Power Unit Circuit
The circuit diagram for the power unit is shown in figure 3.
The mains 220v AC is first converted into 12v AC using a step down transformer. This 12V AC is converted into approximate 17V DC using diodes D1-D4 which are used in bridge configuration. This DC is further smoothened using a 1000uF electrolytic capacitor which acts as a first order low pass filter. Ceramic capacitor C2 is used for further stability of the LM7812 regulator.
A LM7812 linear regulator is used to derive regulated 12v DC which is fed to the relay coil and also to the normally open contacts of the relay K1. A 10-12 volt, 7 watt solar panel is used to obtain electricity from sun’s energy. The electric power from thesolar panel is fed to the normally closed contacts of the relay with a bypass diode D6 in series
As this diode D6 will create a drop in voltage after the solar panel’s output, so it is recommended to choose a diode which has the least forward dropout voltage such as schottkey diode 1N5822.The diode D5 which is connected in parallel to the relay prevents the back emf of the diode to harm the rest of the circuitry.
Now as the solar panel’s output is fed to the normally closed contacts of the relay so when mains power is not available, the relay will be turned off and thus solar panel’s power will directly feed the charging circuitry. When mains power is available, it will turn the relay ON and thus the mains derived 12v DC will pass through the relay cutting off the solar panel’s power. This way during low solar power hours or night times, the user can charge the clock using mains power.
Now the common terminal of the relay goes to the charging circuitry which comprises of the bq24450 IC from Texas instruments. The bq24450 contains all the necessary circuitry to optimally control the charging of valve-regulated lead-acid batteries. The IC controls the charging current as well as the charging voltage to safely and efficiently charge the battery, thus maximizing battery capacity and life.
For our application, the IC is configured as a simple constant-voltage float charge controller for 6v, 4.5 AH battery. The bq24450 IC is so flexible that it can be programmed in various configurations to suit different charging currents and voltages for batteries of different capacities. Only an external pass transistor and minimum number of external passive components are required along with the IC to implement a charger for sealed lead-acid batteries. Here in our configuration, a PNP transistor T1 (BD240) is used to drive the battery charging power. Before going into the programming of battery charger, let us define the required parameters for our battery.
- Final discharge voltage (Vth) = 1.75V per cell = 5.25V
- Float voltage (Vfloat) = 2.30V per cell= 6.9V
- Voltage in boost mode (Vboost) = 2.45V per cell = 7.35V
- Charge rate can be 0.05C to 0.3C
- We will use charge rate (IMAX-CHG) = 0.13C = 600 mA (approx)
- VBAT (MIN) =4V
- Trickle charge rate = 10 mA
Now we will go through the various programming parameters required for implementing a dual level charger for our battery chemistry. The first step is to decide on the value of the current in the voltage divider resistor string in FLOAT mode. This should be substantially higher than the input bias current in the CE and VFB pins and the leakage current in the STAT1 pin, but low enough such that the voltage on the PGOOD pin does not introduce errors. A value of 50μA is suitable.
Let us say,
RA = R14+R15+R16+R17+R18
RB = R19+R20+R21+R22
RC = R23
RD = R24+R25
RP = R27+R28
In FLOAT mode, STAT1 is OFF, so there is no current in RD. The voltage on the VFB pin (VREF) is 2.3V
RC = 2.3V ÷ 50μA = 46kΩ The closest 1% value is 47kΩ.
VFLOAT = VREF × (RA + RB + RC) ÷ RC
RA + RB = 2 × RC = 92.8kΩ.
VBOOST = VREF × (RA + RB + RC//RD) ÷ RC//RD
RD = 474.3kΩ.
VTH = VREF × (RA + RB + RC//RD) ÷ (RB + RC//RD)
RB = 16.9kΩ,We choose closest 16.7k
RA = 92.8kΩ – RB = 75.9kΩ, choose closest 75.7k
IPRE = (VIN – VPRE – VDEXT – VBAT) ÷ RT. Thus, RT = 634Ω,we choose closest 639Ω
The charging current for the battery is programmed by selecting the right value of the shunt resistor R1.Thus
IMAX-CHG = VILIM ÷ R1
R1 = 250mV ÷ 600mA = 0.417Ω. The closest 1% value is 0.422Ω.
RP = (VIN(MIN) – 2.0V) ÷ IMAX-CHG × hFE(MIN) = 7 ÷ 0.6 x 27 = 320Ω.
LED1 in the bq24450 circuitry displays the state of charging. The schottkey diode D7 again serves the role of preventing the back current leakage from the battery and after that the circuit feeds the lead acid battery. After this stage, the battery rail goes to the low voltage detection circuitry and the UCC383 LDO in series with a SPST load switch. The battery low voltage detection circuit is built around the comparator LMP7300 which has an internal reference of 2.048 volts. The low voltage of the battery at which the it should be disconnected from the load is deemed to be around 1.75 v per cell which estimates to 5.25volts for three cell battery.
The comparator output is open collector and the circuit is so designed that its output goes low when the battery voltage drops below 5.25 volts. The output of this comparator goes to the enable pin of the LDO in next stage. Thus whenever the battery voltage goes below 5.25 volts, the output of comparator goes low which in turn switches the LDO off and thus shutting down all the load OFF from the battery and preventing it from getting further discharged.
The potential divider network comprising of the resistors R2 to R6 establishes a voltage of 2.048 volts at the non-inverting terminal of the comparator when battery rail reaches to 5.25 volts which is compared with the inverting terminal of the comparator which is directly connected to the reference output of the LMP7300. Further a combined negative and positive hysteresis of about 300mV in total is programmed to the LMP7300 through R29 and R30 so that the comparator output does not fluctuate when the battery voltage reaches the exact crossing point of the reference potential of the comparator.