Help Solve Dennis's Doubt in Circuits: Battery Charger Circuit

In summary: Q4 is turned off by Q5, so it doesn't matter what voltage is on the Gate, Q4 will not turn on.The charging current should be 500mA, but it is not. Maybe there is something wrong with the circuit?
  • #1
dennis.roshan
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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


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

Regards,
Dennis
 

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  • #3
Most likely the battery is not charging because it is fully charged.

R61 is a pulldown resistor so the Gate of Q4 is not floating when Q5 is Off.

Pin 8, CC, is just a transistor to Gnd. R68 is a pull-up so a voltage can be supplied to U14-2 when CC is Off. R70 and C70, in conjunction with R68, serve as a low pass filter, or time delay, so short noise signals from the battery voltage don't have the circuit constantly trying to correct momentary, inconsequential variations.

R60 and R64 serve to limit the Q3 Base Current (R64) and the Base-Emitter voltage to safe values within the Q3 ratings.
 
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Related to Help Solve Dennis's Doubt in Circuits: Battery Charger Circuit

Question 1: How does a battery charger circuit work?

A battery charger circuit works by converting the AC (alternating current) from a power outlet into DC (direct current) which is then used to charge a battery. It typically consists of a transformer, rectifier, and voltage regulator to regulate the charging voltage and current.

Question 2: What is the purpose of a battery charger circuit?

The purpose of a battery charger circuit is to replenish the energy in a battery by converting electricity from a power source into a form that can be stored in the battery. This allows the battery to be reused multiple times, making it more cost-effective and environmentally friendly.

Question 3: What is the difference between a linear and switching battery charger circuit?

A linear battery charger circuit uses a linear regulator to regulate the charging voltage and current, which can lead to energy loss and generate heat. On the other hand, a switching battery charger circuit uses a switching regulator to improve energy efficiency and minimize heat generation.

Question 4: How can I troubleshoot a battery charger circuit?

To troubleshoot a battery charger circuit, you can check the input and output voltage and current using a multimeter, inspect the components for any visible damage or defects, and test the circuit with a known working battery to see if it charges properly. If the issue persists, it may require further testing or repair by a professional.

Question 5: How can I improve the efficiency of a battery charger circuit?

To improve the efficiency of a battery charger circuit, you can use a switching regulator instead of a linear regulator, use components with lower resistance values, and optimize the circuit design for minimal energy loss. It is also important to use a charger that is compatible with the battery type and size to prevent overcharging or undercharging.

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