12v / 1ohm = 12amp....
right.
12amp x 12v = 144 watts
right.

and 120+ watts or so will become 120C ?
Depends on the ventilation, insulation, etc, but the new max temp isn't going to be linear with power deposited. New max temp will be higher. I imagine there are engineering tomes that say "If you dump ~144 Watts into a heated plate ... yadda yadda."

This is where ol' physicist me waves his hands and says it's an engineering problem.

The good news. Right now you're doing this:

Red Wire: Point A.
No wire: Midpoint
Black Wire: Point B.

R_Total = R_A_to_Midpoint + R_Midpoint_to_B = 1.6 ohms + 1.6 ohms = 3.2 ohms.

Why don't you solder the red wire to the midpoint (on the left) and the black wire to Point A and Point B?

Then R_Total = (R_A_to_Midpoint in parallel with R_Midpoint_to_B ), so, for two resistors R in parallel,

R_Total = 1/((1/R + 1/R)) = 1/(2/R) = R/2,
So R_Total would be 1.6 ohms / 2 = 0.8 ohms.

Of course,
12 V = I * 0.8 ohms gives a current of 15 Amps, so your power supply won't be happy.

But it will give you ideas for a more flexible next board design.

Edited 1 time(s). Last edit at 09/22/2010 04:24AM by SebastienBailard.

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

Why not use this one as a guide: [www.thingiverse.com] ?

Your area is 2.25 bigger so you will need that much more power.

Edited 1 time(s). Last edit at 09/22/2010 04:48AM by nophead.

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[www.hydraraptor.blogspot.com]

Thanks! I did that and then it got to about 60C then I added another one in parallel and it got to 74C or so... I don't think that makerbot psu can do this now...I already just ordered another psu from amazon, it is 500w and says it can do 12v 25A

when it gets here I will just power the heater with it and maybe a LCD, LED, Arduino, thermistor, etc. but mainly just heater stuff not the motors or motor controllers. So hopefully an added psu will be able to get to 120C. I did your test and when I connected it in the middle it was exactly 1.5ohms but like I said it only got to 60C but I'm getting closer!

I wish I had some PLA to test what people say...that you only need 60C for PLA...then I would be done! xD

I'm not sure if other people are aware of this but by using an infrared thermometer (and moving it all over the pcb) instead of just a fixed thermistor...I'm noticing how uneven the trace pattern heat is spreading... some parts get as hot as 77C while others only 50C... I made a pattern randomly..maybe when my new psu gets here I'll flip it over (it is a double sided pcb ^__^) and try the snake pattern...maybe it'll heat more evenly ...and only 1/3 the area of what I have now.

I also ordered a nichrome wire from ebay that is my backup plan when if this fails but it is taking forever to get here...


Anyways, thanks again!

Hi,

did you found a solution ?

I'm trying designing a heated bed since 2 months .. without success ..

The PSU is the problem (I guess).

I try with PCB and with nichrome ..

... you need a fair amount of current with low voltage to build a 'secure' HB - best guess will be a 'really heavy' transformer with line input, maybe 6 Volts output and capable of some to some ten Amps.

Such 'beasts' can be found in old equipment - i have some transformers with 6, 12 and 40 Volts with maybe 20 to 80 Amps salvaged from old broken machines.

Then try with big enamel-resistors or NiCr-wire ...

PC-PSU's can support high Amps too, but the controlling and longterm stability (or 'lifetime' in near-shortcut-conditions) isn't so good ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

VDX Wrote:
-------------------------------------------------------
> ... you need a fair amount of current with low
> voltage to build a 'secure' HB - best guess will
> be a 'really heavy' transformer with line input,
> maybe 6 Volts output and capable of some to some
> ten Amps.
>
> Such 'beasts' can be found in old equipment - i
> have some transformers with 6, 12 and 40 Volts
> with maybe 20 to 80 Amps salvaged from old broken
> machines.
>
> Then try with big enamel-resistors or NiCr-wire
> ...
>
> PC-PSU's can support high Amps too, but the
> controlling and longterm stability (or 'lifetime'
> in near-shortcut-conditions) isn't so good ...


Ok, thanks.

I will try to found this kind of transformers .....

I have glued with Kapton tape a Nichrome wire under a piece of glass ...
And impossible to have more than 40° with a 12V / 300W PSU ...
And no possibility to change the amount of current with this technic ...

Yes should be able to get a lot more than 40C with 300W. I can heat 200x200x6mm aluminium sheet to 140C in about 10 minutes with 290W.

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[www.hydraraptor.blogspot.com]

... it's the resistance of the NiCr-wire, whats defining the possible temperature at a given voltage.

If you can't heat more, then shorten the wire - e.g. 3 parallel wires instead of a 3-fold meander ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Or go to a higher voltage. A standard PC switching power supply will provide up to 20Amps of +12V power. The 12v pushes more current through, and the power is the product of voltage times current, so you get over 5 times more power from the same resistors going from a 5v supply to a 12v. And the MOSFETs should typically handle at least 30v. And with higher voltage, you could increase the resistance and get the same amount of heat with less current flowing through the MOSFET.

Mike

I am currently thinking about a rugged transformer + diode bridge + caps or maybe filtering even, for a heated bed.

To the point: the transformer from a trashed UPS unit (fairly rugged, some outputs choices), a 35A diode bridge, and just some caps (around 50000uF alu electrolitics napkin determined / hopefully enough - trying to overkill because i am not sure how to calculate the best capacitance). That is to drive an 1.7 ohm heated bed with something around unregulated 20-21 (up to 25V), aiming for more than the 200-250W bed.

Q1: I cant make my mind if any further components or maybe a filter (high-pass?) should be used - any suggestions pls if possible? Filters like LC-CLC-Butterworth-Chebyshev or other suggestions ?

Q2: Controlling with a N-mosfet: The before-mentioned IRL3803 which is logic level, ~16.63A in still air, would it switch it properly? Or the unregulated wave form (100-120Hz) would be of an issue - and what should be taken into consideration about it.

Edit:
Q3: A SSR DC-DC relay that says in datasheet "frequency range" something like 45-70 Hz, would work with 100-120Hz unregulated - or what? Sort of speaking, would it have troubles maintaining on-state?

Edited 2 time(s). Last edit at 02/27/2011 10:50AM by NoobMan.

I put my version of a heated bed on the wiki pages: http://reprap.org/wiki/Heated_Bed#Electronics_Design_.232.

It is an extremely simple design with all parts ordered for under 20$ on ebay.
Have a look at it, it might work for you.
I use a 140C switch and this keeps my bed easily warm enough to reliably print ABS on kapton without warping.

Frank

Anybody try simply attaching the MakerBot Thing-O-matic Heater Board v1.1 to an aluminum plate? Would their solution be a plug & play with the reprap or techzone Gen3 electronics?

I'm thinking about getting this, attaching it to my Mendel aluminum plate, with a pyrex-glass build surface on top. I'm assuming that the aluminum will spread the heat to the whole glass platform, since the platform will be 200mm x 200mm vs the 150mm x 150mm of the heater. I might also experiment with attaching directly to the glass surface.

thoughts?

I currently have 4 Heater blocks rated at 2.2 Ohm on my Mendel bed and can get it up to 55C this is on 4mm Aluminium and using a universal laptop adapter running at 12V. I have put a piece of glass on top of the print bed and obviously need to get the temp higher as PLA is not sticking very well. I don't fully understand electronics so would adding more heater blocks be the way to go or could I up the voltage to get more heat?

I have tried using masking tape on the bed but get significant warping on most solid prints

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Experimenting in 3D in New Zealand

I was think about having two heated prusa beds in the following format with a piece of glass or ali on top.

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Would this work with the size modifications obviously? so a roughly 25 x 50 bed

@ NoobMan,

Don't know if this is too late, But you can connect a triac (does the same job as a mosfet for AC) directly Onto your AC transformer and use that to directly heat your build platform with AC (there will be negligible difference in operation, it is a simple resistive circuit after all), (ensure that this circuitry is SAFE, Insulation circuit breakers etc., preferably use low voltages)

You will have to be careful about the interference with the rest of your circuitry but that shouldn't be overly bad if you keep it well insulated and sheilded,

@NelsonRap

You are looking for wattage to heat your build platform so based on the equation P=IV

4 * 2.2ohm resistors placed in sereis will only produce 16.26Watts,

However placing them in parrallel will give you 261.81Watts,

If you double the voltage on your device you will Quadruple the power output.

Adding extra resistors in series will reduce your power output, (not good but may be helpful if you are currenlty blowing up your resistors)

Each extra resistor you place in parrallel will incerease your power output by 65.45 watts

Note you must have a big enough power supply to supply your device of you will blow that up,

The basic equitaions you need are;

P = V^2/R
P = Power V = Voltage and R is resistance
Resistance in series is R + R + R etc.
Resitance in parrallel is R = 1/(R+R+R+R+R+R) Etc.

So plug them together and you will get your answer

@ Kylelnsn

As far as your wattage goes use the info above, The distance between the wires will effect the smoothness of heat conduction and in theory you can increase the size of the build platform indefenately,

If you have the rails (the wires on the end) with approximately 0 resistance it should ensure that there is minumal change in resistance from the start of the first row to the start of the last row this would be the main problem with increase the size of the build platform, If you use nichrome wire for the rails there will be resistance as you move down the bed and you will have a heat reduction over the length of the bed.

Alternatively if you have +Ve in at top centre and -Ve at bottom outsides you should have approximately the same resitance over the hole device, Although the rails will be hotter the closer the are to the +Ve and -Ve rails with them producing upto if you have 10 rails, 10 times the power output compared to the rows and the final section of rail at the other end.

In conclusion as long as the rails are kept to minimum resistance and you have the +Ve and -Ve terminals on the opposite sides of the bed (eg. top and bottom) You should be reasonably safe. To incrase the Build platform indefinately.

Baj Wrote:
-------------------------------------------------------
> Resitance in parrallel is R = 1/(R+R+R+R+R+R) Etc.

Ohms law for parallel resistors reads: 1/R = 1/R1 + 1/R2 + 1/R3 ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I slipped an 18V chip into my universal laptop charger that powers my bed heater and all is good now I get excellent sticking with PLA, I have to wait for the glass to cool before I can remove prints from the bed. So i have 2 pieces of glass and just swap them at the end of each print

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__________________________________________________________________________
Experimenting in 3D in New Zealand