"Don't you remember how hot it was last year?" Dev reminded me. "We need something to cool us down. A misting system will cool us down. Spraying water will reduce the temperature by 10 degrees!" Doug suddenly exclaimed. This is how it all began when we met up for the 2009 Burning Man event. Three friends and I had an amazing experience in 2008 and decided to take another trip to that harsh desert environment. We vowed to make living conditions better than last time, so we started planning early to make sure we were comfortable in the Black Rock City Desert, 100 miles north of Reno, Nevada, USA.
What we were craving for a little comfort was a cooling solution based on a water mist system to overcome the hot, dry air that plagues this desert. This could be done with a pump powered by a voltage source connected to a spray hose with a nozzle. The key to a successful mist system is a power source that can also be used to power LED lights for nighttime illumination or to charge other external devices that require power. Our plan was to use solar panels to charge a deep cycle battery for marine use, and then use this battery to power everything else. I then began designing a solar panel battery charger.
I had three weeks to complete the design. I asked my friend Simon for help. Simon had done solar powered designs before using Linear Technology ICs . Simon gave me a schematic along with a prototype that showed how it worked. This prototype had never been tested with a solar panel , but had been simulated in the lab. I was excited and interested in testing this design with a real solar panel, and we prepared to give the prototype a proper test.
A friend loaned me two BP solar panels (BP380U). Each panel has a peak power of 80W at a maximum output voltage of about 20V and a maximum output current of 4A ( actual specs are 17.6V at 4.55A at 80W maximum power). Combined, I wanted a total current of 8A under peak conditions when the sun was shining directly on the panels. Within minutes of connecting the solar panels to Simon's prototype, the system was fully operational (Figures 1 and 2). Initial testing of the prototype identified several faults that later saved us a lot of time.
Figure 1: Testing BP solar panel, BP380U (0 to 20V output, 4A peak power 80W)
Figure 2: Initial solar charging circuit prototype, using 12V marine deep cycle batteries
The prototype worked well enough that I purchased a couple of Linear Technology demo boards and modified them slightly to better fit the redesigned system specifications. I kept the prototype as a backup and reference while I designed a new system. These modifications improved the original prototype as we worked out some glitches. In short, the architecture remains the same: a 0 to 20V solar panel charging a 12V battery at a constant current of 4A.
Solar battery charger system design
After a few days of playing around with these demo boards, I was able to complete a design that produced the desired results and would be suitable for our trip. A block diagram of the system is shown in Figure 3, which shows some of the ICs and demo board functionality. A photo of the system is shown in Figure 4, which shows the completed solar panel battery charger unit.
Figure 3: System design block diagram
Figure 4: Final solar charger circuit
The solar panel's initial output voltage varies between 0V and 20V, depending on the position of the sun, so a regulator is needed that can accept this wide output range and keep the current draw low (4A maximum input current per panel) while regulating a fixed output voltage. This is achieved on the DC1198A-B demo board using Linear Technology's μModule DC/DC buck-boost switching regulator, the LTM4607.
The LTM4607 is a small LGA package (15mm x 15mm x 2.8mm) chip that includes all the supporting control components required for a complex buck-boost DC/DC switching regulator. The complex switching control circuitry and FETs are built into the micromodule regulator, making the device very easy to use. The result is a clean and regular layout with only a micromodule regulator, inductor , and a few capacitors and resistors. The wide input voltage range of 4.5V to 36V to a fixed 20V output (range 0.8V to 24V) is just right for the characteristics of solar panels (0 to 20V output), and the device can load up to 5A in boost mode and 10A in buck mode. At peak solar panel power, the efficiency of 20V input to 20V/2.5A output is 91%, and the benefits of the wide input range of the buck-boost are actively utilized. For the purposes of this system design, the output is regulated to 20V and is used to power the LTC1435/LT1620 high efficiency, low dropout battery charger system.
The LTC1435/LT1620 demo board (DC133A) controls the charge current to a regulated 4A at a regulated 14V. The demo board is similar to the application circuit on the first page of the LT1620 datasheet, except that I replaced the FB resistor (110k) with a variable potentiometer to achieve output voltage regulation and set the battery float voltage to 14V. The demo board design utilizes the LT1620 rail-to-rail current sense amplifier, combined with the high efficiency and low dropout capability of the LTC1435 switching regulator circuit, to create a battery charger with over 95% efficiency, requiring only a 0.5V input-to-output voltage differential at 4A charge current. A programming current to ground sets the battery charge current (4A), which is regulated until the battery voltage reaches the preset float voltage (14V in this case). As the battery reaches its full charge state, the circuit is programmed to automatically switch to a trickle charge state and slowly reduce the charge current with respect to the battery's output voltage. This reduces the stress on the battery caused by constant overcharge.
An ideal diode circuit design is placed in series with the output of the DC133A charging system, using the LTC4414 for circuit protection and allowing the battery to be used while the charging circuit is running with minimal losses. This automatic PowerPath? control allows external devices to be powered freely from the solar panel or the battery. When the solar panel power is insufficient, the circuit automatically switches to draw power from the battery. The circuit design is similar to Figure 2 on page 9 of the LTC4414 data sheet. The LTC4414 (8-lead MSOP package) controls an external P-channel MOSFET to produce a near ideal diode function for power switching. This allows multiple power sources to be "OR"ed efficiently; in this case, the power sources are the solar panel and the battery. When an external device is connected, the battery and charging system accept the load state. When there is no load, the battery will be charged. Therefore, the design allows the solar panel and battery power to be used together, and the battery charging process to run simultaneously. There is no demo board available for this part, so I designed it according to the application circuit on a custom circuit board.
The current sensing system is connected in series with the battery and uses a parallel sense resistor to measure the input charging current and output discharging current of the battery without disconnecting the circuit. The block diagram of Figure 3 only illustrates the input charging current. The LTC6103 (available in an 8-lead MSOP package and operating from 4V to 60V) is a dual independent current sense amplifier that monitors current through external sense resistors. The device measures and provides a current ratiometric output of the battery charging and discharging current in mV. In this case, it helps indicate how much the battery is charged and discharged. This is a way to read the current with low power loss, which is critical to maintaining an energy-efficient system. I slightly modified the LTC6103 (DC1116A) demo board to achieve this. Pins 8 and 7 are in series with the current path into the battery, +IN_A and -IN_A, respectively. This will provide the charging current into the battery. Pins 6 and 5 are swapped and connected in reverse to measure the battery discharge current path, with +IN_B (pin 5) connected to -IN_A (pin 7) and -IN_B (pin 6) connected to +IN_A (pin 8). The resistor values are changed and adjusted in multiples of 10 so that the output changes by 100mV/A when a 0.1Ω shunt sense resistor is connected in series with the circuit. The multimeter in Figure 5 shows the results of the entire system. The solar panel output voltage is 17.11V, the battery voltage is 12.95V, and the charging current is 3.58A.
Figure 5: The multimeter shows 17.11V solar panel output, 12.95V battery charging voltage, and 3.58A battery charging current.
ADC and micro controller readings
I decided not to use a voltmeter every time I checked if the circuit was functioning properly, as voltmeters are difficult to carry in the desert. To avoid carrying multiple multimeters, I used a microcontroller and ADC to read the voltage values of the system and display the information on a regular LCD display . This method provides real-time data on the circuit performance without having to connect several multimeters.
I used the DC590B demo board and the LTC2418 8-channel/16-channel 24-bit ADC demo board DC571A. My colleague Mark Thoren gave me the Embedded source code sample for the PIC microcontroller, which I fine-tuned to sample the voltages across the different channels of the ADC on the LTC2418 and accurately read the voltage values in the mV range with acceptable resolution. Since the maximum range of the reference voltage is 2.5V, I used a voltage divider method to scale the voltage down to the mV range for proper measurement on the ADC. The channels are connected to a single input and output voltage of interest, including the current sense voltage. This worked very well and eliminated the need for multiple multimeters. Figure 6 is an example of a fully functional system with this LCD display. The final display I got on the LCD provides information about the following voltages: the varying solar source voltage Vs, the charging circuit voltage Vc, the battery voltage Vb, and the input charge/discharge currents C and D on the battery. In this case, it is "C" which is charging. When discharging, the program will change to "D".
Figure 6: LCD readings: Vs (solar panel voltage); Vc (charging circuit voltage); Vb (battery voltage); C = charging current (4.3A),
Controlled with a DC590B PIC microcontroller; voltage read with an LTC2418 Demo Board DC571 ADC powered by an LTM4601 Demo Board DC1041A uModule step-down regulator.
Note that the DC590B demo board is not powered by a 12V rail, but rather a 5V rail. A buck regulator is needed to step down the voltage from the battery's 12V to 5V. This buck regulator will have to be highly efficient because the power will come from the solar panel and the battery, and I don't want to drain a lot of power running the LCD display and microcontroller. I used the LTM4601 uModule DC/DC Switching Regulator Demo Board DC1041A.
The LTM4601 is a 15mm x 15mm x 2.8mm LGA packaged micromodule DC/DC switching regulator with an input of 4.5V to 20V and an output of 0.6V to 5V at a maximum load current of 12A. The design of the LTM4601 makes it very easy to provide a regulated 5V output from a 12V battery. The micromodule includes all control support components such as resistors, capacitors, MOSFETs and inductors. In this system, the efficiency is about 90%, and the battery current is minimized, which greatly extends the battery life. Even easier, the output voltage is set with a resistor, and if I need a different voltage rail (such as 3.3V, 2.5V, 1.8V, 1.5V and 1.2V), then this output voltage can be changed very easily with a jumper wire on the demo board.
In summary, two BP solar panels, each with a 0 to 20V output at 4A, are regulated by the 20V output LTM4607 buck/boost micromodule switching regulator, then to the 14V input LTC1435/LT1620 battery charger, through an ideal diode MOSFET controller LTC4414, a series current sense amplifier LTC6103, and finally into the battery; charging at a regulated 4A current. In this design, the ADC readings are taken by the LTC2418 at different stages and sent to the DC590B demo board microcontroller powered by the LTM4601 micromodule switching regulator to display the results on the LCD. Figure 7 shows the entire system in action.
Figure 7: The entire system design in action
Mechanical design of spray system
With a working solar charger and a steady 12V output, I was ready to start assembling the spray system. A trip to the hardware store provided me with the materials I needed: the bilge pump, hose connector , hose clamp, adapter, and spray system. The hose is about 15 feet long and screws onto the adapter screws, which are attached to the pump with the hose clamp, and the spray system is attached to the end with 5 spray nozzles. The bilge pump runs on a maximum of 12V, and the water pressure can be controlled by reducing the voltage.
For flexibility, I installed a voltage regulator that can accept a 12V input and convert the input to a variable 12V output. This requires the LTM4607 design to have a buck/boost feature. The device uses a feedback resistor to control the output voltage. A 50k variable knob potentiometer replaces the resistor, making it very easy to control the output from 0.8V to 12V. A 5.62k resistor is also connected in series to limit the output voltage, keeping the output below 15V. This design achieves water pressure control by turning a knob.
I was then able to test my fully functional misting system. As it turned out, the pump caused a maximum battery discharge current of about 6A, which meant that at peak output, the pump was drawing about 4A from each solar panel. The benefit of controlling the speed and pressure of the pump was that I could drop the pressure low enough to reduce the battery discharge current and run the pump entirely on the solar panels to save battery power, which was very effective. This way we were able to run the misting system all day at camp without having to worry about discharging the batteries and delaying the LED lighting system at night.
LED lighting
With the power supply complete, I could add the circuitry to efficiently provide lighting at night. LEDs bright enough to illuminate a room were previously unthinkable, but new technological advances have made way for a new era of LED lighting. In particular, Philips Lumileds Luxeon LEDs can provide over 100 lumens of light at 1000mA. I outfitted a LumiLED array using the LTC3475 (16-lead TSSOP thermally enhanced package) Dual 1.5A Constant Current LED Driver DC923A Demo Board. It is designed to drive two channels at 1.5A per channel from a wide range input voltage (4V to 30V). A 12V battery connected directly to the board's input powered three series-connected LEDs per channel, for a total of six LEDs when both channels were on. These LEDs were surprisingly bright, and when covered with a diffuser, were enough to light up our entire campsite. The discharge current at night comes entirely from the battery, since the solar panels provide zero power at night. With a total discharge current of 2A, the lights can be powered all night long. By mid-morning or early afternoon, the battery is fully charged again and ready to power the misting system, which will keep us cool after brunch.
Cigarette lighter adapter for external devices
Our communications wireless radio needed to be charged after heavy use because the cellular phone was not receiving a signal. The radio we used had a car adapter plug that could be charged from the cigarette lighter. To charge it quickly, I added a 12V cigarette lighter female adapter connected to the battery specifically for this car adapter plug. This proved useful because the battery charge only lasted a few hours at a time, so we needed to charge our radio frequently. Figure 8 shows charging the radio from the cigarette lighter connected to the 12V battery output.
Figure 8: Charging a Ham radio at 12Vdc via a car cigarette lighter
Debugging and Pitfalls
During an initial test with the prototype, I discovered a fundamental limitation to the use of solar panels. A varying voltage on a solar panel also means a varying current. I had an epiphany while simulating some real-world elements, such as shadows covering the solar panel or insufficient sunlight. In one extreme case, waving an arm over the panel could cause the system to latch into current limit, which was alarming. When sunlight variations caused the output voltage to drop, the prototype's current-mode architecture caused the system to draw more current from the solar panel, which made sense because power reflects the relationship between current and voltage, and when voltage drops, current will increase to achieve the same power value. The solution is to design a system that varies power because the input voltage and input current vary depending on the amount of sunlight shining on the solar panel.
Because the panel may only provide a maximum peak current of 4A, this undercurrent at the input causes the system to latch off and remain latched off until the system is reset. A simple solution is to reset the RUN pin on the LTM4607 uModule regulator when the input voltage drops below a certain threshold. This can be accomplished by using a comparator with a set reference voltage to trigger when the panel voltage drops. Unfortunately, this solution is not optimal because it causes the system to either turn on or off depending on the amount of sunlight the panel receives. A more suitable solution is to regulate the charge current with respect to the solar panel voltage, so that the 4A charge current varies with the amount of sunlight on the solar panel. As the solar panel output voltage drops, the charge current to the battery should also drop.
One of my colleagues, after struggling with this problem, suggested using an op amp and MOSFET to control the PROG pin on the LTC1435. This pin controls the charge current output value, making it proportional to the current value drawn from this pin. The op amp tracks the voltage of the solar panel, and depending on the value of the panel voltage, adjusts the Rds-on of the MOSFET and controls the current. When the panel voltage is at its maximum, the op amp controls the MOSFET to be fully turned on, allowing maximum current to be drawn and providing a 4A charge current. When the panel voltage is lower, the charge current should also be lower to maintain appropriate power output. I quickly added a circuit using the LT1006 op amp and attached it to the DC133A demonstration board. I was running out of time and was still figuring out the resistors on the bias MOSFET to achieve maximum charge current. Instead of calculating current and resistance, a friend of mine suggested that I use a variable potentiometer to quickly solve the current bias problem. When the solar panel voltage reaches the midpoint of 10V, I need to reduce the charge current by half. At the maximum value of 20V, the charge current should be 4A. He suggested that I set the potentiometer to provide maximum current in the test with the solar panel voltage at 20V, and drop the voltage to 10V while slowly adjusting the potentiometer to 2A charge current without latching up. This suggestion worked, and the circuit no longer latched up when the solar panel output dropped. The system maximized the use of sunlight regardless of the time of day.
The activity was successfully completed and we returned home
Our system and demo board survived the desert, through sandstorms, intense heat and dryness, and a 100°F sun. The batteries held up without failure. The LEDs were covered in a thick layer of dry salt lake dust, but were still functional enough to give enough light for us to pack up. It was midnight when we left, so the lights were the last to go. The misting system performed admirably, spraying a nice blanket of cool water when we needed to escape the heat. The campsite (Figure 9) was cool, comfortable, and a home for our 4-day stay in the desert. The system arrived home intact and functional, ready for use again next year.
Figure 9: Solar panels at camp charging batteries using Linear Technology's demo board
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