How to extend the current measurement range of your handheld digital multimeter up to 10X or more?
Most regular handheld multimeters have a current range that you can measure up to 10A. A problem occurs when there are situations where we need to measure a high current range, (usually in the range 20A to 1000A) and the only measuring instrument we have is a handheld multimeter.
One solution to this problem is to purchase another high rated ammeter or a clamp meter that can measure up to 1000A, but as we can see, this is very costly.
An alternative solution, which is very affordable with higher accuracy compare to clamp meter, is just to integrate what we call a current amplifier, also known as a current sensor or a current shunt.
By integrating this kind of sensor or device to our multimeter, we can now measure a very large amount of current.
In this post, I will share with you the following topics about the current shunt.
- What is a current shunt?
- Current shunt calibration setup
- How to use a current shunt in calibration
- How to verify the accuracy of a current shunt?
- 2 Important things to consider when using a current shunt for calibration
What is a current shunt?
A current shunt is the same as a resistor, thus, also called a current shunt resistor, but it has different material (called manganin) that can handle a large amount of power. It has a very low resistance value that makes it a good conductor for a current to pass through.
If you are familiar with the use of a current shunt resistor, then it is easy for you to understand its principles. It has the qualities of resistors but with different ratings and applications.
The current shunt is used to measure the current in a given circuit (AC or DC). But the current is measured indirectly by utilizing the value of voltage and resistance. It is operating in the principle of Ohms Law (V=IR).
The current shunt is also a sensor, it is used to detect the presence of a voltage which in turn used to calculate the equivalent current using Ohms Law.
Another term for the current shunt sensor is a current amplifier. Since we can measure a high range current just by measuring the voltage using a multimeter, we have extended its measurement range.
The Current Shunt in Electrical Calibration
Now that we know what a current shunt is, I will now explain its importance in calibration especially under the electrical parameter.
The main purpose of a current shunt in calibration is to measure a high current range. If you are looking for a cheaper with higher accuracy standard (above 4:1), compared to a clamp meter, then this is a good choice.
Common accuracy specifications of current shunts are ±0.1%, ±0.25% or ±0.5%. Compared to Clampmeter which is having an accuracy of 1% to 3%.
A current shunt will become part of our reference standard, and therefore must be calibrated before use. This is used with a calibrated digital multimeter.
But keep in mind that its accuracy is also dependent on the accuracy of the multimeter and therefore, we should consider using a highly accurate multimeter.
The rated value of a current shunt is shown above. It is printed in its body. How to use the rated value of a current shunt?
This means that the maximum current that can be measured by this shunt is 100A with a voltage of 75mV.
When a 100A current will pass in this current shunt (current sensor), we should expect to read a voltage value of 75 mV.
And therefore, since current and voltage is given, through Ohms Law, we can calculate the resistance value of the current shunt.
V=IR, R =V/I, Therefore:
R = 0.00075 Ω or 0.75 mΩ
This resistance value is fixed regardless of the current that is present in the shunt (with a little difference usually in the lower range). A current value with corresponding voltage (mV) value.
Or, since the current shunt is calibrated, check its calibration certificate for the exact value of the current shunt (the resistance value) for more accurate measurements at a given range of current.
Again, once resistance is known, by using Ohms Law, the formula I = V/R, we can calculate the current at any given voltage.
Current Shunt Calibration Setup
The calibration setup using a current shunt is very simple.
- We will just connect the current shunt in series with the load or within the circuit.
- Connect the multimeter probes in parallel with the shunt
- Set the meter to mV function
- Measure the voltage across the shunt.
- Calculate the current value using Ohms Law equations.
That’s how simple it is. It has the same principle when measuring a resistor in a circuit. But be careful, because we are dealing with an exposed or open electrical line.
How to Use the Current Shunt for High-Range Current Calibration
In this example. We will be calibrating an Inductive Load Box. We will focus the calibration on its current output. This is where we will use the current shunt.
As per the photo above, the multimeter is only capable to measure up to 10 A and therefore, we will use a current shunt to measure above 10A.
We will calibrate the current parameter of the inductive load box. In order to do this, we will connect the current shunt in series with the circuit, the line going to the load.
Then, at the same time, we will connect multimeter probes in parallel with the current shunt to measure the voltage across the shunt.
We will calibrate the current output of the Inductive load box which is set to an output value of 10.53 A (see the photo above)
Then using Ohms Law, we can determine the actual value of the current based on the acquired voltage reading from the multimeter. Then we can now compare and determine the error.
How to Verify the Accuracy of a Current Shunt?
The current shunt verification is almost the same as a resistor. The only difference is that we need a higher current source in order to achieve the maximum range or the working range of the current shunt.
The current shunt can be verified by generating a known current. As per the photo above, I supplied a known current and measured the voltage.
Base on the photo, a 9A current is generated, and again, through Ohms Law, we will use the voltage reading to calculate the current. (current shunt specs: 100A/75mV)
I = V/R; I = 0.0066V/0.00075 = 8.8 A
We will use the 0.5% accuracy as the basis for a tolerance limit as an example (you may include the error from the multimeter and .current generator for a wider and more acceptable tolerance)
Tolerance limit = 0.005*100 =0.5 A
Tolerance Interval = 9 +/- 0.5 = 8.5 to 9.5
Therefore, the verification result is PASSED
2 Important Things to Consider When Using the Current Shunt
- Use only 66% or ⅔ of the rated output to prevent heating which can damage the shunt.
- If in any case, you need to use above the maximum limit (above 66%), see to it that temperature does not reach above 80 deg C. Usually within 2 minutes of continues current flow.
Read more in this link >> shunts
If you are already familiar with a resistor, then using a current shunt is already easy for you. By using a current shunt, you need to familiarize yourself with Ohms Law. This where the principle of calculating the value of the current shunt is based.
By integrating this device or standard in your electrical measurement, you can extend the current measurement range of your multimeter without purchasing additional costly ammeters.
In this post I have shared with you below topic:
1, what is a current shunt.
2. The Current Shunt for Electrical Calibration
3. Calibration setup using a current shunt
4. Example of Calibration Using a Current Shunt
5. How to verify a current shunt.
6. Safety things to consider when using a current shunt.
I have presented here how to use a current shunt to measure high-range current, please make sure that you observe the safety limit in order to avoid damage or accident.
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