DIY Heated Bed with cement resistors

Hello folks. This is my plan to build a 600x400mm heated bed based in a 600x400x6mm aluminum plate and 20W cement resistors. I ask for help in the analisis and also for some corrections if it needed. I think it can be done.

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Informations:

3D printer: coreXY custom made.
Printed build area: 600x400x400mm.
Total bed area: 60x40cm = 2400cm2
Heating target: 0.5W/cm2
Watts/cm2 options analisis:
- 1000W: 0.42W/cm2
- 800W: 0.34W/cm2 --> Choosed
- 600W: 0.25W/cm2
- 400W: 0.17W/cm2
- 200W: 0.08W/cm2
Energy source: 110V AC.

Formulas:

- Ut = Rt x it
- Pt = Ut x it
- U' = Rr x i'
- Pr = U' x i'
- Rt = (NRS x Rr) / NSP
- i' = it / NSP

Note: for these formulas, it should be considered that all resistors have the same resistance and are distributed in equal numbers in parallel segments.

Subtitle:
Ut: total voltage
Rt: total resistance
it: total amperage
Pt: total power
U ': target voltage of 01 resistor
Rr: resistor resistance
i ': segmental amperage of a series segment
Pr: power assigned to the resistor
NRS: number of equal resistors in series of a segment
NSP: number of parallel segments


Choosed resistor:

As the power of choice is 800W, the cement resistors of 20W are ideal, because 800W / 20W = 40 resistors. Also, by dividing the total area of the bed into segments of 10 cm, it is possible to distribute 39 equidistant resistors, as seen in the figure below. 20W cement resistors are available in Aliexpress and don't costs to much. The following resistors can be found: 0.1 Ohms, 3.3 Ohms, 5 Ohms, 6.8 Ohms, 8 Ohms, 15 Ohms, 20 Ohms, 200 Ohms.



Analysis of viable resistors:

Considering that: Pt = Ut x it --> 800 = 110 x it --> it = 7.3A. This implies that Ut = Rt x it --> 110 = Rt x 7.3 -> Rt = 15 Ohms. Thus, the average resistance should be 15 Ohms. Less than 15 Ohms, the power applied to the resistor exceeds its 20W limit. Therefore, the Rt should be about 15 Ohms for more, but with a small margin.

Of all resistors, the 3.3 Ohms are the choosed one, because:
39 3,3 Ohms resistors divided into three parallel segments: Rt = (NRS x Rr) / NSP = (13 x 3.3) / 3 = 14.3 Ohms. Suitable.

Is the power applied to the resistor viable?
it = Ut / Rt = 110 / 14.3 = 7.69A. We have that i '= it / NSP = 7.69 / 3 = 2.56A. Furthermore, U '= R x i' = 3.3 x 2.56 = 8.45V. Therefore, Pr = U 'x i' = 8.45 x 2.56 = 21.63W. It is above the maximum limit that the resistor tolerates (20W), but it is possible to implement a 10A rheostat to control the current below that limit.

Edited 6 time(s). Last edit at 03/14/2017 03:24PM by renatortb.

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Building a custom coreXY based on BigXY Large Format CoreXY Printer, Vulcanus V2, Vulcanus MAX... ++Brazil++

If you mean the sort of cement resistors that I think you do, the problem I see is that they are not designed to transfer the heat to a metal surface. Rather they are designed to be cooled by convection. The type of power resistor that has a metal case intended to be bolted to a heatsink would be more suitable, but also more expensive.

Power resistors are not cheap. You may find that it costs less to order a silicone bed heater to your own size, voltage and power specifications from one of the Chinese companies that specialise in these, such as Shenzen Ali Brother Technology. The 300mm 230V 350W one they made me cost about $35 AFAIR.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
If you mean the sort of cement resistors that I think you do, the problem I see is that they are not designed to transfer the heat to a metal surface. Rather they are designed to be cooled by convection. The type of power resistor that has a metal case intended to be bolted to a heatsink would be more suitable, but also more expensive.

Power resistors are not cheap. You may find that it costs less to order a silicone bed heater to your own size, voltage and power specifications from one of the Chinese companies that specialise in these, such as Shenzen Ali Brother Technology. The 300mm 230V 350W one they made me cost about $35 AFAIR.

Thank you for your kindly reply.
The total cost of the resistors will be $23 (with shipping to Brazil) in here: [www.aliexpress.com]
Yes, i agree with you. The custom silicon pad is a better way. I checked with the seller and it will cost $48 + $19 (shipping to Brazil) = $67.
I think i will buy the silicon pad anyway, but i'm asking myself if the cement resistors way will work. I'm very curious about that. It can be a cheaper option. Maybe not as functional as the silicon, but...

look for silicone blanket wire on aliexpress..
it is basically silicone encased nichrome wire. u just need to stick it to the bottom of the plate with a liquid silicone gasket and vola instant heated bed... i have 2 such bed running on 220 v... only take care to coat the bed with a insulating material like polymide before putting the wire or in case of short u will have high voltage running through your plate..

Quote
ekaggrat
look for silicone blanket wire on aliexpress..
it is basically silicone encased nichrome wire. u just need to stick it to the bottom of the plate with a liquid silicone gasket and vola instant heated bed... i have 2 such bed running on 220 v... only take care to coat the bed with a insulating material like polymide before putting the wire or in case of short u will have high voltage running through your plate..

Thank you very much for the tip. Could you share some pictures of your project? Also, could you tell what wire resistance did you use and the length? Thanks!

Edited 2 time(s). Last edit at 03/28/2017 02:31PM by renatortb.