DC Current measurement using Shunt resistor and Op-Amp Circuit

Published by MKDas on

Measuring current sometimes become very necessary in electronics circuits. We can measure DC current using current sensors or by using simple shunt resistors. In this article, I’m going to discuss about DC current measurement techniques and different circuits for this purpose.

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DC current sensing techniques:

Based on different circuit configuration and use of current sensing device, we can divide the DC current sensing techniques in the 2 types:

  1. Shunt resistor based sensing
  2. Hall effect sensor based sensing

Now we can get different configuration in circuits. In shunt resistor based circuits, sometimes the shunt resistor is common with measuring circuit. But sometimes, the sensing shunt resistor is not connected with the measuring circuit even sometimes the source is floating. Based on these two configuration we can divide shunt resistor based sensing into 2 part:

  1. Low side shunt resistor based sensing
  2. High side or floating shunt resistor based sensing

On the other type with hall effect sensor based current sensing, we can find another similar type named ‘Magneto-resistive current sensor‘ but it not so popular in DC current sensing at small range.

Now, here in this article, we will discuss about the shunt resistor based current sensing. In the other article I’ll cover the hall effect based current sensing techniques.

Shunt resistor based current sensing:

A shunt resistor is a resistor with very low in resistance. Sometimes it is a metal piece. Sometimes it is a PCB trace, sometimes it is a normal resistor. Whatever it is, the resistance of a shunt resistor is too low so that the power loss across the resistor become very low. As high current flows through these resistances, these resistance should have good heat dissipating system.

These shunt resistors can be connected in any configuration in the circuit. Based on the position of shunt resistor we can design our sensing circuit.

Low side shunt resistor based sensing:

Sometimes, the shunt resistor is connected with GND which is shared with the measuring circuit. That means, both have a common GND. This type of use is called low side shunt resistor based sensing. In this case, we can simply use an Amplifier to amplify the voltage across the resistor. Then we can easily calculate the current flowing through this shunt resistor.

Low side current sensing circuit

Here, the source and the shunt resistor both are connected in common GND with the measuring circuit. This is called the low side shunt resistor based sensing.

Here, we can simply use an Op-Amp based amplifier. Here we used LM358 based non-inverting amplifier.

Calculations for low side shunt resistor based current sensing circuit:

In this circuit, we used DC lamps which internal resistance is 24 Ohms. And current rating is 0.5A. A little less current is obtained due to the shunt resistor’s resistance.

I = 12V / (24 Ohm + 0.1 Ohm)

So, I = 0.496 A

On the other hand, the gain of our Amplifier is,

Gain = (R4/R3) + 1; So the gain = (10K/1K) + 1 = 11.

As you see, we are getting an output of 0.56V. So from this lets find out the current rating through our shunt resistor.

Gain = Op-Amp output voltage/Voltage across the shunt resistor;

So, Voltage across the shunt resistor = Op-Amp output voltage/Gain;

Vshunt = 0.56/11 = 0.051V

So the current through the shunt resistor is,

Ishunt = Vshunt/Rshunt;

Ishunt = 0.051/0.1 = 0.51A

Which is pretty near to our input current. So the final equation for our current sensing circuit will be,

I = Op-Amp output voltage/(Gain x shunt resistance)

Test result
With two lamps load
With three lamps load

According to our equation, here for two lamps loaded circuit the current is:

I = 1.09/(11*0.1) = 0.991A

And for the three lamps loaded circuit, the current is:

I = 1.62/(11*0.1)=1.473A

Which is almost accurate result. Using this circuit diagram we can measure the current when the shunt resistor is connected in low side. But what will happen if the shunt resistor is connected in high side and the source is floating?

High side/floating shunt resistor based current sensing circuit

When the shunt resistor is not connected with measuring circuit in either way of VCC and GND then we can say that the shunt resistor is floating or High side shunt resistor. In this case, we can use differential amplifier to measure the voltage across that shunt resistor. Then using some equations, we can calculate the current through the shunt resistor.

High side shunt resistor based current sensing

As you can see here the shunt resistor R1 is floating. So we used a differential amplifier to measure the voltage across that shunt resistor.

Calculations for high side/floating shunt resistor based current sensing circuit:

The output voltage of a differential amplifier is calculated by this equation,

Vout = Vdif x (R4/R3);

Here Vdif = V2 – V1. Or simply voltage across our shunt resistor. Simply keep in mind that, V2 > V1 and V2 goes to the non-inverting terminal of the Op-Amp. There are details calculation for differential op-amp. But if you just remember this for work, its enough.

As you see in our circuit, the current through the load is 0.49A. Now, we’ll see how can we get this result from our calculation.

If this current is multiplied by our shunt resistor, we’ll get the voltage across the shunt resistor.

Vshunt = 0.49×0.1 = 0.049V

Now, gain of our amplifier is,

Gain = R4/R3 = 220K/10K = 22.

Here, from the differential amplifier calculation, we get this equation:

Vout = Vdiff x Gain.

So, Vdiff = Vout/Gain;

Here in our test circuit, we are getting 1.11V at op-amp output. So,

Vdiff = 1.11/22 = 0.0505V

We know, our shut resistor is 0.1 Ohms. So from V=IR equation,

Ishunt = Vshunt/Rshunt. Here, Vshunt is our Vdiff and Rshunt = 0.1 Ohms

So, Ishunt = 0.0505/0.1 = 0.505A; Which is almost 0.49566A

That means, we can easily find our current through load measuring the output voltage of Op-Amp using this equation:

I = Op-Amp Output/(Gain x Rshunt)

Test result

As you can see from this video, we can calculate the load current keeping the source floating. And the test result is completely accurate according to our calculation.

Test with 2 Lamps
Test with 3 lamps

Summarizing:

After learning about two types of circuit we can now summarize that, we can calculate the load current or shunt current using the same equation in two cases.

Load current/ Shunt current = Op-Amp Output voltage/ (Gain of Op-Amp x Shunt resistance)

So all we need to find first is the gain of used op-amp circuit. Then we can easily measure the load current from Op-Amp output voltage. Now we can use this technique in many circuits…

I hope this article was helpful to you. I hope, this circuit configuration and other information will help you to make your project. If you need any help, feel free to ask. Thank you. Enjoy!


MKDas

Mithun K. Das; B.Sc. in EEE from KUET. Blog: https://labprojectsbd.com

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