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Solar-powered Uninterruptible PSU
C. Tavernier
When you want to power an electronic device from solar panels, broadly speaking there are currently two approaches. The first, very conventional method (described elsewhere in this double issue) consists of employing a combination of solar panels (or an array), an automatic charger, and a battery (or an array). This combination then powers thedevice concerned, which has its own voltage regulating circuits. The second, which we are proposing in this project, consists of building a ‘solar’ PSU directly. It is of course based on the same concept as the one described above, but having been designed for this purpose right from the start, the elements it composes are integrated to a higher extent, leading to improved efficiency. Our suggestedcircuit is intended to power a number of current electronic devices directly, and can provide three different voltages: 3.3 V, 5 V or 12 V, depending on component selection; all at a current of 400 mA, which can even be increased to 1 A if necessary (details below). It’s primarily based around IC3, a high-performance switching regulator from Linear Technology. Depending on whether you choose anLT1300 [1] or an LT1301 (*) [2] you will have a choice of two output voltages: 3.3 or 5 V for the former, and 5 or 12 V for the latter. For both ICs, the voltage is selected by fitting jumper S1 or not, as indicated in Table 1. Look carefully into the output voltages you will require and then select the appropriate ICs for the project. When jumper S2 is fitted, the output current of these ICs isinternally limited to 400 mA. It can be increased to 1 A by omitting the jumper, but we don’t really recommend this as the rest of the circuit has been optimized for an output current from a few mA to 400 mA maximum. The primary power source is the NiMH rechargeable battery pack, which in the
L1

IC1
+PS D1

* see text
R2 22 Ω R5 180 Ω R7 270 Ω S1 6

22µH 33µH V IN

*
SW 7 4 D2

LM317adj.

+V
+V

*

1N5817
solar cell –PS +BATT

*

2

IC3
SEL SENSE

1N5817

R4 100 Ω

R3 1k LED1 3

LT1300 LT1301
SHDN GND

I LIM PGND 8

*

5

IC2
P1

1

TL431
10k

T1

S2 C1 100µ 25V

*

C2 47µ 25V

R8 10k

battery

R1 10k

R6

BC 548C

–BATT
080223 - 11

1k

0

0

COMPONENTS LIST
Resistors
R1,R8 = 10kΩ R2 = 22Ω R3,R6 = 1kΩ R4 =100Ω R5 = 180Ω R7 = 270Ω P1 = 10 kΩ potentiometer

C2 = 47μF 25V

Semiconductors

D1,D2 = 1N5817 T1 = BC548C IC1 = LM317 IC2 = TL431 IC3 = LT1300 (or LT1301, see text) LED1 = LED

Miscellaneous

Inductors

L1 = 22 μH (or 33 μH, see text)

Capacitors

S1,S2 = 2-way pinheader, lead pitch 2.54 mm, with jumper 6 solder pins PCB, ref. 080223-1 from www.thepcbshop.com

C1 = 100μF 25V90

elektor - 7-8/2008

case of the LT1300 will comprise two 1.2 V cells, or three cells in the case of the LT1301. The solar panel should be chosen to deliver a voltage of the order of 9 V at an output current of around 100 mA. Such panels are available commercially. IC1 acts as a constant current charger to limit the current to around 60 mA. To avoid overcharging the battery in the event oflow current draw by the device powered on the one hand and constant sunshine on the other, the circuitry around IC2 and T1 has been added. IC2 is just a variable zener which will turn on T1 harder as the voltage at the wiper of P1 increases. In this way, when the voltage at the battery terminals rises too high, as at the end of charging, T1 will be turned on harder and harder, bypassing part orall of the charging current to ground via R5 and R7, and lighting the LED as it does so. This is simply a contemporary variation of the traditional shunt voltage regulator. The whole of the project fits easily onto a compact printed circuit board of which the component mounting plan reproduced here. The copper track layout is a free pdf download as usual. Building up the board should not present...
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