Built-in battery charger, Li-Ion

Build-in charger for instruments powered by single lithium-ion or Li-Pol accumulator. If external power supply not conected to charger, current back from the accu charger is very small. New version improved temperature stability and Not Needed NTC thermistor. After a simple adjustment can be from initial engagement to launch a thermistor to compensate for temperature dependence. After adjustment (see Figure 1 ) operates as a transistor T4 not voltage follower, but as a switch. If charging voltage is present, transistor T4 is closed and the divider resistors R4 to R6 does not pass any current.
Built-in battery charger, Li-Ion - schematic

Built-in battery charger, Li-Ion - img1

Built-in battery charger, Li-Ion - img2

Base-emitter transition T1 is polarized in the reverse direction, based on the zero voltage, so it will pass through the transistor T1 reverse current. After connecting the charging voltage of at least 4.9 V is based on the T1 will supply about 0.5 to 0.6 V greater than the battery voltage nabíjeného. When fully charged battery (4.2 V) is based on the T1 to approximately 4.7 V. Since the input R TL431 is a voltage stabilizer of approximately 2.5 V, the transistor T4 opened voltage around 2.2 V. At this voltage transistor T4 resistance of closed and open channel of singles, up to tens of ohms and its resistance to practically apply the divider. If the charged battery is discharged, the voltage on the basis of a smaller T1 and T4 may not be fully opened. It does not matter, because at this stage the battery is recharged as the "constant stream" and stabilize the voltage to apply. For proper function of the battery charger must have a small transistor threshold voltage of T4. Well meet transistor BSS138 (SMD) or BS108 for classical mounting. Both types have a threshold voltage of typically about 1 V with a charging current limiter transistors T2 and T3 has been described in detail here and I will not repeat it here. If you use the charger power supply with a small transformer or a small power adapter into the wall, there is no need to limit the current in the charger, it takes care of power supply. You can then use the...

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