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300W 12V Power Supply

osterchrisi

Mar 8, 2011
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Hello guys,

I built a 3D printer that is currently running off the +12V rails of two old computer PSUs which is annoying with all the colorufl cables falling out and the space they are using. The +12V are going to a few microcontroller boards which all have their own 7805, 4 stepper motors and a heated bed which is basically a resistor printed on a self-etched board.

So I was thinking about building a PSU myself but apart from standard 78xx PSUs I never did anything in that league. Do you think it is enough to simply take a toroid transformer, 4 diodes and a few larger and smaller caps? Or will the ripple voltage confuse my motors? I think the 5V regulators should be fine with non-perfect 12VDC and the heated bed of course too, it should simply heat up but I don't know about the motors...

Or what would be the way to go for such a large +12V regulated PSU? A switched power supply?

Very eager to hear your thoughts!!
Thanks!
 

john monks

Mar 9, 2012
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It sounds like what you are thinking about doing is building a straight analog power supply. This certainly would take a lot of space and dissipate a lot of wasted heat.
Power supplies in computers is notoriously efficient and cheap.
So unless you just want the experience building power supplies I don't think you will be able to do any better.
If you want to build your own I would suggest a buck regulator using a toroid. This can be done very simply and efficiently.
 
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Harald Kapp

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Nov 17, 2011
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A switched mode supply surely would be the best solution.

A side note: 300 W from 12 V translates to 25 A. You're currently burning 7 V across eacgh 7805 which means 175 W is converted to heat (apart from the energy lost in the 12 V PSU).
If you go directly from mains to 5V, you will need only 5 V * 25 A = 125 W, a huge savings. But still, if you were to build a linear regulator you would need an input-output voltage differential of at least 2 V, which gives you 50 W of heat to dissipate.
A switched mode supply can work at approx. 85 % efficiency, meaning 15 % losses or 18 W (at 125 W useable power).

However: Building a switched mode power supply in that power range an directly connected to mains is not an easy feat (for the unexperienced) and you need to take into account some safety measures. From my point of view I'd suggest going for a ready made supply as e.g. this http://www.rapidonline.com/Electric...pen-Frame-Power-Supply-Unit-PSF-125-5-90-2554.

Harald
 

osterchrisi

Mar 8, 2011
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Mhm, I see. Yeah the 12V are mainly burnt in the 4 stepper motors and the heated print bed. The motors are around 10A to be sure and the heated bet is around another 10A and the heating element for the extruder another few ampere, so most of the energy _is_ actually necessary on the 12V rail (hence the 12V power supply), the 5V rail is really only for powering the µC and I don't count it into the power comsumption.

But I can also see now that it might be more clever to re-build the two PC PSUs that I have now, simply get rid of all the unnecessary wires and put it into a nice box.

Stupid question then: Is there anything between me and connecting the two +12V outputs of the power supplies together considering I don't know the output stages?
 

john monks

Mar 9, 2012
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Generally speaking it is bad to connect the output of two regulated power supplies together. Sometimes what happens is one pulls most of the load until it overloads and the other takes over then switches back to the first after awhile.

I was just wondering, do your supplies really need to be regulated?
What about the heated bed?
 

Harald Kapp

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The motors can probably run off an unregulated supply.
In my opinion a good solution would use two separate power supplies:
1*12 V for the motors, possibly only a transformer, rectifier and smoothing capacitors. No regulator
1*5V for the electronics. Evaluate the power you really need and buy a cheap 5V power supply.

By having separate supplies you minimize a possible influence of the motors (voltage dips and spikes etc.) on the electronics.

Harald
 

CocaCola

Apr 7, 2012
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I see Harald tossing up some number but do we have the actual current requirements of this?

I agree with Harald that two supplies is ideal, a simply wall wart will likely be plenty for the micros, Idealy one matched to the 5V needed and void the 7805s... Even if you waste by using say a 9V wall wart and keeping the existing regulators on board isn't too bad...

For the 12V supply there are a bunch of open frame power supplies out there that work just fine into the higher current requirements...
 
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