Designing hot bed with Aluminium clad resistors

Hello!

im trying to do my own heated bed

I will use:
3mm aluminium sheet 250mm x 400mm
Aluminium Clad Resistors 50w 1,5 ohms or 25W 1,5ohms

My questions is how many resistors y need and how to calculate the Amperes using 12-24V or is best to use directly to 220V Ac and control them with another circuit.

I do not know if im doing good:

Resistors in paralel is: (Rt = (1/R1)+(1/R2)...) if i use 6 resistors of 1,5ohms Rt= 0,66 x6 = 4 ohms

i want to get 120-150ºC with resistors if i use the 50W resistors. what amperes it will drain? what power supply you recommend me?

Using P = V x I --> 300W (6x 50w) = 12V x I --> I = 300W/12V = 20 A needed???

thx for help!

Edited 2 time(s). Last edit at 02/13/2013 07:22AM by Daimiros.

My 300x300mm bed has 1 ohm total resistance, runs on 12v and will sustain 60C which is ok for pla, however it won't get much hotter than that and takes a while to heat up! In hindsight, something around the 300w mark would have been better!

So yes you need 20amps, using V=IR at 12v you need a total resistance of 0.6 ohms. Your going to need to work out some arrangement of resistors that gives you this! For example you could string 4x1.2 ohm resistors in series to give you 4.8 ohms, and then put 8 of these in parallel to give you 0.6 ohms.

Also this is wrong!

> Resistors in paralel is: (Rt = (1/R1)+(1/R2)...)
> if i use 6 resistors of 1,5ohms Rt= 0,66 x6 = 4
> ohms

It should be (1/Rt) = (1/R1) + (1/R2) ......

So 6x1.5ohms in parallel is 0.25 ohms!!

Depending how many resistors you use, 50w resistors are probably overkill. 25w should be fine since the resistors are connected to a giant heatsink.

Edited 1 time(s). Last edit at 02/13/2013 08:17AM by konwiddak.

Hello! thx for answer

well in have in mind to reduce Amperes, i can up ohms of resistors...

how many resistors of 25W do you recommend to this sheet???

best 12V or 24V or... 220AC? i think 220V AC is dangerous and i want to use 12V-24V but it need high Amperes...

what you recommend to improve it? because i need to transport it.

thx

Read nophead's blog post about this type of heated bed. [hydraraptor.blogspot.com]

But, realize that this type of bed is HEAVY, certainly heavier than most beds, which may limit your speed.

Is 220 dangerous? Sure. But it's more efficient. It will require an SSR (solid state relay) if you enable PID control of the bed, which is a good thing. It will also mean you can wire the resistors in series, which is infinitely easier than wiring them in parallel.

I've actually built a heated bed similar to what you're thinking of doing. I'll post a pic later.

Edited 1 time(s). Last edit at 02/13/2013 11:54AM by akhlut.

---

- akhlut

Just remember - Iterate, Iterate, Iterate!

[myhomelessmind.blogspot.com]

If you reduce the amps then you have to increase voltage.

R = (V^2)/P so choose whatever combination you want, I'd say you want P to be 200-300 watts. So if like above you want 240 watts, then at 24v you only need 10 amps.

What is the problem with high current? 20 amps is easy to control through a MOSFET, 10 isn't even an issue.

Nophead's ever informative blog has this bed: [url=http:// hydraraptor.blogspot.co.uk/2010/01/hot-bed.html?m=1]HEATED BED[/url]

You can judge what kind of spacing and number of resistors from this taking into account how much bigger your bed is.

Весьма полезным. С много хороших идей!

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блиндирани врати решетки метални решетки Цени входни врати метални врати

On my first "RepRap" build I have a 300x260x6mm bed heated with nicrome wire, hold in place by kapton tape, ~10mm rockwool and a 1mm alu sheet. I found that ~240W was about right. It heats to 110 degrees in approx 10 minutes. Below 200W I couldn't sustain 110 degrees.

But I think that 3mm alu is too thin for a bed that big..

Well, here is my heated bed.

As you can see, it is made of (16) 1.8 Ohm 50W aluminum clad wire-wound resistors attached to a 1/8" thick 12" x 12" aluminum plate. I wanted even heat distribution, but I probably could have gone with 9 resistors. More is not better. Lesson learned.

There is a temperature limit switch integrated in series with the resistors to prevent a runaway condition in the event of a loss of signal from the thermistor. This has happened to me. DO NOT MAKE THIS STYLE OF BED WITHOUT INTEGRATING ONE. IF YOU DON"T YOU WILL BURN YOUR HOUSE DOWN. IT IS A FIRE WAITING TO HAPPEN.

This bed runs on 120V AC.
The total resistance is around 29 Ohms, so the average current drawn is ~ 4A
The total output of the bed is ~ 500W

The bed heats up from 30C to 100C in about 2 minutes, and with PID enabled the temp is rock solid (+-1.0C from setpoint).
The bed is HEAVY though...can't go too fast.


Now nophead mentions that he runs his resistors at about twice their recommended rating. I haven't on this bed. If I had only used 9 of the same resistors I would have slightly derated them (890W load into 800W of resistors).

The only thing you need to make a bed like this is Ohms law.



Some tips:

If you're going to use 220V, you need to first determine how much power you want. 500W? 750W?
For example, at 220V, 500W is generated by a total resistance of 97 Ohms while 750W is generated by a total resistance of 65 Ohms

Next, decide how many resistors you want. 5? 9?
So for a 500W bed using 9 resistors you get 10.78 Ohms. Since you're not going to find a 10.78 Ohm resistor we need to find a substitute. 10 Ohms sounds good. So (9) 10 Ohm resistors will generate 538W. So how many watts per resistor? Right around 60W. That's a lot. But something like this should work. Or these.

This is for informational purposes only. Do not build one of there without fully understanding the maths involved as well as the dangers. So if you don't know what you are doing pack up the tools and go another route.

Hope this was informative.

---

- akhlut

Just remember - Iterate, Iterate, Iterate!

[myhomelessmind.blogspot.com]

Wow!!

Thx for you explain... was incredible!! thx! i will post advances of heated bed!

I have a questions.... on how you calculate V and R and I

If i decide to use 10 x 10 ohms resistors of 50W in serial, how can i calculate "I" for 12V? and 24V? what formula of laws did you use?

and if I have total resistance, total power and volts (100ohms 500W and 12-24V) how to calculate I? or i need to use resistors in paralel??

Ok i have made this:

R = (24^2) / 500W = 1,15 ohms

I = square (500W)/ 1,15 = 19,44 A

Is it correct?? wow... a lot of A is needed with 24V i think will use 220V

Ok if use 220V with well isolated circuit i can reduce dangerous... what you recommend for cover connectors and pins for isolate and reduce the danger?

Sorry my mathematics are catastrophic!! ehehe

Another question is:

If i have and old power supply 12V and 8A is there any way to heat this sheet of 400mm x 300mm to 120C??

thx for your help, it's much apreciated

Edited 5 time(s). Last edit at 02/15/2013 04:32AM by Daimiros.

Hi. my upgrade, and problem solving approach on hotbed not enaugh degrees and only heatin up to 40- 60 70 degrees instead of 110C


Actual 9 resistor configuration for hot bet (12ohm diplayed as position under hotbed)

a)Model of Resistance below
---------------------------------
Heater for hot Bed:
arcol 12.40
HS15 12R F

descriere:
Size HS15
Style mil-r: Re65
Power rating : 15
Watts:with no heatsink: 8
resistance range 005-10k , dorit 12R.
-----------------------------------------

b) actual layout , as adrian's prusa hotbed

12 12 12



12 12 12



12 12 12


c) Debug why temperature is not raising above 60-70degrees even after 10-15 minutes.

My PC power source is :
2 * 15A wires.
P=U*I = 12*15= 180 W... so not too much
Bigger power source needed.

Actual using 9 12ohm resitors of 8W :

if 12 V= 15A * R => R can be = 12/15=0.8 ohm, ideal resitance for this PC power source.

Now we have a rezistance in paralel of :

1/ {(1/12)*9 } = 1/0.75=1.3 Ohm and we need to go to 0.8ohm as shown abow for our power source.

d) Modification required for more heat according to the max allowed by my power source.
See below with blue/underlined the new added 6 resistors, for a new total of 15 Resistances now.


We will put aditional 4 ohm resitor in paralel => 13 ohm resistors of 12ohm in paralel
New resistance will be
R=1/ (0.0083 * 13) =1 / 0.9 Ohm. that is good

if using 15rezistors in paralel: R=> 0.803 Ohm. (If using 0.803 ohm than A needed is I=12/0.803 =14.94 A ) wich is still good, and we will go to using 15 resistor instead of 9.

New resistor layout using 15 HS15 12R F will be :


12 12 12

12 12 12

12 12 12

12 12 12

12 12 12

Please do not confuse the resistor power rating (i.e. 25W) with the actual power it will dissipate. My experience is that it is safer to stick to the power rating and not to exceed it (I've burnt a couple of beds made of 10W resistors). For a regular bed I use four 1R5 in series and then in parallel (R+R)||(R+R), or four 6R in parallel for an equivalent resistance of 1.5 ohms. All resistors 25W.

For a bed like yours I woud use 8 resistors, for example 6R in parallel, that would be 0.75ohm. At 12V the power will be V^2/R = 192W and V/R = 16A.

Please note that 16A is way too much current for a RAMPS board (the resetable fuse will blow). What you can do in that case is to use the MOSFET to drive a relay (but do not activated the PID in that case).

For 220V beds I would use eight 25W 1K8 resistors in parallel too. Again, use a relay for switching that load.

At that level of power (around 200W) the bed should be stable enough to prevent a fire (I do not think it will get any hotter than 130C easily).

A 10 to 20% decrease in the resistance values will make your bed heating faster (but a bit more dangerous if something goes wrong).