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10A CAR BATTERY CHARGER

2016-05-10 03:14  
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10A CAR BATTERY CHARGER
The charger is suitable for lead-acid car batteries and it is assembled in two units: a metal box with the toroidal transformer, instrument, lights, etc, and a small plastic box housing the voltage and temperature circuit. Connection between main box and sensor is realized with a standard 3 core x 1mm, electric cable, 4m long. Its resistance is factored in the circuit calculations and it is the limiting resistor against overcurrent. Do not change type or length as it may alter the overall performance and safety of the charger. The sensor box is typically positioned close to the battery to be charged and two short flexible leads, 2mm section, 30cm long, one red and black the other, terminated with good quality clamps make up the connection from the sensor to the battery. This solution assures that the battery is charged up to the correct voltage which depends, in turn, upon the ambient temperature. The final voltage should be set, with the 200Ω multiturn pot, at 14.8V at 20°C-68°F and derated +/-30mV/°C (17mV/°F) at any other temperature. For example, if the prevailing ambient temperature is 10°C then the final voltage should be set at 15.1V, if the prevailing ambient temperature is 30°C the final voltage is 14.5V and so on. Once set, the circuit will automatically adjust the voltage to within 1°C. You have to connect a battery in order to carry out this setting. A thermistor would have simplified the circuit but its correct implementation is not easy and it was preferred to employ a number of diodes. A red led in the sensor box gives an indication of the correct connection to the battery. However the circuit is quite tolerant to mistakes: shorting the output will do no arm as there is no voltage at the output terminals, not until you connect it to the battery. It is the battery voltage that triggers the circuit into operation and once it is disconnected from the battery the voltage too disappears from the output. Only if the battery voltage is above 7-8V then the circuit will operate. A reverse connection of any battery will do no arm either as the circuit will simply not operate. It will withstand a temporary connection of a 24V battery; above this voltage the input circuit is overloaded and could be damaged. Current control is achieved by switching the SCRs at the appropriate time through the BF761 collector current. The blue led, but any other colour will do, gives an indication that the unit is charging the battery. The led will start flickering at the end of the charging cycle so you know at a glance that the charge is coming to an end. You may leave the battery connected after it has fully charged as there will be a trickle charge which will keep the voltage at its optimum level. Switching noise is eliminated by the 85μH choke made up by winding 27 turns of 1mm enamelled wire on a ferrite ring 27x11mm. Due to the way SCRs operate, the common line is positive and not negative as one would expect. Care must be exercised when connecting all the polarity sensitive devices. A toroidal transformer has many advantages: it is small, highly efficient, will tolerate a moderate overload and will consume little power, only 3.5VA when switched on and no battery connected. Cost, at this power range, is surprisingly close to a traditional transformer, yet, the inrush current when switched on can be so high, depending on the exact time with respect to the mains sinewave, that the collapse of the ensuing strong magnetic field will produce mighty spikes up to 500V at the secondary, destroying whatever they find in their path. A few capacitors, the use of fast diodes UF4006 and the high voltage transistor BF761 take care of the problem. The main switch should be rated at 10A. SCRs can get rather hot; the best solution is to mount them on the metal case itself using appropriate insulating kits. As a consequence the box will warm up especially at the beginning of the charging cycle when the unit may be temporarily overloaded. A thermal switch is provided to cut out the mains supply under extreme temperature and overload conditions. This switch is mounted at about 6-8cm away from the SCRs so that it will take care of the heat coming from other sources as well, such as the transformer and the choke. The unit has been tested with batteries from 44 to 100Ah for over a year, from 0 to 38°C (32 to 100°F); the upper temperature limit caused the thermal switch to operate. I should relocate the thermal switch in a cooler place if the designed max operating temperature of 40°C-104°F is to be met. You may have different temperature limits depending on the mechanical configuration of the box and internal components layout. Pay attention to the fact that this charger behaves like a fast charger for the smaller batteries and precautions should be taken concerning gas production and it is good practice to disconnect the battery from the car before charging it.