1. CIRCUIT
IDEAS
FRIENDLY CHARGER FOR S.C. DW
IVEDI
MOBILE PHONES
D. MOHAN KUMAR and the mobile phone. It has features After a power resumption, capaci-
like voltage and current regulation, tor C1 provides delay of a few sec-
M
ost mobile chargers do not over-current protection, and high- and onds to charge to a potential higher
have current/voltage regu- low-voltage cut-off. An added speci- than of inverting pin 2 of CA3130,
lation or short-circuit pro- ality of the circuit is that it incorpo- thus the output of IC1 goes high only
tection. These chargers provide raw rates a short delay of ten seconds to after the delay. In the case of a heavy
6-12V DC for charging the battery switch on when mains resumes fol- power line surge, zener diode ZD1
pack. Most of the mobile phone bat- lowing a power failure. This protects (12V, 1W) will breakdown and short
tery packs have a rating of 3.6V, 650 the mobile phone from instant volt- pin 3 of IC1 to ground and the output
of IC1 drops to ground
level. The output of IC1 is
fed to the base of npn
Darlington transistor
BD677 (T2) for charging
the battery. Transistor T2
conducts only when the
output of IC1 is high. Dur-
ing conduction the emit-
ter voltage of T2 is around
10V, which passes
through R6 to restrict the
charging current to
around 180 mA. Zener di-
ode ZD2 regulates the
charging voltage to
around 5.6V.
When a short-circuit
occurs at the battery ter-
minal, resistor R8 senses
mAh. For increasing the life of the bat- age spikes. the over-current, allowing transistor
tery, slow charging at low current is The circuit is designed for use in T1 to conduct and light up LED1.
advisable. Six to ten hours of charging conjunction with a 12V, 500mA adap- Glowing of LED2 indicates the charg-
at 150-200mA current is a suitable op- tor (battery eliminator). Op-amp IC ing mode, while LED1 indicates short-
tion. This will prevent heating up of CA3130 is used as a voltage compara- circuit or over-current status.
the battery and extend its life. tor. It is a BiMOS operational amplifier The value of resistor R8 is impor-
The circuit described here provides with MOSFET input and CMOS out- tant to get the desired current level
around 180mA current at 5.6V and put. Inbuilt gate-protected p-channel to operate the cut-off. With the
protects the mobile phone from unex- MOSFETs are used in the input to pro- given value of R8 (3.3 ohms), it is
pected voltage fluctuations that de- vide very high input impedance. The 350 mA.
velop on the mains line. So the charger output voltage can swing to either posi- Charging current can also be
can be left ‘on’ over night to replenish tive or negative (here, ground) side. changed by increasing or decreasing
the battery charge. The inverting input (pin 2) of IC1 the value of R7 using the ‘I=V/R’
The circuit protects the mobile is provided with a variable voltage ob- rule.
phone as well as the charger by im- tained through the wiper of potmeter Construct the circuit on a common
mediately disconnecting the output VR1. The non-inverting input (pin 3) PCB and house in a small plastic case.
when it senses a voltage surge or a of IC1 is connected to 12V stabilised Connect the circuit between the out-
short circuit in the battery pack or DC voltage developed across zener put lines of the charger and the input
connector. It can be called a ‘middle ZD1. This makes the output of IC1 pins of the mobile phone with correct
man’ between the existing charger high. polarity.
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