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# How this circuit measure current!?

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#### arvinfx

##### Member
As you can see this is a DIY battery capacity measure circuit, my question is:

How this microelectronic sense the Current? it connected to only 2 nods and both of them don't connect to microcontroller !?

Original Site: http://danyk.cz/avr_aku_en.html

It appears to work by discharging a battery/cell at a constant current and timing how long it takes for the cell to drop to its "fully discharged" voltage.

R16 is the current sensing resistor; with that, T1 and the opamp forming a constant-current circuit directly across the cell, that can be switched on & off by the microcontroller.

Pins PB1 is uses to turn the discharge circuit on or off & ADC0 measures the cell voltage

PC5/ADC5 appears to be another way of turning the discharge circuit on or off, as well as controlling the LED?

There is no charge circuit, it must rely on you charging the cell in a separate charger then this just measures the discharge capacity.

Thanks rjenkinsgb, but i amazed who this circuit measure the current of discharge!? there is a shunt resistor R16 but it seems to connect to nowhere.!

Thanks rjenkinsgb, but i amazed who this circuit measure the current of discharge!? there is a shunt resistor R16 but it seems to connect to nowhere.!

As already pointed out, it doesn't measure the current - there's no need to as it's set by a constant current source (which R16 is part of), so all you have to do is measure the voltage.

Thanks

The opamp compares the voltage across R16 to the refrence voltage set by the voltage divider R10-R12-P2.

Depending if the voltage across the resistor is lower or higher, the amp output changes the gate voltage on the FET and controls the battery current, so the voltage across the resistor is always held the same.

That means the current is always the same, until the battery has discharged to the point it cannot provide that current.

The battery voltage is monitored separately so the device knows when the battery has discharged to the appropriate end-of-test voltage.

For calibration, the discharge current is set during the device commissioning, by adjusting P2 to gt whatever voltage the designed specified across R16 while the test is in progress.

The current set of the ATMEGA 8 (PB1) is an hardware PWM Output of this chip.
So the Current can be set by PWM width modulation.
R10 an C7 built a low pass filter to convert the PWM pulses into a constant voltage.
With P2 You can calibrate the Current source to get correct measuring results by the microcontroller.

Another possible way to measure the real current is to use an isolatet current sensor and measure the output voltage of this.
E.G. the ACS750 from Allegro.

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