Universal Battery Charger for Li-Ion with USB output
       17. Simple electronic devices.        

    17.19 Universal Li-Ion Battery Charger with USB output.

  Another LI-Ion battery charger, which works fine (not broken yet). I opened it just for fun.

  It was designed to charge Li-Ion batteries for Samsung i908i & i8000. It's hard to say which the battery is, but I know for sure what they are lithium.
  Charger working from 120VAC to 240VAC, does not care about battery temperature. A good feature is that it has USB power output, through which we can charge phones and other staff. Additional info. at the bottom of the charger (sorry for the low resolution pictures).

  Internal board has no missing parts. We can move LED to a position LED1, if necessary. SMD components on the bottom side (no picture).

  The device charges the battery up to 4,18 Volts and not afraid of short inside the LI-Ion battery. In the last case, it limits the charge current to 0.45 Amps.

  RED LED has turned ON when the battery is charging. When the battery is fully charged (or missing) the BLUE LED is ON.

  A schematic diagram of, a very simple and easy to understand.
  Converter is based on a transistor Q2. When power is supplied the current through the resistor Rx opens transistor Q2. Additional voltage on secondary winding, through R3 and C2, opens Q2 even more. The saturation of the core T1 or over current through R6 closes the transistor. After, all the power stored in the core of T1 charges the capacitor C3 through the diode D1 and the cycle is repeated.
  The charging circuit on the right side of capacitor C3. Q5?
together with Q3 limits top voltage on the Li-Ion battery to 4.18V. Top charging current limited by resistor R2.
  RED LED is ON when charging current > 0.12A. BLUE LED (unusual LED, with forward voltage ~3V) is ON when voltage on the battery > 3.3V.
  *Since the rectifier diode D1 is far away from the ideal diode, it has a switching time. During this time, the voltage on the
winding of the transformer can rise infinitely high and kill the transistor. Chain of components D8,R12,C7 absorb these short peaks.

 The following 4 waveform are taken from an output winding of the transformer T1 (GND to plus of D1). They show how the transistor O2 is working with different load currents.

  Without load (~120VAC, Vin = +160VDC, free run).


  Average and maximum load (charging Li-Ion battery with current 150mA & 450mA).

To clarify, I powered this circuit from 10VDC instead of 120VAC and I took two waveforms. The first from collector of transistor Q2 (left), and the second, from transformer output where it connected to + of D1 (right).

  And the interesting thing is that the circuit starts to generate pulses at a supply voltage equal +5V. The output voltage is 5V, too. Of course, it does not produce enough power to charge the battery, but enough to light a BLUE LED (LED1).

  Finally, it's all.

August 15, 2016  SKootS


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