Cow Power

Electrical Design

Requirements

1. Regulate current. The XOs at Khairat school have LiFePO4 type batteries. Let us chose a 1A charging current so that with the given capacity of the dynamo, we can charge about 15 XOs simultaneously. See Battery Charging for more details about all kinds of batteries that are used in the XO
2. Regulate voltage. The XO power supply accepts voltage from 9V - 18V . The charging circuit switches on at 9.85V. Let us chose to fix the voltage that we give the XO at 12V

Design approach

1. Using L200 as the central component - it is cheap, easily available and most of all robust
2. Referring to this sample circuit and deriving parameters from this design guide
3. To avoid effects of shorting or reversing of supply from dynamo to the L200 circuit a simple full wave bridge rectifier using 1N5408 diodes has been used. Also, a 100uF 35V decoupling capacitor is being used after the rectifier.

Concept circuit diagram

Deriving circuit parameters

1. For values of R2 and R1, I referred to table in right column of Page 4 of design guide. Selecting Vo=12V +-4% I get R1= 1K ohm +-1% and R2=3.3K +-1%
2. Since Isc = 0.45/Rsc (according to sample circuit design), I chose Rsc=0.47 ohm 1/2 watt
3. Current in R2, Ir2=(Vo-Vref)/R2 where Vref = 2.77V. Hence power dissipation in R2, Pr2 = 25.8 mW
4. Current in R1, Ir1=Vref/R1 = 2.77mA . Hence power dissipation in R1, Pr1 = 7.7 mW
5. Hence R1 and R2 can be chosen to be 0.25W type