Power regulation for solar+laptop

[dpopiz]

Some people on my local craigslist are selling some reasonably priced photovoltaic panels. I'd like to build a device to power my laptop outdoors. The AC adapter uses about 30W when running and not charging.



My engineering question is this:
Although I haven't done any measurement, I'm sure the laptop will only USE power on its input when it's above a certain voltage and current threshold. But the power coming from the panel will be fluctuating and uncertain. So I need a circuit that stores power from the panel until it has enough to supply the laptop for at least a few seconds and then lets the power flow.

So I'm thinking connect solar panel directly to a capacitor and then the cap to some sort of power control IC that opens a switch when the cap has enough charge to meet the laptop's threshold. I know nothing about power ICs and I'd appreciate an introduction.

I also think I could maybe just do away with the the power IC..... because the laptop has it's own power circuitry right? Maybe it doesn't really care what volts/current it's supplied with and will just accept power whenever there is enough coming in.

 

[ZeroNine8]

Find a relatively inexpensive DC/DC converter that takes unregulated DC input and regulates it to whatever voltage your laptop requires. I found some of these on google that have an output that you can plug a cigarette lighter adapter for your laptop into for ~$70.

 

[bobsmith1492]

What you'd really want is a DC/DC converter that can take a broad input range, say, 6-16VDC like the panel may produce, and output the voltage you need.

You could build one easily enough; Linear sells a huge series of switching regulators that require just a few external parts. They even have a CAD program (LTSpice) that designs the circuit for you, picks all the parts, and simulates the circuit if you tell it what input and output voltages you need. You can get it here: http://www.linear.com/designtools/software/switchercad.jsp

I ran the program and the LTC1871 could do 1.6A, 20V out at 6V input and up to 2.2A at 8V input; basically it would give your 30W output anywhere from 6V out of your solar cell and up while allowing for much more power at higher voltages (you could get just about as much as your AC adapter, 68W, with 12V from the panel). Grab the program and take a look!

You'll also also need a diode to prevent leakage back through the cell but this would be just wasting energy. This could be avoided using a properly connected MOSFET, though; I stuck one in the schematic from LTSpice to show how.

One last thing: make sure the solar panels can give you that much power to start with; I don't know if the laptop will control itself to consume only as much power as the supply will give or if it will just draw full load in which case things will break down (not dangerously, mind you... just not working-wise). You'd want at least 40W+ rated panels, especially if you're getting them used as they degrade over time to less than their rating.

Here's a screenie of the schematic (M2 is the reverse polarity protection FET - I swear it won't really dissipate that much power; per LTspice, it's acting like it has an RDSon of .23 ohms when it really is .02, so I don't know what it's problem is; that'll also push the theoretical efficiency up over 95%): http://i4.photobucket.com/albums/y125/bob_smith1492/SolarSupply.png Also, the diode will need a small heatsink (no big deal).

All told, this thing should run probably about 18 bucks for parts (all available from Digi Key) plus whatever you can find for a box. Good luck; I can give you pointers if you actually want to build it. You could probably make it on a perfboard; it's "only" 300KHz switching speed; my friend made a class-D amp switching at 500-750KHz on a breadboard, so this should be doable with some care.

 

[piasabird]

If you go to the link below for Homepower Magazine you may find some interesting articles and some links to stites that sell different kinds of solar technology from sinewave converters to batteries and Solar Panels.

http://www.homepower.com/

Some devices out there incorporate a self contained system that inetgrates a Battery, a solar panel, a changer, and a voltage converter/sinewave A/C converter into a single package. For safety you might be better off with the AC Adapter.

You are more likely to find a cheap Solar Laptop Changer or maybe a Laptop Battery Extender like a Battery Pack that is flat and goes underneath the laptop.

 

[wwswimming]

linear technology
http://www.linear.com/pc/viewCategory.jsp

used to have wonderful data books, haven't looked
at one in a while.

anyway they make a boatload of different devices
(for various categories of power conversion) and
the databooks have schematics.

 

[AgentJean]

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