power supply circuit

Posted on Feb 7, 2014

The SheevaPlug is reputed to have a marginal power supply. It seems to be more of an issue for the people in the UK running at 240VAC, and it also seems to be a problem to load the USB too heavily, as happens if you want to add an external mechanical disk drive. Add to that, the fact that I live in a part of the world where the power is pretty unr

power supply circuit
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eliable in the winter. Storms knock down trees and they take the power lines with them. This project attempts to kill three birds with one stone: The premise of the circuit is simple. The SheevaPlug, USB hub, and laptop hard drive are all designed to operate on 5VDC. The circuit utilizes a DC-DC converter to provide +5VDC for the whole system. The DC-DC converter is designed to take input power in the range of 11-24VDC and convert it to a constant 5VDC output. The only bit of trickiness is to arrange for two separate sources of power to feed the DC-DC converter. The main source comes from 120VAC house wiring through a standard laptop power adaptor. The backup source is from a 12V lead-acid battery. The laptop charger was chosen for easy availability, efficiency, small physical size, and a voltage output of at least 18 VDC. I got a replacement charger for an HP Mini 1000/1100 netbook. It is a 30W adaptor that puts out 19VDC at 1. 6A. It uses a 4. 0mm connector with a 1. 7mm center post, where GND is on the barrel, and V+ is on the center pin. I got a replacement HP Mini 1000 adaptor on eBay for under $10, shipping included. The battery backup power arrives in the form of 12VDC from a lead-acid battery. Even with a charger attached, the voltage across the battery is less than 18 VDC. I will be using a high-quality charger that does not overcharge the battery. In practical terms, this means that you can leave the battery on...

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