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FT-817 Solar Charger

With the purchase of a new FT-817 comes the desire to take it out in the field.  Having a way to charge the batteries while in the field would make this rig even better.  So I set out to do just that.

Being cheap thrifty, I really did not want to spend a lot of money on a solar panel.  I managed to grab three new "5-Watt Solar Powered Trickle Charger" from Woot!, my favorite on-line geek store, for $15 each.  They are rated for a maximum of 350 mA at 15 volts, which in theory, could completely charge the stock FT-817 NiMH battery during a single sunny day.  In reality, it might take a bit longer than that, but for the money, who really cares?

Out of the box, the no-load voltage of the panel is about 24 volts, or potentially enough to fry the FT-817.  What's needed is a way to reduce the voltage to around 14 volts without being too inefficient.  I first tried a linear regulator but that was not very effective in lower light situations.  Next I tried a switching regulator.  This was better, but it was still not as efficient as I would have liked.

Finally, it occured to me that if I hooked the panel cells directly to the rig, there would be no conversion loss.  Of course, I needed to somehow limit the voltage going to the FT-817 to below 16 volts, and preferably around 14 volts.  To solve the excessive voltage problem, I built this simple circuit, which is essentially a very simple shunt regulator.

The circuit consists of a couple of 10 watt, 6.8 volt Zener diodes to clamp the voltage to 13.6 volts and a simple way to monitor the output voltage.  I used extra-heavy duty Zener diodes as a means to protect my FT-817.  If one of them should open up due to too much current, the full 24 volts of the panel could be applied to the rig with disastrous consequences.  Even at full current (350 mA), the Zener diodes would need to dissipate a little over two watts, well below the 10 watt rating.

Installing the rather large Zener diodes required milling down some of the internal plastic ribs of the frame.  This was easily done with a Dremel tool.  With a little planning, there is enough room to fit the rest of the components within the bottom of the frame.

A small rubber grommet provides strain relief at the point where the power cable leaves the frame.  A small piece of heat shrink tubing was used to enlarge the cable enough to provide a snug fit to the inside of the grommet.  A drop of "super glue" on the inside of the grommet finishes the assembly.

One thing missing from the stock panel is an LED power indicator.  This is easily added to the front panel with the help of a small drill and a little hot glue.  The LED will not fully illuminate until both Zener diodes are conducting.  In the above case, this will occur at approximately 13.6 volts, which is a sufficient level to allow the FT-817 to charge the internal NiMH batteries.

Purists may notice the lack of a blocking diode in the above circuit.  This was intentional as I wanted to eliminate the 0.7 volt forward voltage drop of a silicon diode.  Without the diode, the reverse current of the FT-817 through the panel is about 60 microamps.  This level of leakage is not likely to cause any significant degradation of the normal battery life.

One abnormally with the FT-817 timed charger is that it will draw current from the batteries when the external voltage falls below approximately 12 volts.  At this point, the charging circuit will actually discharge the batteries even though the display and orange LED show that it is charging!  This problem does not occur when the radio is turned off and is in the "trickle charge" mode.