a perfect insulation...

For the utilization, it is very important to note that the bed expands with heat, decreasing its distance to the printing head. The head level must be set accordingly!

In conclusion: very good heated bed! I am just beginning to print with it…

Update:
I somehow forgot to include the schematic. Here we go:

(clicking on the image gets you to the .pdf file)

Also here it is the arduino .pde sketch file.

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  1. There where some hiccup in the blog database, and disappeared the comments from this blog.
    I dont remember exactly the post, but basically it said, the voltage drops across the resistance, and is better to insulate each joint, because compared to the table it is 230-x*230/9 Voltage, so 230, 205, 179, 154, 128, 103, 77, 52, 26 Volt.

    And remember above 50VAC you risks your life (and above 120VDC) (single fault limit).
    And above 30VAC it is considered \high\ voltage (EN 60950, 60335), ie no fault limit.

    If you (the original comment author) reads this, could you please confirm, if I missed something from your explanation?;-)

    Laszlo

  2. I’m not sure I’d use mains voltages on a heated table like that. High amperage DC gives more heating than AC, too.