28-08-2017, 12:26 PM
The circuit presented here can charge a 12V lead-acidcide battery tested from 50Ah to 80Ah (even up to 100Ah) capacity and can even be used as up to 18V DC variable power supply of maximum capacity of 5A which is useful for a Test Bench. The circuit can automatically detect the presence of a battery connection and start charging. At that time, disconnect the output supplied as a variable power supply. It also detects the incorrect / reverse polarity connection of the battery terminals and generates an alarm. The charger initially charges the battery to a higher voltage (approximately 14.2V), and once fully charged, maintains the battery charge at a constant voltage (approximately 13.4V).
Circuit and working
The circuit of the 12V battery charger and variable power supply is shown in Figure 1. It is built around two variable voltage regulators (IC1 and IC2) LM138, two optional amplifiers LM358 (IC3), a voltage regulator Of 12V 7812 (IC4), two relays (with 12V normal, 1C / O mounted on PCB and 12V, 1C / O, 10A nominal contact current) and a pair of transistors.
LM138 is a 5-pin 3-pin positive voltage regulator available in TO-220 or TO-3 package. But the continuous flow of 5A current generates a high temperature, which automatically shuts off the output of LM138 due to its internal thermal protection.
This circuit simplifies the higher current handling of LM138 using IC1 and IC2 in parallel, but the output voltage can still be regulated by a single variable resistor. Care must be taken to handle lines carrying 5A current. Two separate relays (RL1 and RL2) are used to reduce project cost.
The transformer X1 lowers the voltage from 230V AC to 15V-0-15V AC, which is rectified by diodes D1 and D2 and smoothed by capacitor C1. This voltage, which is about 20V DC, is fed to IC1 and IC2 which are connected in parallel. Its output voltage is regulated by VR1 (in case of variable power supply) or VR2 (in case of battery charger), which is selected by relay RL2. The output thus obtained is available for the variable power supply or battery charger via RL1.
The double op-amp LM358 (IC3) is used to control the relays and select the type of output, which parts list is either for battery charger or variable power supply. When there is no battery connected to charge, no power is given to IC3. RL1 and RL2 are in non-energized state, and the potmeter VR1 can be used to obtain variable voltage output through CON3 mounted in the cabinet.
When a 12V battery under load (BUC #) is properly connected to terminal CON4 for charging, IC3 obtains the battery power source through diode D10. If the battery voltage is below the "dead" voltage (say, 6-9V and at least 6V), pin 7 of IC3 lowers and LED6 shines. If the battery is healthy (eg more than 9V), pin 7 of IC3 is raised and LED2 is turned on and drive transistor T2, and RL1 and RL2 energize.
The output voltages of IC1 and IC2 are regulated by VR2 and are available for charging in CON4. Once the battery under load reaches its full charge voltage, pin 1 of IC3 goes high, as indicated by the brightness of LED4, and T1 leads, reducing the voltage on pin 1 of IC1 and IC2.
When a 12V battery is connected in reverse polarity, D11 leads, which, in turn, lights the piezo buzzer and LED5 shines. This protection is important when charging the batteries externally.
Circuit and working
The circuit of the 12V battery charger and variable power supply is shown in Figure 1. It is built around two variable voltage regulators (IC1 and IC2) LM138, two optional amplifiers LM358 (IC3), a voltage regulator Of 12V 7812 (IC4), two relays (with 12V normal, 1C / O mounted on PCB and 12V, 1C / O, 10A nominal contact current) and a pair of transistors.
LM138 is a 5-pin 3-pin positive voltage regulator available in TO-220 or TO-3 package. But the continuous flow of 5A current generates a high temperature, which automatically shuts off the output of LM138 due to its internal thermal protection.
This circuit simplifies the higher current handling of LM138 using IC1 and IC2 in parallel, but the output voltage can still be regulated by a single variable resistor. Care must be taken to handle lines carrying 5A current. Two separate relays (RL1 and RL2) are used to reduce project cost.
The transformer X1 lowers the voltage from 230V AC to 15V-0-15V AC, which is rectified by diodes D1 and D2 and smoothed by capacitor C1. This voltage, which is about 20V DC, is fed to IC1 and IC2 which are connected in parallel. Its output voltage is regulated by VR1 (in case of variable power supply) or VR2 (in case of battery charger), which is selected by relay RL2. The output thus obtained is available for the variable power supply or battery charger via RL1.
The double op-amp LM358 (IC3) is used to control the relays and select the type of output, which parts list is either for battery charger or variable power supply. When there is no battery connected to charge, no power is given to IC3. RL1 and RL2 are in non-energized state, and the potmeter VR1 can be used to obtain variable voltage output through CON3 mounted in the cabinet.
When a 12V battery under load (BUC #) is properly connected to terminal CON4 for charging, IC3 obtains the battery power source through diode D10. If the battery voltage is below the "dead" voltage (say, 6-9V and at least 6V), pin 7 of IC3 lowers and LED6 shines. If the battery is healthy (eg more than 9V), pin 7 of IC3 is raised and LED2 is turned on and drive transistor T2, and RL1 and RL2 energize.
The output voltages of IC1 and IC2 are regulated by VR2 and are available for charging in CON4. Once the battery under load reaches its full charge voltage, pin 1 of IC3 goes high, as indicated by the brightness of LED4, and T1 leads, reducing the voltage on pin 1 of IC1 and IC2.
When a 12V battery is connected in reverse polarity, D11 leads, which, in turn, lights the piezo buzzer and LED5 shines. This protection is important when charging the batteries externally.