A Friendly Charger Schematic for Mobile Phones

A Friendly Charger Schematic for Mobile Phones

Circuit Description

Most mobile chargers do not have current/voltage regulation or
short-circuit protection. These chargers provide raw 6-12V DC for
charging the battery pack. Most of the mobile phone battery packs have a
rating of 3.6V, 650mAh. For increasing the life of the battery, slow
charging at low current is advisable. Six to ten hours of charging at
150-200mA current is a suitable option. This will prevent heating up of
the battery and extend its life.

Circuit diagram:



  • P1 = 10K LOG
  • R1 = 1K
  • R2 = 1K
  • R3 = 1K
  • R4 = 1K
  • R5 = 3.3K
  • R6 = 16R/2W
  • R7 = 220R
  • R8 = 3.3R
  • R9 = 1K
  • C1 = 470uF/25V
  • C2 = 10uF/25V
  • C3 = 1KuF/25V
  • D1 = Red LED
  • D2 = Green LED
  • Q1 = BC547
  • Q2 = BD677
  • ZD1 = 12V/1W
  • ZD2 = 5.6V/1W
  • IC1 = CA3130

The circuit described here, provides around 180mA current at 5.6V and
protects the mobile phone from unexpected voltage fluctuations that
develop on the mains line. So the charger can be left ‘on’ over night to
replenish the battery charge. The circuit protects the mobile phone as
well as the charger by immediately disconnecting the output when it
senses a voltage surge or a short circuit in the battery pack or
connector. It can be called a ‘middle man’ between the existing charger
and the mobile phone.

It has features like voltage and current regulation, over-current
protection, and high- and low-voltage cut-off. An added specialty of the
circuit is that it incorporates a short delay of ten seconds to switch
on when mains resumes following a power failure. This protects the
mobile phone from instant voltage spikes. When short-circuit occurs at
the battery terminal, resistor R8 senses the over-current, allowing Q1
to conduct and light up D1. Glowing of D2 indicates the charging mode,
while D1 indicates short-circuit or over-current status.

The value of resistor R8 is important to get the desired current
level to operate the cut-off. With the given value of R8 (3.3 ohms), it
is 350 mA. Charging current can also be changed by increasing or
decreasing the value of R7 using the ‘I=V/R’ rule. Construct the circuit
on a common PCB and house in a small plastic case. Connect the circuit
between the output lines of the charger and the input pins of the mobile
phone with correct polarity.

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