Please help me analyse this circuit

[mahaju]

how to analyse a circuit

Could someone please help me analyse this circuit?

Thanks in advance.

What I mainly want to know is whether opamp 301 is only a comparator or something else.

 

[banjo]

opam lm 350

The opamp appears to be configured as just a comparator. It does not control the regulator. It just seems to control the full charge LED.

However, the location of the "start" switch is odd. Why it is needed and why the opamp's output is shorted to ground, it not clear.

The opamp appears to really be comparing two scaled current. The positive terminal monitors the scaled current through R4. The negative terminal monitors the scaled current through R7. When a discharged battery is attached, the charging current is high, the voltage drop across R7 is high and the negative terminal of the opamp is lower than the positive terminal. When the battery reaches a good charge level, the drop across the current sense falls, the negative terminal exceeds the positive terminal, and the LED turns on.

The charging current is fixed by LM350T output characteristics. The 500 ohm resistor across the LM350T is a trickle charge path for when the battery is fully charged.

 

[Audioguru]

lm350 short circuit

The output of the opamp reduces the output voltage of the regulator when the charging current drops to a certain low value. It also turns on the transistor which lights the LED.
Positive feedback through R5 and R6 latches the opamp so that the charging stays off.

The START button turns on the charging.

 

[mahaju]

op-amp balance pin

Thank you for your explanation

I still do not understand the function of the start switch

The output of the LM350 regulator is normally around 14.3V, but when the start switch is pressed it rises to around 20V, and the output of opamp goes high and the RED led goes off.

The switch does not connect the output of op amp to ground, rather it is the pin no 1, which is offset. I dont understand what happens when you ground the offset pin of an opamp.

 

[Audioguru]

lm301 open loop gain

The LM301 has a special offset/compensation pin #1.
The offset pin #1 on most opamps works differently.

Unlike most opamps, the LM301 has inputs that work when they are near its positive supply voltage.

 

[banjo]

lm350 opamp

When the opamp's output goes low, R6 is paralleled across R2. This reduces the output voltage.

When the opamp's output goes high, D1 is reverse biased and the opamp cannot effect the output voltage. In this condition, the output voltage should be approximately 1.25V(1+(R2/(R4+R5)) or about 16.6V. [ I am neglecting the error term of the adjust current.]

Pin 1 of the LM301 is called the balance pin on the datasheet I looked at. It is used for AC compensation. Grounding this pin has a similar effect as placing the inverting terminal of the opamp at the negative power rail, gnd. This circuit appears in the app note section of the LM350 datasheet. Since both the LM350 and LM301 are National Semi parts, the designer used his/her insider info to know the tricks that can be done with the balance pin.

This circuit's safe operation requires the use of the LM350. This part is current limited to 3A and thermally protected. If you attach a shorted or very low battery, only this regulator chip limits the current. This regulator IC's output is short circuit protected. Many others are not. Substituting for the LM350 could cause problems.

 

[harii74]

opamps balance pin ofset

The opamp is in comparator mode(no feed back gain only open loop gain(infinite).
The Pin3 is reference and pin2 is a measuring point.
During charging pin 2 is less than 3.When battery is charged for enough the voltage is raised and the pin2 gets more than pin3 and outputs negative and diode conducts.
This negative output from diode will reduce /or increase the output of LM350( i need to study its function).
But in circuit nature The voltage at pin 1 of LM350 is reduced as diode conducts.

Added after 12 minutes:

Hi,
Th offset of any opamp is connected to + or - supply to balance the output to get zero output for zero input.(This you know very well)
Know one thing, When offset pin( either one) connected to any one of the rail(+ or - supply), then output goes to any one of the rail.
In this case as offset connected to ground (or - supply) the output goes to + rail,so that diode is reverse biased and Pin 1 of LM350 gets more + voltage and gives higher + voltage as output of regulator.