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Circuits designed by David Johnson, P.E.
Last Updated on: Monday, July 11, 2016 01:59 PM

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Solar Circuits

24v NiMH Battery Charger Using a 12v Solar Panel   -  A while back I received an email from a guy in the UK.  He wanted to use a small solar panel, designed to charge 12v lead acid batteries, but use it to charge a 24v battery pack.  His 24v battery pack used 20 AA size NiMH cells.  He went on to explain that his 20 cell NiMH battery pack was used very infrequently . . . [Hobby Circuit designed by David A. Johnson P.E.,  10/12/11]


8-Solar Cell Lithium Ion Battery Charger Version 2  -  Jim Wilber looked at my design for a battery charger and sent me some suggested changes.  He suggested using the LTC4412 ideal diode IC to switch off the current path from the solar cell to the battery in darkness.  He also noted that the 2.5v regulator from Seiko I had selected was no longer in production and suggested using the shunt reference . . . [Circuit by David Johnson P.E.,  05/21/13]


Broken Solar Panels  -  As illustrated below, one way to salvage a large number of broken solar cells is to wire them in parallel, not in the more conventional series connection. Broken cells of various sizes and shapes are grouped together into panels of about one quarter to on . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/01/12]

Cell Phone Solar Powered Charger  -  The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas Instruments . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/02/11]


Convert Solar Powered Light to Flasher  -  The circuit converts an off-the-shelf solar powered LED path light from producing a steady state light to one that produces a flash.  This circuit includes a tiny dual Schmitt trigger inverter IC which form the pulse generator and the daylight sensor circuit.  The circuit produces a positive pulse, which feeds the constant current LED driver circuit. . . . [Circuit by David A. Johnson P.E.,  03/02/10]


Eight-Solar Cell Lithium Ion Battery Charger Version 3  -  A single lithium ion battery cell needs 4.2v from a current source to insure it is fully charged.  A solar panel with just enough individual cells wired in series to produce the needed voltage would be more efficient than one with more cells for a given surface area.  If you divide 4.2 by eight you come up with a figure of 0.525 volts . . . [Circuit by David Johnson P.E.,  05/21/13]


Ideal Blocking Diode Circuit for Photovoltaic Solar Panels  -  Most photovoltaic solar panels are used to charge a battery during the daytime. Nearly all panels come equipped with a blocking diode.  The diode prevents DC current from flowing backwards from the battery bank into the panel at night.  The usual blocking . . . [Circuit by Dave Johnson P.E.,  03/06/11]


Ideal Diode for PV Panels  -  Solar panels usually include a diode at their outputs.  The diode prevents current from back feeding into the panel at night from the battery being charged by the panel.  But, this diode does waste some power and does reduce the available panel voltage . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/06/11]


Ideal Diode Function  -  This circuit uses a low power op Amp and a p-channel FET to form a diode function with a very low 0.05 volt voltage drop.  With the selected FET, the circuit can handle up to 2 Amps of current.  Higher currents are possible with a FET with a lower channel . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/22/08]


Improved Solar LED Spot Light  -  Nearly all the Chinese made solar powered LED lights I see being sold at Lowe's and Home Depot home improvement centers are poorly designed.  Their solar panels and battery packs are all undersized.  The light from their Chinese made LEDs fade rapidly and the devices never make it through a complete night before the light dies . . . [Circuit by David A. Johnson P.E.,  02/03/13]


Improved Solar Path Light  -  The circuit below was designed to replace the guts of an existing path light assembly with one which has a much better performance.  I suggest using a quality 10 cell solar panel with at least a 4 square inch area. Buy a panel with solar panels that fills . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/13/11]


Modified Cheap Solar LED Path Light  -  Modification of a typical solar path light with a standard 1000ma-hour NiMH AAA cell.  I then replaced the small 4 cell solar panel with a larger 4 cell unit, which produced 150ma in the sun.  Next, I simply changed the 470uH coil on the circuit board to a 22uH coil.  The lower inductance increased the battery current to 40ma when the LED light was on, which translated to about 13ma LED current.  The light was much brighter . . . [Circuit by David Johnson P.E.,  10/26/12]


Photovoltaic Solar Panels-Ideal Blocking Diode Circuit  -  Most photovoltaic solar panels are used to charge a battery during the daytime. Nearly all panels come equipped with a blocking diode.  The diode prevents DC current from flowing backwards from the battery bank into the panel at night.  The usual blocking . . . [Hobby Circuit designed by David Johnson P.E.,  03/06/11]


PV Solar Cell Test Fixture  -  This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/01/06]


PV Solar Panels-Ideal Blocking Diode Circuit  -  Solar panels usually include a diode at their outputs.  The diode prevents current from back feeding into the panel at night from the battery being charged by the panel.  But, this diode does waste some power and does reduce the available panel voltage . . . [Hobby Circuit designed by David Johnson P.E.,  03/06/11]


Reduce voltage to Photovoltaic Solar Panel with Ideal Diode  -  Solar panels usually include a diode at their outputs.  The diode prevents current from back feeding into the panel at night from the battery being charged by the panel.  But, this diode does waste some power and does reduce the available panel voltage . . . [Circuit by David Johnson P.E.,  03/06/11]


Safety Light  -  My 1200 square foot metal shop only has one small entrance door and no skylights.  If the lights are turn off while inside, there is not enough light to safely find the door to exit the building.  On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking . . . [Hobby Circuit designed by David Johnson P.E.,  11/29/04]


Salvaged Broken Solar Cells  -  As illustrated below, one way to salvage a large number of broken solar cells is to wire them in parallel, not in the more conventional series connection. Broken cells of various sizes and shapes are grouped together into panels of about one quarter to on . . . [Hobby Circuit designed by David Johnson P.E.,  01/01/12]


Shop Safety Light  -  My 1200 square foot metal shop only has one small entrance door and no skylights.  If the lights are turn off while inside, there is not enough light to safely find the door to exit the building.  On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking . . . [Hobby Circuit designed by Dave Johnson P.E.,  11/29/04]


Solar Cell Test Fixture  -  If you are considering building your own solar panel, you might try building the solar cell tester shown below. When constructing a solar panel it is important to make sure that all the solar cells used in the panel be closely matched for their short circuit . . . [Hobby Circuit designed by David Johnson P.E.,  11/25/07]


Solar Insulation Monitor  -  Owners of photovoltaic solar panels often like to monitor how bright the sunshine is. They can use this information to gage how well their solar panels are doing. On a typical clear day, the sun energy at sea level is about 1000 watts per square meter . . . [Hobby Circuit designed by Dave Johnson P.E.,  05/01/12]


Solar Lithium Ion Battery Charger Using LT1129  -  I have been designing a number of solar powered devices lately.  Many of them use just a single 3.6v lithium ion battery.  These batteries like to be charged from a fixed 4.2v source with a charging current limited to about C/8, where C is the milliamp-hour rating of the part . . . [Circuit by Dave Johnson P.E.,  12/30/12]


Solar Path Light Improvement  -  Use the circuit below was designed to replace the guts of an existing path light assembly with one which has a much better performance.  I suggest using a quality 10 cell solar panel with at least a 4 square inch area. Buy a panel with solar panels that . . . [Circuit by David Johnson P.E.,  03/13/11]


Solar Powered Cell Phone Charger  -  The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas Instruments . . . [Circuit by David Johnson P.E.,  01/02/11]


Solar Powered Flood Light Using NiMH Cells  -  There are many solar powered garden lights available.  Most of the devices offered at the home improvement stores have very poor performance. Their batteries are too small, the light launched is weak and the solar panels are undersized. None of the devices I have tested would operate through a typical night . . . [Circuit by David Johnson P.E.,  12/20/13]


Solar Powered Infrared 1W Security Camera Illuminator  -  Many security cameras are equipped with an array of infrared LEDs to illuminate an area in front of the camera with invisible light for night time surveillance.  However, there are times when a remote area, well away from the camera, needs additional infrared light.  The circuit below uses a modified solar powered path light to bathe a fairly large area with 850nm infrared light . . . [Circuit by David Johnson P.E.,  06/22/14]


Solar Powered Lithium Ion Battery Charger  -  The circuit below feeds a controlled current and voltage to a 3.6v lithium ion battery. The current is limited to 300ma and the voltage is limited to 4.2 volts. The circuit uses a LTC1734 IC from Linear Technology. No diode is needed between the circuit a . . . [Hobby Circuit designed by David A. Johnson P.E.,  05/15/07]


Solar PV Cell Test Fixture  -  This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions . . . [Circuit by David Johnson P.E.,  10/01/06]


Solar-Powered Pump Motor Controller  -  A Discover Circuits visitor had a problem.  He needed a simple on/off controller for his solar powered water pump.  His system used two 12v 50 watt solar panels wired in series.  The power from the solar panels drove a submerged water pump.  However, during . . . [Circuit by David A. Johnson P.E.,  10/03/10]


Variable Isolated AC Voltage Spans 0Vac to 280Vac  -  I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel . . . [Circuit by Dave Johnson P.E.,  07/09/00]


Water Pump controlled by Solar Power  -  A Discover Circuits visitor had a problem.  He needed a simple on/off controller for his solar powered water pump.  His system used two 12v 50 watt solar panels wired in series.  The power from the solar panels drove a submerged water pump.  However, during . . . [Hobby Circuit designed by David A. Johnson P.E.,  10/03/10]


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