Off topic electronics/electrics question ... sorry!

[Julian]

3D printing efforts are progressing slowly but proving very rewarding.

Three times now my creations have been thwarted by a power cut and what appears to be an over zealous RCD in the fuse box.  The computer side of things continues to work fine because it's running off a lap top with a good battery, but the power for the printer comes from the 12v side of a large ex PC power supply, which shuts down, ruining the print.

I suspect I know the answer if not the reason why, but could I run the 12v from the power supply direct to a couple of sturdy 12v batteries and then on to the printer?

If I can't, in terms a 6 month chimp could understand, what would I need to make the arrangement work.  I know UPS's exist and would do the same job, but finances won't stretch to a decent one at the moment.

[greasemonkey]

I can't see any reason why not. How many amps does the printer need? Your battery is going to need to be able to supply this.

Connecting the PSU to the batterys might be a different story, coz it's not really a charger. Someone else will have to advise about that.

Say your printer draws 10 amps, then a 100 amp/hour battery should in theory keep it going for 10 hours. If it draws 20 amp, it'll keep it going for 5 hours, and so on. In reality, it will be somewhat less than theory, but more than half. You could really do with a deep cycle battery.
It should say on the printer how many amps it draws.

Don't take my word for it though, I'd rather someone else be responsible for any damage.........

[Julian]

I can't remember exactly what current it uses.  I've got a couple of reasonably servicible Disco batteries sitting in the garage.  I'm sure they would keep the kit going long enough to reset the trip or outlast a power cut (they tend to be quite short round here.

As you say it's the connecting the battery to the power supply bit that strikes me as a bit dodgy.

[greasemonkey]

I can't imagine its pulling huge power. I would have thought it would be fine. Just run it by it's self, without any charging.
Charge it up well before use, and let it get on with it. You'll soon know if it's going flat.

[Julian]

I can't imagine its pulling huge power. I would have thought it would be fine. Just run it by it's self, without any charging.
Charge it up well before use, and let it get on with it. You'll soon know if it's going flat.

Ah, I see where you're coming from.  Only problem is this plastic printing lark is a bit like bio, you don't sit there for hours and it can be several hours if you're aiming for a quality print, you tend to set it going then leave it to it's own devises.  So you may not be there if the batteries start to go flat.  I'll have a think around it but I'd like to try and used both mains and battery backup.

[greasemonkey]

Hmmm, so it could easy be pulling 10 amp, what with the heater and everything, and five hours at that could easy drain a 100AH car battery to the point where it was struggling.
I'd say a deep cycle, plus a battery charger, but then I'm guessing it has to have a pretty clean electric supply, so a battery charger might not be giving that.
Dunno, some that knows more than me will have to suggest something. Out of ideas.

[julesandtash]

Depends on how voltage critical the printer is.
In my caravan I have a 13.8v switched mode power supply in parallel with the leisure battery. All the time the mains hookup is on, the powersupply produces output and the battery floats at 13.8v

When there is no hookup, the battery supplies power, slowly running down to somewhere near 12

The power supply is one of these....
http://www.ebay.co.uk/itm/350W-13-8V-25-3A-Switching-Power-Supply-RADIO-K30-/380836025652?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item58ab989534

I have used them in the last couple of vans I have had and they work very well

If needs be, you could always put a 12V regulator after the battery(s) and before the printer although, to be honest, I suspect it would be perfectly happy anyway

[Bill]

Or is it easier to get the RCD problem sorted out?
Solve the problem, not the effects of the problem.

[Tony]

Got to be careful with charging lead-acids, particularly where deep cycle is expected.

If they don't have much charge they will try and draw a LOT of current on recharging which can lead to damage, so have to initially be current limited until the voltage hits 13.8V.  Then they can be switched to just voltage charge (aka float charge) as your regime suggests.

Car batteries and charge system are designed to keep batteries at the mostly-charged end of the scale which is how cars they get away with just a float charge system without current limit - and why running a car battery down is bad for it, as the alternator will happily provide the battery with damaging current when it does recharge.

Having designed charge circuits for lead acids I'm not sure there is a simple way to do this.  The only way I can imagine is an intelligent lead-acid charger and then hanging your load off the battery at the same time (which isn't great as the charger won't know about it and may make incorrect assumptions about the battery, and a lot of smart chargers use a PWM pulse system for the charge profile which may interfere with your rep-rap).

Tricky one.

[Dickjotec]

Off post but how is the 3d cad going?

[Julian]

Bill ... Yes, certainly replacing the RCD is a must but it won't cover power cuts.

Jules ... how does that unit switch?  It looks for all the world like a straight forward power supply I have for the LEDs in the kitchen.

Dick ... I persevered with Sketchup for the time being.  I think the issues I've been having may be a bug in the software, but I've managed to produce some reasonable parts ...

This is a modified printer part to part embed and fix in place a little servo motor to automatically sense bed level.




And this in the same part, modifying it and adding a couple of cooling fan ducts.



I hadn't forgotten about it, but I'd like to hang on to your SolidWorks for a little longer until I'm confident in my abilities with Sketchup, if that's OK with you.  I perhaps need to try and upgrade Sketch up to see if the "bug" is solved, but in the latest version the gearwheel add-on doesn't work ... nothing is ever simple!

[Dickjotec]

3d is looking good. Keep the solid works as long as you like, when / if it comes home I will only feel the need to try it out,

[Julian]

3d is looking good. Keep the solid works as long as you like, when / if it comes home I will only feel the need to try it out,

You should ... try it out that is.  I'm an absolute novice at Sketchup, but it's incredibly rewarding drawing a set of parts intended to fit together and then printing them to find that they do!

It's a strange feeling.  Ordinarily when you want to make an item and have to hunt around to find a bit of suitable stock or perhaps weld two bits together.  With this you dream up an idea, draw it and make it.  Kind of liberating really!

One other thing, the ability to look at the back of an object is amazingly intuitive.  I find myself on the web looking at photos (not that type of photo) and trying to spin them round to get a better view.

[Dickjotec]

I use sketchup for building work and have found it very easy to use. I have not tried it on mechanical parts. I must get a 3d printer one day and have a go. I used to teach pro desktop and found that good for engineering projects but have not got a personal copy.

[Julian]

When using sketchup, have you had planes that split into unbounded triangles or shoot unbounded triangles off into space?

Let me know f you want to build a printer.  I recon with a lot of hunting around, you could put one together for around 180 quid.  I think component prices aredropping all the time.