Heated bed

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.

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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.

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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reprapworld.com
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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.

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?

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I'm building it with Baling Wire

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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reprapworld.com
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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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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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.

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[www.hydraraptor.blogspot.com]

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).

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

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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.

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I'm building it with Baling Wire

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.

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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I created a spreadsheet to help me with the calculations. Maybe it's useful for someone else too

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19

jgilmore Wrote:
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> 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.

I have two old dead laptops with good power supplies. Hard to beat "The Right Price™" which is, of course, free.

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I'm building it with Baling Wire

I used IRL3803 mosfet which can carry 15A with no heatsink, and lots more with one

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Wooden Mendel
Teacup Firmware

Thanks for the tip, I've ordered them and now have a working heated bed

Some pictures:
[www.flickr.com]

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19

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.

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

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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.

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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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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?

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.

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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?

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.

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
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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.

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

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reprapworld.com
PLA, ABS, PETG multiple colors (3 and 1.75 mm) €16,49 / kg
Megatronics 3.0 €79.99 / Minitronics €37,19

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??

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.

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.