# Current measurement circuit help needed

#### mishra87

Joined Jan 17, 2016
947
Hi Guys...

Could any body explain me how attched circuit is working.

How does it measure the motor and battery current.

Thanks !!!

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#### Audioguru again

Joined Oct 21, 2019
4,096
Current in the two series 10m resistors produces a voltage acrosss them according to Ohm's Law. This voltage is amplified and inverted by the opamp.
EDIT: 10m resistor Rx does nothing but act as a piece of wire and 10m resistor Ry has the motor and battery current in it. The opamp is not powered so it will do nothing.

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#### AnalogKid

Joined Aug 1, 2013
9,619
This will go much better if you add reference designators to each component.

Are you sure the left-side 10 m resistor is in the circuit? In series with a 1.5K resistor, it does nothing.

What is the opamp part number and what are its power rail(s)?

ak

#### KeithWalker

Joined Jul 10, 2017
2,091
What is the power supply to the op-amp and how is it referenced to the circuit common shown in the diagram. How much current does it take to drive the motor? Without that information we have no way of knowing if the circuit would work.

#### Audioguru again

Joined Oct 21, 2019
4,096
I edited my post #2. The opamp is not powered. I think on another forum it is a lousy old 741.

#### mishra87

Joined Jan 17, 2016
947
Hello Guys,

I have updated the schematic.
I do not understand how this circuit is amplifier is working.

Power supply of opamp is 4V around and mcu is working with same supply level.

Battery charger is connected with R1.
Still I don't understand when does R1 and R2 come into picture . As per my understanding Charger is only connected when battery comes lower cutoff voltage.
I am not sure when these resistor are used for charging current and motor current.

Regards,

#### Attachments

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#### AnalogKid

Joined Aug 1, 2013
9,619
What is the external battery charger output voltage?

Also, again, what is the part number of the opamp. You show a part powered by 4 V trying to process a signal of 4.2 V. There are *very* few opamps that can do that.

Your understanding might be correct if R1 and R2 were diodes.

ak

#### mishra87

Joined Jan 17, 2016
947
What is the external battery charger output voltage?

Also, again, what is the part number of the opamp. You show a part powered by 4 V trying to process a signal of 4.2 V. There are *very* few opamps that can do that.

Your understanding might be correct if R1 and R2 were diodes.

ak
Hi ak,

Sorry but information is not clear to me as well.
Let me try explain more about it.

STM8S003F3 microcontroller is used to measure the opamp voltage. The supply voltage for this controller is 2.95V-5.5V so now i assume the opamp supply would be 5V. sorry again for wrong infromation.
https://www.st.com/resource/en/datasheet/stm8s003f3.pdf

Opamp part number MCP6001UT is used.
https://www.mouser.in/datasheet/2/268/21733j-740845.pdf

Edit :
Battery is 4.2V Li-Ion 2x4.2 =8.4V
Charger is around 9V

Attached is the partial part of the schematic.
These two resistor R1, R2 are used to monitor battery charging current and motor driving current but i am not sure.

Regards,

#### Attachments

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#### ronsimpson

Joined Oct 7, 2019
1,750
It looks like:
When there is no charger and is a batter; The motor current is monitored.
When there is a charger and no battery; The motor current is monitored.
Then there is a charger and a battery; The battery charging current and the motor current are mixed together.

Do you want to only watch the motor current?
Do you want to watch the battery charging current?

#### mishra87

Joined Jan 17, 2016
947
It looks like:
When there is no charger and is a batter; The motor current is monitored.
When there is a charger and no battery; The motor current is monitored.
Then there is a charger and a battery; The battery charging current and the motor current are mixed together.

Do you want to only watch the motor current?
Do you want to watch the battery charging current?
Thanks
ronsimpson

Yes, i wanted to understand these three cases.

When there is no charger and is a batter; The motor current is monitored.
Yes, correct since opamp + terminal is at higher potential and - terminal is at lower potential so there will be output at opamp terminal 4. For negative output opamp is grounded.

When there is a charger and no battery; The motor current is monitored.
opamp + terminal is at lower potential and - terminal is at higher potential so there will be no output at opamp terminal 4.

Then there is a charger and a battery; The battery charging current and the motor current are mixed together.

when charger is connected -: Inverting terminal voltage is higher at higher potential and non inverting terminal at lower potential so there is no output voltage at op-amp output.

This the case when motor is not working . e.g. saving trimmer

Still i am not clear the use two shunt resistance use case and the working of this circuit.?

Thanks !!!