Heated bed hack - currents, voltages and heat output...

Hi to all on the forum,

Please could forum users be able to share their knowledge about electricity. I'm in the process of adding a heated bed to my printer and need some input to stop anything going up in a cloud of smoke.

I want to add a heated bed to my printer which is running on gen_3 electronics. I am not sure of how to get this added through the electronics as it requires additional circuitry to be built and I'm not sure of things on the firmware side either so for this reason I'm planning on powering my HBP with a laptop power supply.

I have tested the heat bed and it draws 9 amps at 12v. This heats my print bed to shove 80C which is bad because it is dibond. For this reason I'm looking to buy a laptop charger rated at 85-90 Watts.

Question time....

1. For a given power rating, would a power supply with a lower voltage and higher current heat the heat bed wicker/better/more than a power supply with a higher voltage and lower current (eg 12v 7.5a vs 15v and 6a)

2. Because this will be a hack, could a variable resistor be placed between the power supply and heated bed to control the power going to the bed?

3. How does the amount of heat vary based on the amount of power? Does the heat output increase in a,linear fashion (eg, twice the power equals twice the temperature) or is it closer to being exponential (twice the power equals four times the

Sorry gang,

Writing on a phone and I have fat fingers...

just missed off temperature from q3. No more questions.

Thanks to all on the forum.

Raj

1) The PSU voltage and the resistance of the bed dictate the current: I = V / R. The PSU should have a current rating the same or higher otherwise it will be overloaded.

2) It would have to be a very high power resistor, known as a rheostat, and it would waste a lot of power and get very hot itself. For example if it was the same resistance as the bed the bed power would be one quarter, but the rheostat would also dissipate the same power. Not really practical. Pulse width modulation, or simply turning it off when it is too hot, is much more efficient.

3) The temperature rise above ambient is roughly proportional to the power. The power is VI, which equals V^2 / R, so temperature rise is proportional to the square of voltage.

Note that with a PCB heater R is far from constant, it goes down with increasing temperature.

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[www.hydraraptor.blogspot.com]

Hi Nophead,

Thanks for your reply.

I have checked the resistance of the heated bed plate and it is between 1.2 and 1,3 Ohms. With 10v and 8 amps, I could get 80 watts going through the HBP which is a start. I think this can be done through my ATX PSU by using the +5 and -5 wires (http://reprap.org/wiki/PCPowerSupply). I will see if I can get this working and if so report back on what kind of temperature it gets to.

BTW - I'd just like to say a personal thanks to you as I bought a set of Mendel parts from you. Since then I have printed and built a child which I then used to print and build another printer. Couldn't have done it without you!
Thanks.

Raj

i use a dibond printing bed too, are you heating that, i would be worried about it getting weaker with the heat. i just wish that stuff was cheaper, its great stuff to work with

You can't use the -5V rail because it is very low current.

Good to hear that my Mendel has great grandchildren.

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[www.hydraraptor.blogspot.com]

RepRapRaj83 Wrote:
-------------------------------------------------------
> .... With 10v
> and 8 amps, I could get 80 watts going through the
> HBP which is a start. I think this can be done
> through my ATX PSU by using the +5 and -5 wires
> (http://reprap.org/wiki/PCPowerSupply). I will see
> if I can get this working and if so report back on
> what kind of temperature it gets to.

As NopHead says, forget the -5V line, it is rated for very low current. E.g. on my 500W ATX supply, the -5V line is only rated to 0.5 A, so you would overload the power supply for sure if you hook it up to such a low resistance.

Why not just use the +12V line and Ground as suggested in the Wiki? I run my whole Prusa (including the heated bed, extruder, steppers and RAMPS) off the 12V line, having hooked a few of the yellow 12V wires together to give me plenty of current capacity without any of the wires getting too hot.

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

dissidence - Hi, yes, the bed is heating the dibond too. With regards to cheap dibond, I got two sheets for about £15 from [www.sheetplastics.co.uk].
The price includes delivery. Much flatter than MDF but since I placed it on my printer, my warping has got very bad hence the need for HBP.

Nophead/julianh72 - Checked the PSU and my -5v is .2A.,,

Julianh - Hi, The reasons i'm not powering the HBP from my PSU are because my PSU can only handle 15A on at 12V, with a HBP on board, it would be pretty much at its limit and may cut out. The second reason is that even if i change the PSU to a better one, i'm running gen3 electronics (The red ones - i'm still not sure if these are officially known as techzone remix or something else) which require additional circuitry to be built in order for the HBP to work with it. I have looked at the wiki and have trouble interpreting the circuit diagrams into an actual breadboard circuit. I will give this another look as i may have a eureka moment.

A further possibility is to (after getting a better power supply) install a thermal switch which would open at 70c and close at 55c.

[www.ebay.co.uk]

These have a maximum current load of 10A but in the specifications, the contact voltage outlines a maximum of 250v AC, im not sure if a DC load would damage these but it seems like an option to me.

Regards to all.

Raj