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4550Re: [ScoutRadio] Blinker Morse App (USB charger)

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  • Robert Bruninga
    Aug 7, 2014
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      My design approach is to look for 6V panels.  Then shunt-regulate to 5v with a zener and single power transistor so there is 100% efficiency at full load.

      My unknowns are how USB chargers work.  Does each one just try to draw MAX rated current?  I asssmue they gracefully handle any voltage drop?  Then, I'd just run raw (dioded isolated) power from the single 5.7v shunt regulated bus.  An LED on the 5.7v bus with cut-out at 5.2 volts woiuld indicate if charge is still available.

      Still, 2 foot solar panels still cost near $100.  Though going really DIY, the big 6" square cells are only what ? $2 each? and probably 16 are needed (to give about 8v unregulated) and about 6 amps for a 2' square panel.  THen just cover it with a piece of glass.  DOne

      Bob, WB4APR


      On Thu, Aug 7, 2014 at 2:09 PM, n5gui@... [ScoutRadio] <ScoutRadio@yahoogroups.com> wrote:


      I am glad my previous comments sparked the idea of a Solar USB charger for Scouting events.  If it continues much more, then perhaps it would need its own thread.


      Last year I was an advisor for a crew at Philmont.  As part of my gear, I had a camera that needed a charger for the batteries.  Being a retired engineer I quickly determined that it was not practical to bring charged batteries for the entire trek, so I got a second battery and a 12 volt charger.  I tested the charger for current draw ( less than 200 ma ) and minimum supply voltage ( less than 6 volts ), and from that I put together a solar panel and AAA battery pack that would allow me recharge the camera batteries.  Simple weight comparison, and the fact that I was using a headlamp and backup flashlight that needed AAA cells so that the spares were common, brought the data that I needed for the final decision.  After coming home, I found that I could easily have left half my spare AAAs in base camp.


      With that as my recent experience, I am going to suggest an engineering approach for those considering this as a project.  ( Besides, exposing the Scouts to engineering is one of the four legs of the STEM program. )  The USB ports may be rated for more than one ampere, but is that really what you need?  First decide if you plan to use the port exclusively to charge the devices or if you need to be able to run devices on external power.  Find out what charging current is needed, both peak and average.  You might find that you can charge three devices with less current that it takes to operate one.  And you may find that the first part of the charging process draws more current, like 400 ma tapering to 100 ma in the first hour dropping to 50 ma at full charge after three hours.  That suggests you might need to stagger the times you start charging various devices.


      There are lots of design features that you might want to consider, such as separate regulation circuitry for each port,  isolation diodes (if one device draws down the power source, the other devices would stop charging, but they will not be discharged by back flow to the troubled device), and current limiting the individual ports.  There are high efficiency regulators that might serve well in a project like this:  A solar panel in full sun might provide 1000 ma at 19 volts. Ten 100 ma charging circuits that output 5.1 volts using linear regulators dissipate 13.9 watts in heat for 5.1 watts delivered to the load.  More efficient regulators might charge more devices.


      A "smart" charging circuit might monitor each device's current demand so that it can divide the available power.  The process of deciding how to use the resources could be allocated would be as much of an engineering lesson as the hardware system design.


      I have a set of solar panels, so I am familiar with how they work, but I would suggest you research what you can get in your area rather than try to more of what I have.  I bought them in sets of 3 and each panel is about 1 foot by 3 feet and puts out more than 1 amp at 19 volts in full sun.  I usually wire OR them to a deep cycle battery to power a transceiver, but that doesn't seem like a good starting configuration for a USB device charger.


      From that you might be able to get a rough idea what you would need for each patrol in your unit to build the charger.  It might also let you decide if the project should be worked as a troop project instead.


      James

       n5gui






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