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I've built several of these and they work great. The LEGO NXT charger is 9 VAC, 800 mA. The maximum current draw when charging an NXT, however, is actually only 700 mA. I found a 3400 mA power "brick" that has a 6ft AC power cord on one side and a 6ft cord on the other (9V) side. "All you have to do" is to cut off the existing charging connector and replace it with multiple charging cable assemblies. Here are the parts I used:
I really hate power bricks / wall warts / whatever-you-call them. I've got a classroom with 16 NXT kits and the prospect of having 16 power bricks plugged into 5 or 6 power strips really bothered me. So, I came up with an alternative, pictured at right. Besides being more convenient, they're also a lot cheaper. My three 6-NXT chargers cost me $221, which comes out to about half of the cost of the official LEGO chargers. I used the difference to buy 9797 education kits with extra light sensors instead of the charging transformers.
If you want to build a charger like this, you'll need to find a source for a high-current 10 V DC power brick - and be careful about polarity.
WARNING: In 2010, LEGO Education switched to using batteries that charge with DC rather than AC. You can easily tell the difference: the older W979798 batteries (AC charge, discussed here) are dark gray. The newer W979693 batteries (DC charge) are white.
Allied Electronics Stancor STA-5790 transformer
Input: 120 VAC
Output: 9 VAC, 3400 mA
$27.01 ea
These things are heavy - a couple of pounds. FedEx Ground shipping is about half of the cost of UPS (from Allied, at least). This will save you about $5.
Again, this is an AC transformer - not for use with the newer, white, DC-charging NXT batteries.
Digi-Key CP-2185-ND Cable Assembly
2.1MM power connector
6' long cable, 24 AWG
$1.74 ea ($1.51 for 10 or more)
These were in-stock the last time I ordered them; just now they had a one-month lead time. Order early!
In the remainder of this article, I'll refer to this as an "NXT cable", since "small cable assembly" gets tiring.
Heat-shrink tubing. For each transformer, you'll need two small 1-inch pieces to cover each of the solder joints, and one bigger 2-inch piece to cover the whole thing.
(click to enlarge)
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(picture coming soon)
Five or Six-Headed Hydra?
If you're being careful, you would attach five cables - 5 x 700 mA ⇒ 3500 mA. That's 100 mA over the stated capacity of the transformer, but I'm not worried about it. In my situation, I have 18 NXT bricks, of which two are spares. I decided to attach six cables to each of three transformers, on the assumptions that (a) I'd rarely be charging 6 battery packs from empty on each transformer and (b) that the transformers have a bit of extra capacity anyway, and they'll have adequate cooling. I've never noticed them getting the slightest bit warm. WRONG!
Well, they didn't get warm, but apparently charging 6 completely-drained NXTs is beyond the limit of the 5-amp fuse built into the Stancor transformer. It'll be 5 heads from now on.
Cable Trimming and Wire Stripping
If you add up all of those cord lengths, you could have up to 18 feet from the power outlet to the charging plugs. I'm a careful guy, but even I can see that that's way more than anyone could possibly need - or untangle. So, I cut a foot or so off of the 9V transformer cable, and kept only 18-24 inches or so of the NXT cables.
After you've cut off the excess cable, split the two wires apart and cut 3/4" from one of the them – on both the transformer and the NXT cables. This offsets the two solder joints so that (a) they don't make a huge lump and (b) there's less chance of a short, since the bare part of the wires aren't anywhere near each other. Strip all wires to expose 1/2" of bare wire.
VERY IMPORTANT: You might think that, because this is AC current, you don't have to worry about polarity. You don't, but you still have to wire the NXT cables consistently, so that of the outer shells of the charging connectors are all connected to the same side of the transformer. If you don't, then when two oppositely-wired outer shells touch, you'll get a short-circuit and, after a while, a yucky burning smell, and then maybe worse. Be very careful about this. It's not hard, you just have to be attentive to detail.
The above image shows the trimmed and stripped wires. On the left is the (large) cable from the transformer. On the right is one of the NXT cables. Notice that the top wire has a faint white stripe on it. That's how you tell which wire is which. If you make sure to cut the 3/4" off of the striped wire on all of the NXT cables, you'll be fine.
If you've got a pair of good wire cutters, it's easiest to cut the striped side before peeling the two wires apart.
Notice the stripe?
The short length of wire is then easy to peel off.
Here are six cables, ready to go. Except... there's one problem. Can you spot it?
After you've got all of the wires trimmed and stripped, I recommend taking an ohmmeter and testing to be sure that the center and shell of the connectors are connected to the wires you expect. Over the course of building 9 of these things, I actually found one bad NXT cable. I'm very glad I tested them all. (Digi-Key was great about it - they shipped me a replacement very quickly.)
Soldering and Heat-Shrink Tubing
One of the Most Important Things to remember are to slide the big heat-shrink tubing onto the transformer cable first, and to slide the small heat-shrink tubing onto the long wire that you're going to solder. It is really annoying to have a perfectly-soldered connection that you have to take apart to put on the heat-shrink tubing you forgot the first time around.
I like to solder the short ends of the NXT cables first. Tie them together tightly with a twist-tie after stacking them on top of each other. This makes them much easier to control when twisting the wires together.
Bend the long wires out of the way.
Twist the wires together. Make sure to put the heat-shrink tubing on the longer transformer wire first! (Of course, you already put the large heat-shrink tubing further up on the transformer cable, right?)
When the heat-shrink tubing is this close to the joint, you need to do the soldering quickly, or the tubing will shrink early. It doesn't hurt to peel the wires back a little further if you're unsure about your soldering skills.
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