NiMH Charger For Up To Six Cells

  
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The circuit presented here is intended for charging NiMH batteries. The MAX712 IC used here contains all the necessary functionality to make sure that this happens in a controlled manner. Figure 1 shows the schematic of the charger. The heart of the circuit is easily recognized: everything is arranged around IC1, a MAX712 from Maxim. This IC is available in a standard DIP package, which is convenient
NiMH Charger For Up To Six Cells - schematic

for the hobbyist because it can be directly fitted on standard though-hole prototyping board. IC1 uses T1 to regulate the current in the battery. R1 is used by IC1 to measure the current. While charging, IC1 attempts to maintain a constant voltage, equal to 250 mV, across R1. By adjusting the value of R1 the charging current can be set. The value of R1 can be calculated using the formula below: R1 = 250 mV / I charge For a charging current of 1 A, the value of R1 has to be 250 mV / 1 A = 0. 25 ©. The power dissipated by R1 equals U G— I = 0. 25 G— 1= 250 mW. A 0. 5-watt resistor will therefore suffice for R1. Transistor T1 may need a small heatsink depending on the charging current and supply voltage. IC1 needs a small amount of user input regarding the maximum charging time and the number of cells in the battery to be charged. IC1 has four inputs, PGM0 to PGM3, for this purpose. These are not ordinary digital inputs (which recognise only 2 states) but special inputs that recognise 4 different states, namely V+, Vref, BATT or not connected. To make this a little bit more user friendly, we`ve brought out the necessary connections to 2 connectors (K3 and K4). A number of dongles have been made (Figure 2) that can be plugged into these connectors and set the number of cells and the maximum charging time. When determining the maximum charging time we have to take into account the charging current and the capacity of the cells...



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