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Fast Charger with Auto cut off

2018-01-21 16:01  
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This is themodified version of the Fast Chargercircuit posted earlier. This circuit has Auto cut off facility also. This High current charger can be used for the Fast charging of sealed lead acid batteries used in Automobiles, Inverters etc. This charger can give 5 Ampere current for quick rejuvenation of the battery. It uses an Adjustable voltage regulator so that 6 volt and 12 volt batteries can be charged. When the battery acquires full charge, charging process will be terminated.
LM338Kis a high current variable voltage regulator that can provide2 to 25 volts DCwith high current output. Its important features are7 Ampere peakoutput current, adjustable output down to1.2 volts,thermal regulation etc. It is easy to use and require only 2 external resistors to set the output voltage. The time dependent current limiting ability of the device allows peak current of up to 12 ampere to be drawn for short time. So that it is useful in heavy transient loads and speed start up.

LM 338 Variable Regulator

Design considerations

Use of external resistors

LM338 Kis available in metal can packageTO3.K suffix metal can. It has only two pins – Vin and Adjust. The body of the device acts as the output pin. On the bottom side, the first pin is adjust and the second pin is Vin. LM338 requires heat sink to dissipate heat since it is draining high current. Two resistors are required to set the output voltage.Resistor R1(220 Ohms) act as the programme resistor. Usually1.25 voltsreference voltage (Vref) develops between the output and adjusts pins. The reference voltage is impressed across the programme resistor R1.The voltage across R1 is constant so that constant current flows through the output set resistor VR1.Therefore output voltage is

Vout = Vref (1 VR1 / R1) I Adj VR1

Use of capacitors

Two capacitors are usually soldered close to the input and output terminals. C2 is the input bypass capacitor. Output capacitor C3 gives stability for the output current.

Circuit operation

Input voltage is obtained from a0-15 volt 5 Amperesecondary transformer with the10 Ampere KBPC Rectifier Module.Capacitor C1 makes the DC ripples free and around 16 volt DC enters into the Vin input of the regulator. VR can be used to set the output voltage from 2 volts to 15 volts. A digital panel meter is used for measuring the output voltage. Ready made LED or LCD panel meters are available at reasonable cost. This can be directly connected to the output. So by adjusting VR, it is easy to see the output voltage in the meter .If the output put voltage reduces due to line voltage drop, it can be easily detected through the panel meter. The panel meter also can be used to measure the terminal voltage of the battery. If the power supply is switched off, meter shows the terminal voltage. If the voltage remains steady, it indicates that the battery is fully charged and holding charge.

Auto cut off circuit is built around the 5 Ampere Darlington NPN transistorTIP122. When the terminal voltage of the battery rises above 12.5 volts, Zener diode ZD conducts and inhibits the working of the Regulator LM338. When the battery voltage drops below 12 volts, Zener turns off and the regulator again provides charging current.

Fast Charger with cut off Circuit diagram

Setting the output voltage

A fully charged 12 volt battery should have a terminal voltage of13.5 voltsand that of a 6 volt battery6.5 volts. To charge the battery, set the output voltage for charging as 14 volts for 12 volt battery and 8 volts for 6 volt battery.

LM338 on Heat sink

Useheavy gauge Red and Black wiresin the output and usecrocodile clipsfor connecting the battery. Before the charger is connected to mains, connect the clips to the battery terminals with correct polarity and see the terminal voltage. If it is less than 13.5 / 6.5, switch on the charger and charge up to one hour. Wait for 10 minutes and again check the terminal voltage. If the voltage remains steady, the battery can be used.

LCD and LED volt Meter

TIP 122 Pin connection

TIP122 Datasheet

LM338 datasheet
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