Custom power supply - an idea

[Sariel]

See, I like to build compact & small. Yet I'm severely limited in this, primarily by the size of certain elements that can't be broken into smaller pieces. The most annoying of these are the LEGO power supplies: the battery boxes or the 8878 batteries. All of them are really huge, while providing just as much power as the small 9V battery. There is, in fact, an old LEGO box for this particular battery, but it's much bigger than the battery itself, and doesn't work with most PF elements.

So, despite my being purist and all, it eventually occurred to me that there is no reason to stick with the bulky LEGO boxes. If the power is the same, why not bypass all the needless stuff and plug directly into the 9V battery? It doesn't affect the performance of the mechanism powered this way - it's still 9V, after all - it just cuts down on the size and weight of the power source. The 6LR61 type battery is roughly 4x2x6 studs big and has a convenient, rectangular shape, and it's just as "juicy" as the LEGO power supplies, except it runs dry sooner due to lower capacity.

I asked the talented M_longer to help me carry this idea out (I don't know which end of soldering is which), and this is the result (click for full size):

The whole thing is really simple and can be easily reproduced by anyone more skilled at tinkering with electrics than myself: it's just the regular PF plug connected to a wire with a switch on it (in this case, a tiny NC contact switch), and with the 6LR61 battery plug on the other end. It's tiny and works flawlessly. M_longer says he's not going to take orders, so you're on your own with this.

Now, my goal is to use it only when necessary - I will be using standard LEGO power supplies whenever possible. Which doesn't change the fact that I already have a whole list of MOCs to use with this thingy...

[Junpei]

Will a 9v battery fit inside the two plates of this part?

[Sariel]

No.

[legomuppet9]

As a purist myself, the limits of the standard battery box do annoy me, so I'm intrigued by this solution, and may just give it a go thanks for sharing Sariel

[peter_m]

I used this idea (of cutting PF extension wire into two halves) to connect M-motor to Arduino Motor Driver Shield - works perfectly fine.

[freakwave]

Interesting approach!

have a look at this solution that "inof" used to power his Monorail:

http://www.galaxiede...php?f=20&t=1069

Picture by info

Those were cell-phone batteries that give a great flat-pack, just one brick thick. Find the construction details behind the link I posted above.

Edited June 19, 2013 by freakwave

[DLuders]

@ Sariel & others: Perhaps you could consider the various tricks for 9V batteries displayed on this previous Eurobricks topic.

[Blakbird]

While it is true that a 9V battery has a similar voltage as an 8878 battery box, it is not true that it has the same power. The problem with 9V batteries is that they have pathetic current output. If you ever take apart a 9V battery you will see that it actually has 6 tiny AAAA size 1.5V cells inside. These tiny cells just can't pass much of any current through them. This means that an unloaded M-motor will spin just as fast with a 9V battery as with 8878, but a loaded XL motor might not turn at all. For Technic models, 9V batteries are almost worthless. Jennifer Clark used them for her famous DeMag AC-50 crane for the reasons you mention (size mostly), but she had to put two in parallel to get it to move so the end result was not really a saving of overall space but rather a distribution of power from multiple smaller locations.

To get more power per unit volume you need to go to a different style of battery altogether (other than alkaline). Using lithium cells works really well because they are flat and have a high power density. You can tell from the weight of 8878 that it is mostly empty. Since this battery was made for children and since lithium cells are inherently dangerous, most of the internal volume is taken up with thermal controls and charging logic circuits. To really minimize space, you should use lithium cells from R/C hobbies with no protection circuitry (protection is in the charger). I can get enough power to drive a 4 kg R/C car at 100 kph using a battery volume no bigger than 8878. Using such a battery, you would have no trouble getting the power you need and you could adapt it easily enough to PF connectors using the same methods you showed. You would, however, need to have a dedicated lithium battery charger to charge it. These are relatively inexpensive.

[aqaz]

Blakbird:

Could you give us a product number or illustration of the Li battery you are talking about, please.

I was going to solder a PF connector onto a phone battery, it fit perfectly in a 2 plate high gap, but then realised that if I did I would be unable to charge it.

[Junpei]

Is it the same size as the phone battery in the monorail?

[Doc_Brown]

Interesting thread! I have thought about this a bit, and believe that batteries designed for RC aircraft are the way to go!

58 x 19 x 6 mm. 19.5g

2 of these flat.

http://www.helipal.c...r-heli-25c.html

Edited June 20, 2013 by Doc_Brown

[zux]

I clearly understand that LEGO is made for kids. And as we can see power sources isn't a primary item/feature for TLC. I don't know if in R/C battery world battery connectors are standardized, but I belive it would be nice to have TLC made us some "PF>3rd party battery" adapters.

[DLuders]

On this previous Eurobricks topic, one can see that a simple 9V Battery clip can be soldered onto the underside of a Lego Power Functions Extension Wire's electrified 2x2 plate, like this:

[1974]

9V batteries have a rather high internal resistance and are not good for high current needs. They work great for stuff like monorail, but I'm not sure I'd use them for Technic motors

Here's a good site/list regarding internal battery resistance :

http://www.learningaboutelectronics.com/Articles/Battery-internal-resistance

[RohanBeckett]

BIG grin at the 'Concerned Purist' fig... esp in the biohazard suit!

Personally, I remember loving the small 9V battery box... especially after growing up with the old 4.5v 3xC Cell Battery rods! they were huge!

I've only got one of the PF AAA boxes, and it's a very nice size.. ..can't afford/justify the ~AUD$100 for the LiPo one :(

Quite open the idea of hacking a cheap PF AAA box, and stuffing some Heli batteries in there - they are very cheap!

[sten]

Just a heads up if anyone is planning on using LiPO - do not run them down all the way. In order for them to charge they need to retain close to 3 V per cell. 8878 includes low voltage cutoff to prevent one from draining the batteries, but RC ones do not.

[Meatman]

I seen Paul(crowkillers) using a custom powerplug/9v contraption for his power functions motor at the Kidsfest show. I think he said that he bought the stuff at Radio Shack and it had 5 or 6 different voltage settings(I think up to 12v) and then can be unplugged from the wire lead and it had a 9v battery adapter in case he didn't have a power source. You guys come up with some great ideas.

[Blakbird]

Blakbird:

Could you give us a product number or illustration of the Li battery you are talking about, please.

There are literally hundreds of brands, sizes, and cell combinations. Just go to any R/C hobby website and pick what you like. I like to use cheapbatterypacks.com or go to a local hobby store. The specific packs I was talking about were 3 cell (11.2V) 5000 mAh packs. You can pull over 100 Amps through these.

I clearly understand that LEGO is made for kids. And as we can see power sources isn't a primary item/feature for TLC. I don't know if in R/C battery world battery connectors are standardized, but I belive it would be nice to have TLC made us some "PF>3rd party battery" adapters.

That would be nice, but I'd hardly call the R/C world standardized. Different model manufacturers have their own proprietary connectors (Traxxas, Tamiya) and there are also 3rd party connectors (Deans) that are good for high current. Many high end packs come with no connector at all because they assume the user will solder on what they like. If you don't like soldering, you can usually specify a connector and they will solder it on for you for an additional charge. Making your own R/C to PF connector would be very simple. Just watch the polarity! PF connectors are not made for high current though. No more than an amp.

Just a heads up if anyone is planning on using LiPO - do not run them down all the way. In order for them to charge they need to retain close to 3 V per cell. 8878 includes low voltage cutoff to prevent one from draining the batteries, but RC ones do not.

A very good point. In R/C use, your speed controller usually has a build in low voltage cutoff that protects the battery by shutting down the model at a predefined (or user specified) low voltage. If you were going to plug a Li-Po directly into a LEGO model, you would have to be careful not to discharge too far. If you had the initiative, you could put a low voltage cutoff in series. They are available for R/C.

[1974]

You can pull over 100 Amps through these.

I'd like to add, that this welding territory! If something goes wrong, you may easily end up with a bunch of plastic in flames - DO TAKE CARE!! (Especially indoors)

Also, LEGO will never aproach even a cheap Tamiya RC car, due to lack of bearings and structual integrity (plus proper RC control). LEGO will wear out fast with hotrodded batteries and motors. It might be better to just abandon ship and jump to _real_ RC cars. They too are somewhat modular/constructable/diy

[Junpei]

Well that would be too easy! How about using custom batteries and motors for RC Lego so there is no danger of overloading the motors and pf stuff?

Edit: Also if the wheels are connected straight to the RC motor axles, there will be no danger of wearing out the Lego (except the tyres maybe).

[1974]

You still need suspension and connecting points for the motor, even if the wheels are directly (and thus not suspended) connected to the motor. Also, if you get it to drive 50kmp you still get a lot wear on the steering wheels and all other parts. Might work for a day or two, but don't enter any RC competition. And it'll be outta (LEGO IR) reach in a split second btw

LEGO is NOT comparable to real RC cars with current parts. It never will be. TLG have to design a boatload of new parts for that to happen. And then it'll still be poorly performing and very expensive if they do (and they wont, they did have _real_ RC cars a few years ago btw. They sucked) And the real RC guys will still laugh

We are still waiting for that flying helicopter and the massive plane (wazzup Carsten?) to lift off

LEGO is not about that. We can have tons of mechanical (and electrical/pneumatic) functions in many weird variations, combined with wonderfull unique designs that you _don't_ find in real RC world. Let's explore that ... imho that is

[Phoxtane]

I don't think we're likely to see any sort of official third-party to Lego adapters. Similar to Apple products, Lego is a closed ecosystem in order to ensure everything works the way intended.

[piterx]

that's a good idea sariel!

but:

i'm 99% purist and i think that your idea could be nice if everyone could have one of those :)

as long it isn't so, it's something that only people with some experience (and tools) can have

[aqaz]

BIG grin at the 'Concerned Purist' fig... esp in the biohazard suit!

It made me laugh.... I imagine a 99% purist has the visor open.... just a little bit.

...., LEGO will never approach even a cheap Tamiya RC car, due to lack of bearings and structual integrity (plus proper RC control). LEGO will wear out fast with hotrodded batteries and motors. It might be better to just abandon ship and jump to _real_ RC cars. They too are somewhat modular/constructable/diy

I don't think that I am alone in this particular opinion; There are some things that Lego simply do not make that enhance a good MOC. This thread seems to me to be in this category.

For instance, again, personal opinion, I cannot tolerate the lack of realism of missing out windscreen or mirrors, Lego don't make them, so I make them... The MOC is none the less still an essentially Lego model... ("Purists" are happy to miss out cylinder heads!)

So my interest here is to make the power supply a little more discrete but still compatible with PF components.

Blakbird: Thank you for the link.

Doc_Brown: Helipal is very interesting... Easy to envisage some set up with these batteries...

But still at square one with making a Lego friendly holder and PF compatible adapter.

I did some more experiments with a phone battery, it fit quite snugly on edge in a one stud gap. (this is a Samsung battery, it is 5 x 4 x 1 studs. 3.7v, 800mA.) So I imagine 3 would be required to run PF...

But, how to link them in series to a PF output.

[Doc_Brown]

I think its important to note that you should only attempt to modify lipo batteries if you REALLY know what you are doing.

They are dangerous and a friend of a friend of mine DID set his house on fire!! after doing stupid things with them.

After all they recommend you charge them in fire proof bags!

For Lego applications I think they hold good potential, but only in the hands of experienced electronic hobbyist or professionals!