DIY 12V supply for the budget-conscious

I was just looking at some Youtube vids of a DIY spot welder, and thought the same technique could be used to make an analogue DIY 12V (or higher) PSU very cheaply. It uses a transformer from a discarded microwave oven, which commonly have their primary and secondary windings separated. In the oven it is a step-up transformer (and very lethal) to supply the magnetron with a few thousand volts, but by hacking off the secondary winding completely (whilst leaving the primary intact), you can wind a few turns of thick wire in the gap created by the missing secondary to make a high current low voltage transformer capable of delivering 800Watts or so (Look at [www.youtube.com] to see what I mean). That's more than 60 amps at 12V! By the looks of things you'll get about 1 volt per turn, so you only need to wind about 10 turns of thick insulated copper wire (thick automotive wire should work fine) - you can easily experiment to get the desired output voltage. You might as well choose the thickest wire that will physically fit into the gap to get the lowest voltage drop (and least heat). Then you'll need a full wave bridge rectifier capable of handling the current and a smoothing capacitor, though I shouldn't think it needs to be all that smooth as the regulator will iron out the low spots for the electronics, and I don't think the stepper motors are that fussy (BICBW), so a 1000uF 25V capacitor would probably be adequate (that's a guess). The rectifier in the microwave oven is not suitable, as it will be a high voltage but relatively low current device. You won't be drawing anything like the rated power from the transformer, so it should run reasonably cool. You could probably print the parts necessary to make a reasonable enclosure to mount the PSU components and a switched mains inlet socket (or switch & cable) perhaps also salvaged from the microwave oven, but keep an eye on its operating temperature to make sure nothing gets hot enough to soften the plastic, make the walls thick so it won't break and expose mains wiring if knocked - and position ventilation slots so that metal objects (and fingers) cannot fall onto or touch any exposed wiring.

Obviously anything connected to the mains can be dangerous, so I would not advise anyone to try it unless they have reasonable experience with electrical work and understand how to make a safe design.

Dave
(#106)

Hi Dave, those things can be fun (if you REALLY know what you're doing!) I don't think it will work too well as you describe it though - the 15A draw with the heaters on alone will deplete 1000uF way before the next half cycle of 50/60Hz AC. (around 1ms) 20,000uF as a minimum I'd think. Also it would be in desperate need of a current limiter to prevent any unintentional Arc Welding. Managing surge currents and heat in the 40A rectifiers you'd be using could be a little tricky. Maybe not as easy at it sounds!

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RS Components Reprap Ormerod No. 481

I haven't looked at the schematic for the power here but if it was just powering the heater then a simple transformer rectifier supply would suffice, the caps have no purpose in a heating circuit, a simple heatsink with a meaty rectifier would mean no fan requirement and high reliability.

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Ormerod No 128 the half Byte

Quote
Mickyblueeyes
I haven't looked at the schematic for the power here but if it was just powering the heater then a simple transformer rectifier supply would suffice, the caps have no purpose in a heating circuit, a simple heatsink with a meaty rectifier would mean no fan requirement and high reliability.

Oh yes, if it was just the heaters and the +12V connections to these were separated from the motor supply then any rectified mains around 12~14V RMS (i.e. car battery charger) would do. However, the "holy grail" for many of us is a single cheap, safe (i.e. pre-wired for mains connection) PSU that holds up well under load. The ATX supply is tantalizingly close to that but just leaves us a couple of volts shy of reaching the higher temperatures required for ABS.

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RS Components Reprap Ormerod No. 481

I was looking only at a cheap and viable way of powering the Ormerod adequately rather than a high quality smooth regulated PSU. The Ormerod does not require a regulated 12V supply. Regarding the smoothing, it will be powering the fan, the stepper motors and possibly the Duet board's electronics. I therefore feel that we might get away with an extremely rough output. I have very little experience with steppers, and do not know how well they would cope with a rough input - presumably their constant current drivers will compensate until the voltage is insufficient to maintain the set current, and I have no idea what that voltage might be. It will be powering the Duet if there is no USB connected, but in that case it feeds a 5V regulator & smoothing circuit that may well be able to handle a very rough input. My idea was to provide a bit more than 12V average output so the bed heater gets up to temperature quicker - I understand the Duet can handle up to 35V. If it is much more than 12V, you'd need to fit a small 3-terminal 12V regulator in the fan supply - but that would not present a problem, just hang it on the feed wires inside a sleeve.

If smoothing proves to be an issue you could use a big smoothing capacitor - RS sell a 22000uF 63V cap for under £3. Inrush current then becomes an issue, so the circuit complexity must increase to cope with that. One way is to put a low value resistor in series with the secondary that is shorted out by a slow-acting relay (e.g. coil of relay fed via a simple RC delay), though there are several other solutions, such as fitting a suitable NTC thermistor in series with the mains input. It still ends up being a very cheap and rugged high power PSU.

Dave
(#106)

Why not use a second hand server PSU? 60 A @ 12V for £9.99 delivered? [www.ebay.co.uk]

It is by far the best value for money, and has safe mains wiring. It's very easy to jumper the control pins to turn it on. All that needs doing is soldering wires to 12V outputs, it's not worth trying to find a mating connector.

There are loads of 30A units (ESP113, PS-3381) which are easy to use as well, similar or cheaper prices.

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Quote
bobc
Why not use a second hand server PSU? 60 A @ 12V for £9.99 delivered? [www.ebay.co.uk]

It is by far the best value for money, and has safe mains wiring. It's very easy to jumper the control pins to turn it on. All that needs doing is soldering wires to 12V outputs, it's not worth trying to find a mating connector.

There are loads of 30A units (ESP113, PS-3381) which are easy to use as well, similar or cheaper prices.

That's what I'm already using - we have a store-room full of them. The issue is that with a regulated 12V supply the bed takes over 30 minutes to get to ABS printing temperature in a warm room, and it won't get there at all in an ambient below 17 deg C or so, so I'd like a PSU that puts out a couple of volts more. I expect I could find a way of adjusting the voltage on the PC supply, but having a completely DIY supply appeals to the hobbyist in me. I was also thinking of those people who live in countries where new PSUs are not as cheap (relatively speaking) as they are in the UK, but recycling junked household appliances is just as feasible.

Dave
(#106)

The problem with using a home-made non-switching PSU is that you will need a very large smoothing capacitor. With both heaters going, you would need about 25000uF to keep the ripple down to 6V. The steppers should be OK with this ripple, but the fan probably wouldn't be. A better way would be to run the two heaters from unsmoothed DC straight from the bridge rectifier, then use a diode and a smaller capacitor to get smoothed DC for the fan, steppers and switching regulator.

I recently bought a new 12V 300W switching PSU for £21.80 on eBay UK, so I won't be going down this route.

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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].

I might adopt this power supply idea in to my carry case. Thank you. You've given me more ideas.

Kim..

We've been trapped in this loop for a while now. Even El~Cheapo ATX supplies are fine for Ormorod's motors and feed to the 5V regulator but the 12V just doesn't quite it for the heated bed element. I think the hot end is quite usable as well..

If the heated bed PCB design was tweaked to perform at 10V most of us would be happy bunnies.

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RS Components Reprap Ormerod No. 481

The Ormerod appears to use a standard (Mk2?) PCB heatbed, it should take less than 10 minutes to reach 100 deg. If it doesn't, then there is something wrong with it, or the wiring. There are duff heatbeds around with resistance too high because they were not made properly, the resistance is quite critical. It should be between 1 to 1.2 ohms.

I would fix the heatbed before considering DIY power supplies.

It would be better to use 24V for a heatbed, but I would not try that with the 12V heatbed. There is a Mk2b version designed to run at 24V.

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I think the ATX PSUs supplied are entirely adequate for PLA printing. For those printing ABS, 12V from a better-regulated ATX PSU is sufficient (see [forums.reprap.org]), although 13V from a 12V adjustable supply such as the one I mentioned a few posts ago will heat the bed up faster. But yes, if the heater resistance were a little lower, then even cheap ATX PSUs would be good enough for ABS.

Edited 2 time(s). Last edit at 01/31/2014 07:57AM by dc42.

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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].

Lots of people in RepRap community use the Mk2 heatbed, most of them use 12V PSU, and many of them print ABS. There is lots of experience with this setup.

Experience shows it works fine, but you need a *good* quality PSU, and the el cheapo ATX PSUs are generally not good enough. Their ability to deliver enough current on the 12V is too far below spec.

I don't really know why RepRapPro would ship a setup that doesn't work well for ABS, apart from cutting corners on the cost. They should really know better.

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Quote
bobc
Lots of people in RepRap community use the Mk2 heatbed, most of them use 12V PSU, and many of them print ABS. There is lots of experience with this setup..

I'm using my own PC PSU (the RRP one was DOA) and printing ABS. With all heaters on the voltage measured at the Duet input is 11.72V - slightly low but not excessively so. I print ABS fine BUT have to wait a heck of a long time for the bed to reach 100 deg C and have the ambient >17deg or so (which suits me anyway, so that's not an issue). I put an insulating pad over the bed to improve heating time, but it still takes over 30 minutes. The only way I can see to improve the heating time is to either raise the voltage or change the heater PCB to one with a lower resistance. Lowering the temperature of the first layer to 90 deg (which I print subsequent layers) makes the first layer slightly more problematic to get right, and false starts can take up more time than waiting for the extra 10 degrees to be reached.

Dave
(#106)

OK, so we probably all know this but it's worth spelling out: The reason it's so persnickety is that It's a square law thing because Power=I*I*R.

So with a fixed heater resistance of 1R:

Vsupply => Power in bed
@10V => 100W
@11V => 121W
@12V => 144W
@13V => 169W
...and of course @14.14V => 200W

With a 9.16% drop in voltage from 12V to 11V the power in the bed drops by 16%

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RS Components Reprap Ormerod No. 481

I also print solely ABS, using the shipped PSU (alpine I think). It is a little slow to get up to 100°C, but it seems quite adequate in my climate (UK "winter" with or without heating).

I'm not sure that there's much advantage to it getting much past 100°C for ABS. Sometimes it would go higher than 100°C , generally it wouldn't according to the thermistor that is... until I recently rebuilt my bed, and I'm getting higher temperatures now (it reports reaching 120°C today). I guess I've put the thermistor in a place that gets hotter, or it's in better contact with the heater.
The temperature at the thermistor's position isn't going to be the surface temperature of the glass, unless you find a place that has similar heat loss to the surface (since the heat is being radiated and convected from that surface,or conducted into a print once underway, and glass is far from being an ideal conductor. I haven't repeated my surface temperature measurement using an external thermocouple, but I did do that before rebuilding the bed (with the thermocouple covered in insulation) and the two temperatures at that time agreed (my hunch is that the thermocouple will now report lower temperatures).

On the power/voltage/temperature thing, my bed resistance measures around 1.2ohm at 20°C, calculating the resistance at 100°C I come up with a prediction of around 1.6ohms presuming that all the resistance in the bed is due to copper (my multimeter pretty much agrees but isn't reliable for measuring such low resistances - the short circuit resistance across the probes is 0.2 ohms). Presuming that the PSU COULD produce 12V without droop, then the heater would be drawing 10amps at 20°C but only 7.5A @100°C - ie the heat it produces diminishes with temperature from 120W when cold to only 90W when it's up to temperature - unfortunately, at the higher temperature differential to ambient, there's a much greater rate of heat loss (both through conduction to the bed support, convection from air gaps under the heater, and radiation/convection from the glass surface). As an aside, I'd imagine that the regulation of the supplied PSU would lead to a higher voltage at the higher temperature (since the current draw is significantly lower, but I haven't measured this).

Dave successfully speeds up his heating time by limiting convection from the upper surface using an insulator (a habit I've picked up too!), it's worth also bearing in mind that the bed support itself requires good insulation (preventing both conduction and convection) sealing the ends of the cardboard' flutes, or replacing with another low thermal mass insulator with closed cells would help - particularly if people replace the mdf "support" with aluminium which will sink some of the heat if not well insulated from the heater.

Ray

Quote
rayhicks
I also print solely ABS, using the shipped PSU (alpine I think). It is a little slow to get up to 100°C, but it seems quite adequate in my climate (UK "winter" with or without heating).
Ray

What time does your printer take to get to 100, Ray? Mine takes just under 35 minutes from 22 deg ambient - and my previous measurements under the glass show that the Duet's temperature reading is reasonably accurate - although as that temperature setting was arrived at by trial-and-error to get a good first layer and no warping, it really doesn't matter whether it is accurate at the surface of the glass - whatever it really is is what I need! I am now using the 12mm thick white foam square that was part of the original PSU packaging to insulate the bed as it heats (it's practically the same size as the bed). As said, voltage at Duet is 11.7V. My multimeter reads the bed heater's resistance as exactly 1 ohm, but like yourself I do not trust the accuracy at such low resistances. I haven't measured the voltage at the bed. Somewhere in our production area is a milliohm meter (used to locate shorts on PCB tracks)which I might try, though knowing the exact resistance is not really going to get me any further. I had not realised that the heater is ordinary copper PCB - is there a layout image anywhere? I could try making a new one with slightly thicker tracks - it should be a simple matter to calculate how much to increase to get, say, a 15% increase in power.

Dave
(#106)

Hi Dave- I haven't really timed it, I guess around 20-30minutes (time enough for some slicing, a cuppa and a fag before checking Z height anyway .I'm on my kindle now so not so easy to post links etc, but search reprapwiki for pcb heater [edit, I love my kindle, but typing on it is like typing in boxing gloves - here's the link from a computer [reprap.org]] to get descriptions and schematics (as well as caveats) on the various models.
Cheers
Ray

Edited 2 time(s). Last edit at 01/31/2014 02:21PM by rayhicks.

I posted some voltage measurements using the Alpine PSU at [forums.reprap.org], some more with a better ATX PSU at [forums.reprap.org] and some timings (again with the better PSU) at [forums.reprap.org]. The timings were done without covering the bed. Using the better ATX PSU, it took 13 min to reach 100C indicated.

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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].

Don't know if anyone had seen this but for those using the the Dell 12v server supply you can tweak the voltage a little higher (not a lot but maybe enough to help).

https://sites.google.com/site/tjinguytech/my-projects/server-ps

This thread suggests an external nod for 12.5v and a way to quite the fan.

http://www.rcgroups.com/forums/showthread.php?t=1750411

I have not swapped my supply from that provided in the kit but am researching it all now.

Dan.

Edited 1 time(s). Last edit at 02/06/2014 01:18PM by FalseIdle.

I purchased a 12v 300W PSU for £21.80 from [www.ebay.co.uk] and this is also adjustable. I've printed a box to cover the live terminals and house the mains inlet connector/fuse/switch, neon, and outlet connector (all sourced from Maplins).

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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].

There is loads of great info on converting server PSUs on the rcgroups forums, they cover a lot of models, although not all of them.

I've found for models that don't have adjustable fan speed, the Zalman Fanmate is perfect because it provides 5-11V which ensures the PSU always turns on, and is also easy to fit by swapping a couple of connectors.

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Quote
dc42
I purchased a 12v 300W PSU for £21.80 from [www.ebay.co.uk] and this is also adjustable. I've printed a box to cover the live terminals and house the mains inlet connector/fuse/switch, neon, and outlet connector (all sourced from Maplins).

PSUbox.jpg

I have one of these psus, which I use for charging my model aircraft batteies. I put mine in a big enclosure but that looks like a much neater solution!
I would get another of these but I am only printing PLA at the moment.

30 minutes for the bed to heat up does sound a lot though. On mine I used the packing foam that came with the Ormerod (used for the glass plate) and stuck it under the cardboard to provide extra insulation

To raise the temperature quicker why not add some device above the bed. Could be just another blanket of foam/corrugated cardboard or an active device with a heater in it? (You could then double up using another PSU while its heating)


regards
Andy

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

Quote
kwikius
To raise the temperature quicker why not add some device above the bed. Could be just another blanket of foam/corrugated cardboard or an active device with a heater in it? (You could then double up using another PSU while its heating)

regards
Andy

That's what I already do - using the foam from the PSU packing as you have done.

Dave
(#106)

With all this talk of PSUs, can someone explain whether a new PSU 12V can go straight in to the duet pcb or does it need to go into the small pcb with the multiple wires and if so is there a wiring scheme somewhere please?

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Regards

Chris H

The 12V from the PSU can go straight into the Duet terminal block. Be sure to get the polarity right, or you'll wreck the Duet.

Edited 1 time(s). Last edit at 02/08/2014 06:41PM by dc42.

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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].

Ignore - trying to see why I'm not seeing page 2 or last message on this topic