Voltage regulator

Introduction

The XO is designed for children in developing nations, kids who may not live with easy access to a reliable power grid they can charge their laptops from. Most laptops need a very precise voltage and amperage into them to have them charge properly (and not explode). This is like only being able to eat purple-colored fruit - you can survive, but it's somewhat inconvenient compared to being able to eat fruit of any color.

The ability to plug any power source (a solar panel, a diesel generator, a car battery, etc.) into a computer and have it "just work" would be awfully handy, and greatly expands the types of conditions under which the XO can be used. While the XO is already designed to accept a range of power inputs (11-20V), anything that can be done to extend that range would help; in particular, some types of power sources a child might want to use (human power, animal power, car batteries, solar panels) can fall outside that range or have current spikes greater than the XO's power circuitry can handle.

We're building a voltage regulator that will help people using off-grid power peripherals for the XO. It's a device that sits between the laptop and a power source, takes in whatever random power input it's given (within limits - but very broad limits), and gives you two things: an 18V, 0.944A output to charge your XO by, and an "overflow" output of excess wattage which can be dumped into a battery, a motor, or (in the default worst case) a dissipator/heatsink. (Hey, it's better than frying your laptop.) To mangle the earlier analogy, it's like a magic hat that enables you to eat red, green, and blue fruit instead of just purple fruit when you put it on.

Requirements

Input

We used this information on possible power input to guide out input requirements for the voltage regulator:

Attribute	Bicycle Power [David Gordon Wilson. "Bicycling Science: Third Edition" The MIT Press. Cambridge, MA. 2004]	Cow Power [1]	Solar Power [2]	Car Battery
Voltage	~15V steady, 60V max	12V steady	14-17V steady	12.7V steady
Current	20A max	10A steady	0.35-3.6A steady	Adjustable
Power	40-200W steady, 400W max	~120W steady	5-60W steady	-

Steady State Requirements

• 17-18V
• 10A

Design Requirements

• 60V
• 20A

Output

• (single) XO power input - ideal is 18V at 0.944A. (Laptop can accept 17W maximum, 11-25V and 2Amax but lower current = lower resistance losses.)
• "overflow" wattage should get switched out to another place - by default some sort of dissipator (whonkin' power resistors with a big heatsink?) that's easily bypassed in favor of something useful like a battery.

Parts

Current recommended

The current recommended voltage regulator is a L200.

• Digikey: 497-1382-5-ND ($1.92 USD each when purchased individually)
• datasheet (pdf)
• application notes (pdf)

Benefits

• can handle up to 60V, which is around the surge you'd expect when you start up a human-powered source (for instance, the spike from a bike-power generator when you start to pedal)

Open questions

• what is its behavior at spikes over 60V? constant input?