APC Back-UPS Pro 550 BR550GI - Brak ładowania akumulatora

... sheet of UC3842B, the 22µF capacitor seems to be underestimated.
At least in RS500 230V version REV05 one of resistor around eats permanently about 2 Watts and generates a lot of heat. That is why I decided to do a more radical change: Replace the capacitor with much larger one: 220µF. Remove the parallel 3.3kΩ resistor R139 without any replacement. Replace large hot 50kΩ resistor R28 by 220kΩ of the same size (probably even more is possible). It decreases power current by 2 Watts and temperature there by 40°C at cost of 1 second delay after connecting the power plug. Purpose of such modification: Original design used a very hard divisor that divides the plug voltage to supply ~10mA. It is nearly enough for running the oscillator, so 22µF was (nearly) enough. But the divisor should be used just to start oscillation. Once the oscillator runs, it is powered from the transformer and the divisor is not needed. You can verify this fact by disconnecting the large divisor resistor, touch, and remove. Oscillator runs. The datasheet says, that when there is no power to run oscillator, the IC eats less than 0.5mA. It means, that the divisor could be designed to provide only 0.5mA, not 10mA. In this case, capacitor must be larger (e. g. 220µF), because the complete start of the oscillator is powered nearly exclusively from it. It must have enough juice to keep voltage above the minimal voltage until the transformer start to give power.

VOLT_SELECT, which is a discrete signal and is generated by a microprocessor. This signal changes the parameters of the resistive divider that sets the output voltage of the stabilizer, and in particular the resistance of the "lower" resistor (R2 in Fig.4). Setting the VOLT_SELECT signal to a high level opens the transistor Q12 and locks the Q7. As a result, the" lower " resistor of the divider becomes the resistor R15. Setting the VOLT_SELECT signal to a low level leads to the opening of the transistor Q7 and closing Q12, resulting in a "lower" resistor divider becomes R17 C other resistance rating, which ultimately leads to a change in the output voltage of the charger.

Switching on and off the charger is carried out by the signal on / OFF and transistor Q18, when you open the control terminal of the stabilizer LM317 (cont.1) shunted to the "ground". Current limitation, as usual, is carried out by the transistor Q19, which, in turn, is controlled by the current sensor – resistor R35.

In the diagram shown in Fig.11 you can also see the presence of a charger sensor consisting of R53, R45 and C19. This sensor generates a CHRG_ON signal as soon as the supply voltage of the primary network appears at the UPS input. This signal with its high level informs the microprocessor about the presence of the mains voltage and the possibility of starting the battery charging process. It is on this signal that the microprocessor sets the on/OFF signal to a low level, which leads to the start of the charger. In principle, this sensor could be called a network voltage sensor.