Nophead(Chris Palmer) has confirmed the values of resistor used to achieve regulation on ATX power supplies for the 12v Mendel 90 as being 10R and 4.7R on the 5v and 3.3v rails. Whereas RRP have used 30R and 22R. Apparently this type of supply regulates the 5v rail and other rails do not have their own regulation? The Mendel 90 electronics are different but the bed is the same, motors may be more powerful and more of them. Draw you own conclusions as to what this means.

I am sure many people will prefer the suggestion of using a 12v only regulated supply even if the mains needs a filter socket adding ideally.

Rory

Hi,
Most of ATX power only provide 5V and 3.3V which are well regulated, and 12V rail not directly regulated. PC needs accurate 5v and 3.3V for electronics but as for 12v , it just for fan or maybe build in audio amplifier etc, that's why precise regulated 12V rail not really necessary in PC.
ATX power supply can be use in most 3d printers without problem , because ATX can supply enough current and most of 3d printers already build-in with regulator for 5V for electronics, but some 3D printer PCB design might sensitive to voltage noise, fluctuation, interference etc. which will be best operate with well regulated power.
As an industrial maintainance guy for the past 20 ++ years, I still prefer regulated power supply just because of its more reliable , especially for heavy users who operate it for whole day long.
That's just my personal point of view. ATX power supply can be more easier to install than industrial one.

Edited 1 time(s). Last edit at 01/04/2014 02:00AM by tru168.

Google "Power Supply Design Guide for Desktop Platform Form Factors Intel 1.2" and this returns a useful document (probably based on an EN standard?) This Intel doc gives the following minimum loads:



Note that there is also a minimum rating for the 12V supplies, not sure what the Ormerods consumption is with the heaters off.

Treth

It would seem from your figures that the Ormorod just fails to provide the minimum load required. However it is a 550W ATX so may have different specs.

Rory

Quote
Rory166

It would seem from your figures that the Ormorod just fails to provide the minimum load required. However it is a 550W ATX so may have different specs.

Rory

Yes unfortunately, 450W was the highest spec'd part I could find in any doc. The V1.3 is very different. The minimum currents were the same for the range 220 to 450W but you are correct "what happens outside that range"?

A Legend 550W PSU [www.legend.com.au] has the same minimum currents quoted above, so that's my best details so far.

Think I might add some LEDs to illuminate the bed and use up a little more current!

Has anyone the circuit for the power pcb or know which 12V's is used?

Treth

I have traced out the power PCB myself and it is very simple, all like supplies are connected together and the resistors to ground added. There are 2 leds and link for psu enable and 5v stb. Our ATX has only one 12v rail but the two rail type might not appreciate being so connected, just beware if replacing the ATX.

I like your idea of connecting LEDs to the unused 3.3v rail. We could design an LED holding strip which would slide into the unused extrusion slots, fit multicolored LEDs and create the fairground version of Ormerod! Some suitable cheesy organ music playing on the computer speakers would add to the spectacle.

Rory

Edited 2 time(s). Last edit at 01/04/2014 07:09AM by Rory166.

Minimum load isn't really the issue (although it may shut down if they are not observed), it's about cross regulation and load balancing. There is only one regulator in a typical ATX PSU. They rely on the voltage rails being related by the transformer secondary turns ratio plus some magnetic magic done with a smoothing choke that has windings for all the rails on one core which helps them track each other under different loads.

If the 12V rail is more loaded than the other two rails (relative to their max specs) then it will be low and the other rails will be high. The more load you put on the other rails the more you get from 12V. On a typical Reprap machine the regulation of the 12V rail is not important because everything it powers has some form of regulation downstream but you can't afford it to be too low if you want to hit the ABS bed temperature. The values I picked are a compromise between the 12V being too high when the heaters and motors are off and too low when they are on. I also used 15W AL clad so that they don't get dangerously hot with only a Dibond panel for a heatsink.

---

[www.hydraraptor.blogspot.com]

Nophead

What is the internet version of the we are not worthy salute. Whatever it is I am doing it now.

Clearly you picked sensible values for your resistors, and RRP are using much higher values, I would like to think this is down to their having thoroughly researched and tested their choice.

It does seem a bit of a compromise all this power supply business. The ATX can be very cheap and has a mains socket and filter built in. For about 25 quid one can get a regulated 12 supply but need a box and mains filter ideally. I must have some 12v psus used for powering Amateur Radio transmitters kicking about somwhere , come to think of it they may not be regulated and may give out more than the nominal 12v to replicate a charging car battery.

Rory

Edited 1 time(s). Last edit at 01/04/2014 08:04AM by Rory166.

What would be nice would be a sweet little PSU that slides in under the front of the machine under the print bed. The separate box with the ATX PSU is a PITA.

Only saying But if anyone knows of one that would fit...

regards
Andy

---

Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

I wasn't involved in choosing the resistor values, as my electronics knowledge is not that great. However, I think these figures are correct:
nophead's load resistors:
5 volts, 10 ohms = 0.5A, 2.5W
3.3 volts, 4.7 ohms = 0.7A, 2.3W

RepRapPro load resistors:
5 volts, 30 ohms = 0.17A, 0.83W
3.3 volts, 22 ohms = 0.15A, 0.495W

Ideal as per spec from Intel (for 450W ATX PSU):
5 volts, 0.2A = 25 ohm, 1W
3.3 volts, 0.1A = 33 ohm, 0.33W

So perhaps the 5 volt load resistor needs to be a bit lower ohms - I'll consult our engineers. But you can also see what the problem is - the heat (Watts) these things produce. As nophead says in his post, he uses a heatsink and dibond ground plan to keep them cool, and others use active cooling, e.g. a fan. I believe our values were chosen, like nophead, as compromise between heat and power output.

Regarding the choice of an ATX PSU over a dedicated 12V PSU, I have already posted on another thread about our PSUs from our Mendel and Huxley kits. We wanted to make the wiring as simple as possible, hence the hot end, proximity sensor and motor looms - in our experience it is the MOST difficult part of the build, for most customers. And wiring up a PSU is a nerve-inducing moment for many kit builders. We also wanted to remove any soldering from the kit. The ATX PSU does all this, gives a power switch and a sensible connector built-in, for half the price, and we can get the printer CE approved, too.

Ian
RepRapPro tech support

Edited 2 time(s). Last edit at 01/04/2014 09:42AM by droftarts.

Ian

Your maths looks spot on to me. As you say CE approval must be a big plus.

Rory

Quote
kwikius
What would be nice would be a sweet little PSU that slides in under the front of the machine under the print bed. The separate box with the ATX PSU is a PITA.

Only saying But if anyone knows of one that would fit...

regards
Andy

Once you move away from ATX form factors then costs start to rise.
Meanwell do a 12V 29A that could be of interest. Here is a link for reference [uk.mouser.com] at £60 then there is VAT and delivery, plus it is meant to be enclosed.......

@Rory, a "Fairground Ormerod"! Go for it!!!!

Somewhat less expensive is this range. I've recently ordered the 25A version.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

If you want a cheap but beefy PSU for DIY use (and mains safe) try a second hand server PSU, e.g. [www.ebay.co.uk]. The fan on these is a bit noisy but can be fixed with an internal mod.

I also have a 750W unit which gives 62A (!) and the fan is speed adjustable, only £15 delivered.

---

What is Open Source?
What is Open Source Hardware?
Open Source in a nutshell: the Four Freedoms
CC BY-NC is not an Open Source license