# Differential amplifier help needed ?

#### mishra87

Joined Jan 17, 2016
906
Hi Guys,
Hope you are staying safe.

I have two questions regarding attached schematic

1. Battery Mode : Opamp will measure DC load current at R7 and 9V charger is not connected
2. Charging mode : DC load will be disconnected and opamp will measure the charging current of battery

Now i want to understand attached differential amplifier circuit.

For case 1 : Opamp output = (R6/R5) * (V+ - V-)
so this correct because V+ is always greater than V- hence Vout opamp is +Ve

For case 2 : Opamp output = (R6/R5) * (V+ - V-)
In this case V+ is less than V- hence Vout opamp is -Ve. Since opamp is at GND level so Vout opamp is 0V

I have a doubts on case 2 and may be my understanding on case 1 is correct.

Could anybody clarify on above two points.
Note : Attached two circuit are combined for for DC load current measurement and Battery charging current measurement. When battery is in charging mode DC load will off and hence opamp will measure battery charging current. When battery is charged and charger come out and DC load works with battery only so opamp will now measure DC load current.

For simulation purpose i have created two circuit

Best Regards,

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#### Audioguru again

Joined Oct 21, 2019
2,890
At www.batteryuniversity.com they explain everything that a Lithium (two series cells, max 8.4V) battery charger and discharger must do, that is in a battery charger IC. Your circuit will overcharge and unbalance the cells causing an explosion.
Also a low voltage detection and disconnection circuit is missing.

#### mishra87

Joined Jan 17, 2016
906
At www.batteryuniversity.com they explain everything that a Lithium (two series cells, max 8.4V) battery charger and discharger must do, that is in a battery charger IC. Your circuit will overcharge and unbalance the cells causing an explosion.
Also a low voltage detection and disconnection circuit is missing.

I wanted to understand a very narrowed area i.e. opamp working to measure charging current and DC load current. My intention not to understand the BMS system for now.

Anyway thanks !!!

#### Audioguru again

Joined Oct 21, 2019
2,890
The opamp is destroyed because its input voltages are higher than its maximum allowed voltage of 6V and higher than 0.3V above your 4V supply for it. Use an opamp that works with a 9V supply and with inputs that work up to the supply voltage.

Why is the charger charging a 9 ohm resistor instead of charging the battery to 8.4V maximum?

#### mishra87

Joined Jan 17, 2016
906
The opamp is destroyed because its input voltages are higher than its maximum allowed voltage of 6V and higher than 0.3V above your 4V supply for it. Use an opamp that works with a 9V supply and with inputs that work up to the supply voltage.

Why is the charger charging a 9 ohm resistor instead of charging the battery to 8.4V maximum?

After checking information.
Charger used for charging purpose is 9V only.
Yes opamp used is MCP6001UT-I whose differential input voltage of pin is VDD-VSS i.e. 4V in our case.

Since we are measuring differential signal Op-Amp will not get destroyed at any case.

Regards
Stay Safe

#### Audioguru again

Joined Oct 21, 2019
2,890
You changed the opamp to an MCP4001 but it will still be destroyed since its absolute maximum allowed input voltage is Vdd +1V (5V with your 4V supply) and the simulation shows the inputs have a voltage of 8.8V when the battery is charging and 8.4V when the fully charged battery is discharging.

#### mishra87

Joined Jan 17, 2016
906
You changed the opamp to an MCP4001 but it will still be destroyed since its absolute maximum allowed input voltage is Vdd +1V (5V with your 4V supply) and the simulation shows the inputs have a voltage of 8.8V when the battery is charging and 8.4V when the fully charged battery is discharging.
Correct me if i am worng,
Since we are measuring differential signals through op-amp input terminal and datasheet says VDD-VSS i.e. 4V .
Now in our case we will get only few mV differential voltage at op-amp input terminal so it will not get damage at any case.

#### OBW0549

Joined Mar 2, 2015
3,566
Correct me if i am worng,
Since we are measuring differential signals through op-amp input terminal and datasheet says VDD-VSS i.e. 4V .
Now in our case we will get only few mV differential voltage at op-amp input terminal so it will not get damage at any case.
WRONG.

The differential input voltage is not the only thing that matters; the absolute input voltage on the inputs also matters, and as has been pointed out several times, you are exceeding THAT absolute maximum rating.

#### Audioguru again

Joined Oct 21, 2019
2,890
I remember that you are the video guy. Please post a video of your over-voltage opamp burning up.