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# Heated bed

Posted by brupje
 Heated bed January 27, 2011 01:47AM Registered: 13 years ago Posts: 384
So I ordered a thick sheet from mendel-parts (and it got delivered in time ) and some resistors (2.2 ohms with aluminium heat sink) from RS. I have a PC power supply with 12 volts. I put some cooling paste between the resistor and the heated bed.

When I put 12v on one resistor it gets very hot quickly. It did blew it. When I put 12V on two resistors they get warm, but not much.

So should I get other resistors? Should I run them in parallel? I would really like to keep using the pc power supply and I do not prefer using 240V.

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19
 Re: Heated bed January 29, 2011 02:08AM Registered: 13 years ago Posts: 1,780
Did you hook up the resistors in series or in parallel? You will get more power to heat the bed if you hook them up in parallel. The formula is Power = Voltage*Current = Voltage^2/R. So, if your resistors are in parallel, R=1.1 ohm and the power would be 12^2/1.1=131 Watt and the supply current need is 11 A. This should be plenty for PLA (50-60C) and maybe good enough to go to 100 C for ABS. If your power supply has enough current output, you can go to smaller resistors for more power. If you want to distribute the heat more evenly over the whole bed, you may need to increase the number of resistor. It would also help to insulate the bottom side so most of the heat goes to the top of the bed.
 Re: Heated bed January 29, 2011 04:30AM Registered: 13 years ago Posts: 384
So if i have 4 2.2 ohm resistors 12²/2.2 = 56.45 , 56/12 = 5.45 A , 5.45 * 4 parallel resistors = 22A. Correct? My PC power supply says it has 6A on 12V. So that is a no-go then?

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 Re: Heated bed January 29, 2011 10:08AM Registered: 13 years ago Posts: 1,780
Yes, I don't think 6 A is enough. You might be able to get away with 4 x 4.4 resistors in parallel and a 12V supply with at least 11 A. Anyway, I've been looking at PC internal power supply and there are a lot of cheap ones available.
 Re: Heated bed January 29, 2011 11:11AM Registered: 16 years ago Posts: 438
You might be able to use the 5V rail. On PC PSUs the 5V rail usually has more power, but you'd have to use smaller resistors, and probably hook them up in parallel too. And use heavy wire of course.

What's the PSU rated for on the 5V rail?

--
I'm building it with Baling Wire
 Re: Heated bed February 05, 2011 06:41AM Registered: 13 years ago Posts: 384
The 5+ line has 22A.

I tried hooking up to the 12 rail with 3 resistors. They were getting hot and the power supply seemed to be ok, although it kinda makes high noices.

When I put the vnp20n07 in things are not ok. The vnp20n07 is getting very hot. I am pretty sure I hooked it up alright, so I don't understand why.

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PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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 Re: Heated bed February 05, 2011 07:40AM Registered: 13 years ago Posts: 384
with the same resistance, 2.2 x 3, running at 5 volts, things seem to be better. The vnp20n07 is still getting very hot, which is what worries me. But the bed is getting warm, not hot. S next step would to get lower resistors I guess.

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PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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 Re: Heated bed February 05, 2011 08:29AM Admin Registered: 17 years ago Posts: 7,881
With 3 2.2R resistors you get 3 * 5 / 2.2 = 6.8A. The vnp20n07 has an on resistance of 0.07R with 5V gate drive. That will dissipate more than 3W so it will need a heatsink.

[www.hydraraptor.blogspot.com]
 Re: Heated bed February 05, 2011 08:48AM Registered: 13 years ago Posts: 1,780
You need to put a heatsink on the power mosfet to keep it from getting too hot. I used a 511-STP55NF06 which is rated at 55 A with an Ohmite EA-T220-38E heatsink (http://www.mouser.com/ProductDetail/Ohmite/EA-T220-38E/?qs=sGAEpiMZZMtffRBI%2f27p9Cs%252b%2fWQ0OLlr5NlE5w7uHbo%3d) and it seems to work well with the Prusa Heated bed (1.1 ohm) and a 12V ATX supply with 18A.

You may not be able to get enough power to your heater from the 5V supply with the vnp20n07. It's only rated at 20 A. With a 5V supply, that's only 100W (5 x 20) at the maximum rating of the mosfet. At that current, you definitely need a big heatsink. The one I used is rated at 55 A so it would be better 5V. Also, your effective resistance should be 0.25 ohms to get 100 W. You need more resistors in parallel or use lower values.

If you have two power mosfets, you can use both the 5V and the 12 V supply and run one set of resisters with one mosfet on the 5V supply and the other one with the 12V supply. You'll need to select resistance values so that the power dissipated on the ones on the 5V comes out equal to those on the 12V. You can get a total of 172 watts this way. If you used a different mosfet, then you can use 22 A from the 5V supply and get a little more power: 182 watts.

Also, the 5V circuit would need bigger wires than the 12V since it will handle more current (20A vs 6A).
 Re: Heated bed February 05, 2011 08:49AM Registered: 13 years ago Posts: 384
Thanks. I guess you are right. I put small heat sink on it, but still overheats. It's ridiculous, the power supply is getting hot faster than my heated bed

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19
 Re: Heated bed February 05, 2011 10:34AM Registered: 16 years ago Posts: 438
Another option would be to get a different power supply, preferably something a little higher than 12V. I think 19V or 20V laptop supplies are becoming common.

--
I'm building it with Baling Wire
 Re: Heated bed February 05, 2011 01:11PM Registered: 13 years ago Posts: 384
I was thinking about that, I have ordered another pc power supply, which has 2x12v lines. According to my calculations that should be 2x10A, with 2x2 resistors (2.2ohm). I have two vnp ic's left, after blewing one up, so no problem there Thanks for the suggestion.

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19
 Re: Heated bed February 05, 2011 01:16PM Registered: 13 years ago Posts: 384
I created a spreadsheet to help me with the calculations. Maybe it's useful for someone else too

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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 Re: Heated bed February 05, 2011 04:23PM Registered: 13 years ago Posts: 1,780
jgilmore Wrote:
-------------------------------------------------------
> Another option would be to get a different power
> supply, preferably something a little higher than
> 12V. I think 19V or 20V laptop supplies are
> becoming common.

Yes, higher voltage supply would allow you to use higher resistance in the heater and more power. The advantage of using an ATX power supply is that they are a lot cheaper than the laptop adapters. The one I bought with two 12 V, 18A rails costs less than \$25 (US). I use one rail for the heated bed and the other for the motherboard and controllers.
 Re: Heated bed February 05, 2011 10:06PM Registered: 16 years ago Posts: 438
I have two old dead laptops with good power supplies. Hard to beat "The Right Price™" which is, of course, free.

--
I'm building it with Baling Wire
 Re: Heated bed February 07, 2011 12:34AM Registered: 16 years ago Posts: 1,094
I used IRL3803 mosfet which can carry 15A with no heatsink, and lots more with one

-----------------------------------------------
Wooden Mendel
Teacup Firmware
 Re: Heated bed February 20, 2011 08:32AM Registered: 13 years ago Posts: 384
Thanks for the tip, I've ordered them and now have a working heated bed

Some pictures:
[www.flickr.com]

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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 Re: Heated bed February 20, 2011 08:38AM Registered: 13 years ago Posts: 1,780
What's under the white tape? Is that the thermistor mounted on one of the resistors? It would probably be better to mount it in the middle of the bed.

Edited 1 time(s). Last edit at 02/20/2011 08:39AM by brnrd.
 Re: Heated bed February 20, 2011 08:58AM Registered: 13 years ago Posts: 384
I figured if I keep the resistors at 70 degrees, the heated bed will have a nice temperature of 55 degrees. Also the resistors won't get very hot, which should give them a longer lifespan

reprapworld.com
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 Re: Heated bed February 20, 2011 09:12AM Registered: 13 years ago Posts: 1,780
If you're using PID control to set the bed temperature, then it would take longer for the bed to reach the desired temperature if you mount the thermistor on the resistor instead of the heater. Also, you' ll be more accurate. At a bed temperature of 55 C, you shouldn't need to worry about burning up your resistors. But, I haven't build one of these beds myself. I went with the PCB heated bed from Prusa.
 Re: Heated bed February 20, 2011 09:26AM Registered: 13 years ago Posts: 384
It is heating faster than the extruder nozzle, so that's no problem for me. I see why it might be less accurate, but I have no problems so far

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PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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 Re: Heated bed February 20, 2011 09:53AM Registered: 13 years ago Posts: 1,780
With so much mass, I'm surprised. My tip reading reaches temperature within 1 minute. I haven't timed it exactly. How long does yours take?
 Re: Heated bed February 20, 2011 04:07PM Registered: 13 years ago Posts: 384
I haven't timed it yet, but from cold start it takes a few minutes. But I am using a alu block. The nichrome wire was much faster.

The nozzle runs on 24W, while my heated bed takes 260W. That probably makes a difference too.

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19
 Re: Heated bed February 20, 2011 11:23PM Registered: 13 years ago Posts: 1,780
You might think that your bed heats up fast, but since you have the thermistor on the resistor, you're not really measuring the bed temperature. In fact, you really don't know when your bed reaches the desired temperature. It's really better to put the thermistor on the bed. Are you controlling the bed temperature with the extruder controller? Or are you using Gen 6 or RAMP?
 Re: Heated bed February 21, 2011 01:12AM Registered: 13 years ago Posts: 384
After two minutes I can't touch my heated bed, so it's probably hot. It's true that I don't know what temperature my bed is exactly, but the temperature should be less fluctuating, because the delay between heater and thermistor is lower.

I am using the extruder controller.

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19
 Re: Heated bed February 24, 2011 12:37AM Registered: 14 years ago Posts: 1,092
If you have the room on your electronics for the inputs, you could consider having 2 thermistors for the bed.

One like you've said with the resistors, to control the power.

One in the centre of the bed so you know when it's near operating temperature.

It'd also take a bit of code work to 'use' both effectively so you get the right temp.
 Re: Heated bed February 24, 2011 02:35AM Registered: 13 years ago Posts: 384
I like that idea, but I don't think I have any analog pins left on my extruder controller. And as long as I can print Warp free I am happy about it :p

reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19
 Re: Heated bed February 27, 2011 01:27PM Registered: 13 years ago Posts: 10
Hi there

I try to use a heattbed from Pruso on the BFB3000.
I bolted it to built base and ..... heatbed bends when heated (80 degC).
I know it's physics ..... how do you solve this??
 Re: Heated bed February 27, 2011 07:05PM Registered: 13 years ago Posts: 1,780
How did you attach the PCB heatbed? If you screwed it down tightly on the four holes in the corners, then there won't be room for the board to expand and it will warp when heated. One simple way is to use bulldog clips to hold the the PCB on your bed.
 Re: Heated bed February 28, 2011 02:55PM Registered: 13 years ago Posts: 10
bulldog clips will not help, since print head will hit them as RepRaps building platform is way smaller than BFB3000
I tried to loosen bolts and make holes bigger to allow expansion, but not much of improvement
May be the heatbed is bending by itself without fixation ....ex. because of different expansion of base material and cooper lining??? I'll try...
On the other hand Reprapresource design is faulty - not allowing more point of fixing.
The better one is being made by MakerBot ind. , with 9 point fixation, I'll try to follow this way by making my own.
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