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shunt placement current sensor

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yassin.kraouch

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Hi, i have i servo-moteur that i will drive from an msp430, i would like to insert a shunt in order to mesure the current, if the current exceed 300 mA the mcu will stop the servo, the problem is that when i place the shunt between the 0 of the servo and ground, the servo will vibrate and this is the easest way to measure current, and when i place the shunt between the VCc of the servo and the supply voltage the servo run normally but i don't know how to measure current in this way, HELP PLEASE
 

emresel

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Hi;
You need to measure the voltage at the VCC input of the servo. Since you know the supply value (let's say Vsup) then current is Vsup-(measure value at the VCC port) divided by shunt reistor value. To measure voltage, you need an ADC.
 

yassin.kraouch

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and what about the current measument of the high side using the 5 ohm shunt resistor ?
 

FvM

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and what about the current measument of the high side using the 5 ohm shunt resistor ?
Are you asking about the resistor value? I fear 5 ohm is bad both for high and low side as well, because the unwanted voltage drop. Just apply ohms law.
 

yassin.kraouch

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Yes but the voltage drop through this reisitor will be the mirror of the current, and i calculate the 5ohm is enough a good value, but the problem is when i place it in high side, i don't know how to calcule the voltage that will input ADC
 

emresel

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Which point is not clear to you? explain please.
 

alexan_e

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This is not the proper way to calculate the resistor value, you should use a very low value like 0.1R since your current is only 300mA , of even lower.
The shunt resistor should be as low as possible so that it doesn't disturb the circuit operation.
Then you have to use some kind of amplifier to amplify the small voltage of the resistor, a differential amplifier can work fine in the input level can be handled by the opamp

DiffAmp.jpg

I suppose you had tried with the 5R in the low side so you may have had problems because of the high value.
Make another test with a 0.1R and see if you have the same problems.

Alex
 

yassin.kraouch

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Good answer alex i replace the shunt with another shunt of 0.2 ohm and the i input the drop voltage to the ADC and it works fine now :), without adding an amplifier
 

alexan_e

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0.2R *0.3A=60mV
normally you need an amplifier because this value is very low, in a 10bit ADC with a ref of 5v it will give a result of just 12, lower currents like 50-100mA will be very close to the ADC error range.
I would suggest an amplifier, a value of 50 can amplify your voltage to 3v for 300mA.
An alternative is to use a low ADC reference , like 500mV so that the resolution gets much higher and the result for 60mV is 120

Alex
 

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