Posted in Gear on September 26th, 2008 by Northwest Astro
For those who have started to accumulate bits of gear for long-exposure photography via a laptop, and are running out of ports, usually the first thing you consider as a fix is a hub. What if powering the hub over USB using the laptop isn’t enough? Then you are in trouble because you can’t just go out and buy the ‘car adapter’ for your USB hub. Doesn’t mean you can’t make one though.
In my case, I came into a Belkin USB 2.0 4-Port Hub on sale, and I liked the design that it had two USB ports on the side, and two on the top. It would help me later on if I decided to put the hub and a couple other USB devices into a single hobby box. Building a ‘hub’ which instead exposed 2 USB ports, a Serial port for scope control, and an autoguiding port for long-exposure adjustments could be an interesting side project down the road.
The problem is that this Hub will be expected to be used in the field, with all 4 ports in use. Total current would be very close if not a bit over the 500mA limit the laptop is allowed to provide. Also, this sort of drain would shorten the battery life of the laptop in a noticeable way. Instead, why not try to power it with 12V batteries and power tanks commonly being used by amateur astronomers to power their gear?
First, you need to track down a 12VDC car adapter at the right voltage and amperage rating for your purposes. For a USB hub, it is safer to get a high-amperage car adapter with a fuse, instead of trying to go for something more common but rated for lower current use. I found this High Power Car Charger for under 10$ and makes a good target for a quick DIY project. It is rated for up to 2A, and when it arrived, I found out that it also uses a 250V/1.5A fast-blow fuse in the plug. Perfect for my uses, and easy to modify because it already provides the right voltage to the output, it just needs the right plug.
To do the mod with this particular hub and charger (other hubs may be a bit different on the plug it accepts), you will need one more part from Radioshack: a size H DC plug. If you are attempting to power a different 5V hub, then you will probably want to get the outer diameter (OD) and inner diameter (ID) measurements from the manual, or take the AC adapter with you to a Radioshack to get the right part. Read on for the beginner’s Do-It-Yourself instructions.
This Do-It-Yourself is actually fairly easy, since we are just removing the old power plug that comes with the adapter, and soldering on the DC plug from Radioshack. You will need the usual gear for soldering wires, but you can probably avoid using heat-shrink tubing in this mod if you are careful enough.
The first step is removing the old plug by cutting it off at the base, and removing about 1/8″ to 1/4″ of the plastic cover, exposing the wires. Also, we will need to cut the third wire (looks a bit like a garbage tie) down the same amount. It helps the cord keep its shape, and will just get in the way when soldering the new tip into place.
Before you solder the plug on, you will want to do a couple things. First, we need to check if the plug for the hub is expected to be positive or negative for the tip and make note of it. You normally can find it on the hub itself near the power port, or the AC adapter that comes with it. Second, we should remove the outer shield for the plug and widen the opening a bit. The shield around this power cable is rather thick, and it is easier to widen the plug to accept it than to ’shave’ the shield down to fit.
Now, we want to slide the outer shield for the plug onto the cable, away from the exposed wires. Down maybe a couple inches if possible. We then want to strip the exposed wire for the tip, knowing that with this cable, black is negative, red is positive. Don’t strip very far (1/16″ is enough), or we could introduce shorts in the finished cable and we have to start over. Solder the wire into place, using the hole on the mounting tab to help hold some of the strands on the other side as well, the solder will then help lock everything into place and ensure a good join. For the outside connection, we want to cut a bit off the wire first, then strip away 1/16″ again. The two joints are at different distances from the plug, which makes this a bit tricky, but otherwise is the same as soldering for the tip.
Hub with Completed Power Cable Attached
One thing I should mention at this point is to not use too much solder. It will make it hard or impossible to get the plug’s outer shield into position. If you use too much, use a bit of copper-braid solder wick to remove the excess. Other tools will probably remove too much, while this will let you keep just enough solder on the joint to hold it firm.
Once the solder has cooled, you should notice that you may have melted the shielding around each wire a little bit. This is actually okay as long as you haven’t been hovering over the work closely in a poorly ventilated area. There should be no exposed wire strands that can come into contact with the other strands and cause a short. If any strands escaped becoming part of the solder joint, you can pluck them with tweezers or a fine pair of needle-nose pliers.
Now you should be able to move the outer shield for the plug up and screw it into position. If you did everything right, it should (with effort) make it into position, and the cable’s outer shield should not have any areas where you can see the wires underneath (cracks in the shielding being visible are fine). Now, with care, plug it into your power tank or other 12VDC source with a cigarette port, and then plug it into the hub, it should come to life.
The cable tends to hold fast to hits coiled shape, and will come unplugged from the hub under very little pressure. So you will want to keep distances to the hub from your power supply fairly short, or put together an extension cable of some kind. The upside of this Belkin hub is the cable guide seen in the image, which helps keep tension off the power plug itself, and will let the cable stretch a little bit, but I wouldn’t recommend putting it under tension during normal use.
