This circuit was designed to charge Nicad battery packs in the range of 4.8 to 15.6 V from a convenient remote power source , such as automobile battery. When power is first applied to the circuit applied to the circuit , a small bias current supplied by R1 via winding L1 , starts to turn on the transistor TR1.
This forces a voltage across L2 and the positive feedback given by the coupling of L1 and L2 causes the transistor to turn hard on , applying the full supply across L2. The base drive voltage induced across L1 makes the junction the necessary base current to hold Q1 on.
Portable Battery Charger Circuit Diagram

Resistor
R1 = 1M
R2 = 120R
R3 = 10R
R4 = 39R
Capacitor
C1 = 100uF 25V
C2 = 0.01uF
C3 = 4700pF
C4 = 100uF 25V
Diode
D1 = 1N4148
D2 = BYV27-5
D3 = BYV27-5
Transistor
Q1 = ZTX650
Transformer
L1 = 12T 36awg
L2 = 13T 36awg
L3 = 20T 30awg
L4 = 40T 30awg
Core FX3437 with gap of 0.08mm
Former DT2492
Hi,
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