I was plagued by the myriad cables that were running to my telescope from the laptop. I had the cable for the DSLR, a cable for the guide-head and a serial cable (from a USB->serial dongle). It was time to clean things up. I found an inexpensive 4 port USB 2.0 hub and a 5m USB 5 high-speed cable. The hub could either be powered from the USB bus or from a 5v 'wallwart' that came with it. As I don't have mains out at the dark site, and everything is powered by my car I thought that a stable 5v power supply for the hub would be a good idea.
I had a bag of LM340AT-5.0 positive voltage regulators so I grabbed one and a few other spare parts and put together a small power supply. Now, the regulator I'm using is rated at 2.4A continuous (the 78xx series are 1A parts in TO-220). I was pulling about 400mA with a voltage drop of 8.1v so the regulator got pretty hot. With a big heatsink it got VERY hot to the touch; time for another idea.
There is a 'Simple Switcher' single-chip switching power supply available, part number LM2576. This seemed to be the way of the future for the 5v power supply. The datasheet says that it's 77% efficient and will handle 3A of current. It is a 5-pin TO-220 part. I picked one up.
The hardest part to come by on this build was the 100uH inductor. Nobody had one but I found an old AT power supply that had 2 coils on the 5v outputs. I pulled them and started building.
Parts List
- 1 x LM2576-5 5v positive Simple Switcher
- 1 x 1N5401 diodes
- 1 x 100uF 16v electrolytic capacitor
- 1 x 1000uF 10v electrolytic capacitor
- 1 x 1N5822 Schottky-barrier diode
- 1 x 100uH inductor
- A small piece of perf board or a home-made PCB
On the first prototype the LM2576 was getting fairly warm and I had a heatsink on the chip. I think this was due to a bad solder joint on the Schottky diode. This is fixed that on this board and the LM2576 JUST gets sensibly warm with a 400mA load; maybe a couple of degrees C over ambient. You'll notice all the hot-glue; it's ugly, but it works really well for strain-relief and to hold things down.
If your hub uses a different voltage you can use the adjustable voltage LM2576 part. Google the datasheet for more information
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