Clanking Replicator Project

 

Getting an affordable power supply for Tommelise

A few things about electricity and Tommelise and safety

Tommelise uses 12 volt direct current (DC) power. This grew out of the original RepRap specification that a 3D printer for everybody anywhere on the planet should be able to run off of a standard 12 volt car battery. This particular specification has a number of wonderful features. First and foremost is that it is nearly impossible to hurt yourself with 12 volt direct current power. Given enough amperage you might strike a spark and burn yourself if you are not careful, Mostly though, if you accidently complete the circuit between the 12 volt line and ground you won't even notice that you've done it, never mind hurt yourself. This technology is inherently pretty foolproof.

Mind, you could conceivably mash a finger by getting it caught in one of the positioning axes. The gearmotors, albeit small, pack a lot of torque. You could as well burn your hand on the heated extruder barrel of the Mk 2.1 extruder. That device runs at about 12-15 watts and never more than 24 though, which means that you'll get burns no more serious than if you picked up a low wattage soldering iron by the wrong end. Indeed, soldering irons operate at considerably higher temperatures than the Mk 2.1 extruder, so you probably won't get hurt nearly as bad.

The bottom line is that Tommelise is a far, far safer device to work with and around than virtually any powered tool that you will find in a typical garage workshop. I'll get into the sorts of fail-safe features that I have built into the Tommelise 3D printer in another document.

Probably the cheapest and most reliable power supply for 12 volt direct current (DC) power for a 3D printer like Tommelise can be had in the form of a common PC ATX power module. If there is an outdated PC lying around your house that you no longer use you already have your power supply for free. If not, you can pick up a shiny new one for $15-25 from any computer parts discount site on the Web. Mind, you can pay a lot more than that for an ATX power supply, but it is not at all necessary that you do so.

Getting a power supply for free

If you have an obsolete PC it is a simple matter to unplug it and remove the detachable power cord and then unscrew and remove the cover of PC's enclosure. If you look where the fan exhausts out of the back of the PC chassis you will see a little grey box about 6 x 6 x 4 inches that contains the exhaust fan. That is the power supply. Disconnect (don't cut) the various cables from that box to the motherboard of the PC, the hard disk(s) and other units that require 5v and 12v power.

Once you've got it disconnected, unscrew it from the chassis of the PC and you have your power supply. They typically look something like this.

Here you see the back side, the part that faces into the inside of the PC enclosure. It is unusual in that it has two fans, one to draw air in from the inside of the PC enclosure and another on the other side of the power supply to expel it into the room. Just below the fan you can see two bundles of wires; one wrapped in nylon mesh and the other just a bundled set of yellow, black and red wires.

Now you see the side of the PC ATX power supply that is exposed to the room on the back side of your PC enclosure. I've circled the on/off switch in yellow and the power cord receptacle in blue. Many cheaper power supplies have a volltage supply switch which allows you to switch the power supply from 110-120 volts input to 220-240 volts input. Since you are in the US this button should be set to 110-120 volts. More expensive PC ATX power supplies like mine, not that it's that expensive, often lack this voltage set switch because they have more sophisticated circuitry inside that automatically detects the incoming voltage. Typically, if you don't see one of these little switches you have an automated voltage sensing unit. The little red circle denotes the fan speed switch. It is a triple pole switch which has a high, medium and low setting. These little slide switches tend to be cheaply made so that if you do not have the switch firmly set on one of the three positions the fans won't operate. I've bumped my power supply several times and caused the fan switch to slip a little resulted in the cooling fans not coming on when I powered it up. If the fans don't come on in your power supply, you should always switch it off immediately and then check the fan switch to make sure it hasn't slipped a bit. This will save you a bit of anxiety. Mind, I've thought of epoxying or taping mine down to one position. I haven't got around to actually doing that, though.

This one I bought new for $24.95. It is rated at 400 watts and can deliver 25 amps of 12 volt (actually about 12.4 v) power. This quite small PC ATX power supply actually supplies vastly more current that Tommelise will ever require. I doubt that Tommelise will ever draw more that 5-6 amps of 12 volt electricity at most.

WARNING!

DO NOT OPEN THE LITTLE GREY BOX! There is nothing in there that need concern you and if you do open it and start poking around one of four things can happen.

  1. Nothing.
  2. You may not be able to put it back together again.
  3. You may break it.
  4. You may short out one of the very large capacitors in it and either burn or kill yourself.

Item #4 is the one that I worry about for your sake. PC power supplies are designed to provide steady electrical power to your PC in spite of the vagarities of the electricity they get out of the power point in your wall. To do that they use several quite large capacitors that look like big cylindrical firecrackers. Those store electricity when there is a lot and feed it back into the circuits when there isn't. The ones in your PC power supply are quite large and can stay charged with electricity after you've unplugged your PC from the wall sometimes for several hours or longer. Just because a PC has been unplugged does not mean that it can no longer electrocute or burn you.

The power supply is locked up in that little grey box for a very good reason ... your safety.

Making it run outside of your obsolete PC

ATX power supplies have one little thing that can make them a bit unsafe to run outside of your PC. They are rigged to where a thermostat in the PC turns the cooling fans on and off as they are needed. If you just plug one in you will get the 12 volt power you need out of the power supply but the cooling fans won't work.

There is a simple way to fix this. Look over the electrical connectors coming out of the back side of your power supply till you find one with a green wire going to one of the connectors like this.

 

Cut a piece of single conductor wire a few inches long, strip the insulation from both ends and either fold over the protruding wire to thicken it or tin it with a bit of solder with your soldering iron so that it is a bit thicker. Use that piece of wire to jump the GREEN wire which ordinarily goes to the thermostat with one of the many BLACK wires, which are ground wires. That shorts out the thermostat loop and turns the cooling fans on which keeps the power supply cool. Please make the connection while the power supply is turned OFF!

For reference, the wires coming out of an ATX power supply are color coded

  • Black: Ground
  • Red: +5V
  • Yellow: 12V
  • Orange: +3.3V
  • White: -5V
  • Blue: -12V
  • Gray: power on indicator
  • Purple: standby power output
  • Green: power on input

I've highlighted the colours of the wires you will need for Tommelise. You can safely ignore the rest.

NOTE

These instructions apply only to ATX type power supplies. Converting other types, and including the ATX 2 power supply can be a bit more involved. Mind, ATX Switching Power Supplies are far and away the most common ones being sold by discount computer supply websites in the US.

Powering your Tommelise

Tommelise needs 12 volt power to run. To supply that you will need one of the YELLOW lines (12 volt) and one of BLACK lines (ground) for this.

Shorting out your power supply: a cautionary tale

There will come a time when you cross one of the live DC voltage lines, usually one of the YELLOW (12 volt wires) or RED (5 volt wires) with one of your BLACK (ground wires). This can happen in a variety of situations. You can cross lines directly or accidently wire up your controller or one of your sensor boards incorrectly or accidently short out your extruder barrel. Trust me, I've done almost all of these things.

What usually happens after a bit of sparking is that the fans in your power supply go off and you lose power. Since I'm of Scots-Irish ancestry and as tight-fisted as they come with money, my heart gives a little flutter and I think I've destroyed my power supply. In fact, I've found that if I switch the ATX power supply off for a few minutes then switch it back on, without the short condition still being in effect, mind, I find to my immense relief that in fact I still have a working power supply.

This is not to say that you can not short out and ruin an ATX power supply. I'm sure there must be some way to do it. However, I haven't managed to so far, knock on wood. Given how clumsy and accident-prone I am that speaks well of the ATX power supply that I bought and possibly even for ATX power supplies in general.

Alternatives

There are two alternative sources of 12 volt power that you can use for Tommelise.

Regulated Power Supplies

You can get one of these for 12-13 volts rated at the amperage you need (~6 amps) for anywhere from about $75 at a place like Radio Shack to $200-300 from a traditional electronics supply house.

Wall Warts

These are the little black power bricks that you can get for things like laptops. They're called wall warts because the smaller ones plug directly into the 110 -120 volt power point on your wall. I've found that finding one for 12 volts and 6 amps rather difficult and that the ones that I have found typically are meant for laptops and can cost as much as $75.