# Doubt in Circuits

1. Jul 26, 2011

### dennis.roshan

Hello, my name is Dennis; and I am rookie engineer. I have a doubt in the above circuit and I was wondering if any of you could help me.
I have attached a schematic for a power/ battery charger circuit. Right now My concern is on the battery charger circuit marked with a red box. According to the design and circuit the charging current should be 500mA, pratically it is not so.Can anyone explain why? On the multimeter I read current of 30mA and keeps decreasing; the source is +6Vdc and current reading on it is 0.1A .

This is a brief explantion of the circuit of what I think it is.

bq 2002 IC and party analyzed circuit

BQ2002 IC decides wheather charging current flows from the DC source to the battery or not.

pin 4 is GND and pin 6 is Vcc. for the ic to work pin 6 is given to 6V and pin 4 to GND

Input pins
Vbat(pin 3) is monitored if this voltgae exceeds 2V then bq2002 stops the charging current. (i.e. max voltage in battery per cell. )
Vtemp(pin 5) : the battery temp is measured via thermistor. this has a negative temp cofficient, so if temperature increases voltage decreases. if this voltage drops below 1.5v(excessive heating of battery has occurred) then bq2002 stops the charging current.
INH (pin7): this is an input pin, if this goes high then charging is stopped. if this is low then charging is continued. (bassically works like an intterupt).

if the charging is stopped by meeting any of the parameters then powers has to be restarted for the charging to continue.

Output pins
if BQ2002 decides to allow the charging current then the output pin8 CC goes to high impedance state(5.9V).
if BQ2002 decides to not allow the charging current then the output pin8 CC goes low (0V)
Ignore LED pin
TM decides the charging rate (C) in our case TM is connected to Vcc/2 to achieve C/4 charging current.
In the C/4 charge rate, if the charge time exceeds 320 min or 5hrs and 20 min bq2002 terminates the charging current.
in our case as seen in the schematic TM (pin1) is attached in between 24Kohm resistance between 6v and GND. so if the charging time exceeds 5hr and 20min the charging current is stopped.

U14B is a comparator circuit. battery temp voltage is connected to pin 5(+ve terminal) if U14B, 2V is given to -ve terminal. this generates a low on pin 7 of U14B which is connected to INH pin. If a non rechargable is connected here then INH will go high, else it will be low.

D11,R73, R74, R75 is used to generate 5.5V which is given to pin 3 of U14A.

we know that CC (Pin 8) - if high impedance(5.9V) to allow charging current - if low stop charging current.

U14A is a comparator. it compares cc(Pin8) with 5.5V. if CC is high impedance then the output on pin 1 of U14A goes low which turns on the pnp power transistor which allows the charging current flow from the dc source to the battery. if CC is low then the output on pin 1 of U14A goes high which makes the pnp power transistor off which stops the charging current flow from the dc source to the battery.
U14A also works as an opamp as a current source. follow the link and compare with U14A circuit
http://www.eecs.tufts.edu/~dsculley/...s/opamps7.html
current = (+6V - Vref)/1ohm = .5A or 500 mA
Vref = 5.5V which is labeled as 0.5V(pin 3 of U14A) which means Vref is 0.5V below the +6V

this is the rought idea, the link below is the data sheet for BQ2002 it tells u what I summarized above
http://www.datasheetcatalog.org/data...ts/bq2002t.pdf [Broken]

In addition I would like to know the purpose of R60, R61, R64, R68, R70 and C70

Regards,
Dennis

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Last edited by a moderator: May 5, 2017