The SCT013 series are sensors of non-invasive, current transformers that measure the intensity of a current that crosses a conductor without needing to cut or modify the conductor itself. We can use these sensors with a processor, like Arduino, to measure the intensity or power consumed by a load.
The SCT013 sensors are current transformers, instrumentation devices that provide a measurement proportional to the intensity that a circuit crosses. The measurement is made by electromagnetic induction.
SCT013 sensors have a split core (like a clamp) that allows the user to turn it on to wrap electrical equipment without having to cut it off.
In the SCT013 series there are models that provide the measurement as a current or a voltage output. It is more preferable to use voltage output because the connection is simpler.
The sensor accuracy can be off by only 1-2%. To ensure highest accuracy, it is critical to confirm that the core has been properly closed. Even a small air gap can cause a 10% deviation.
As a disadvantage, being an inductive load, the SCT013 introduces a variation of the phase angle, whose value is a function of the load that passes through it, being that it is able to reach up to 3º.
Current transformers are common components in the industrial world as well as in electrical distribution, as they allow the points of consumption to be monitored, whereas another form of measurement does not exist. They are also considered multiple measurement instruments, even in portable equipment such as perimeter clamps or network analyzers.
For example, in our electronics and home automation projects, we can use the SCT013 current sensors to measure the electrical consumption of a device, check the status of an electrical installation, and to record the consumption of electricity in home energy monitors. an installation or even access through the internet in real time.
PRICE
The SCT013 series has a variety of models that can change the measurement range and output shape. Physically, they are the same, although it is possible to identify them by the text written on the product shell.
| Model | SCT013-000 | SCT013-005 | SCT013-010 | SCT013-015 | SCT013-020 |
| Input current | 0-100A | 0-5A | 0-10A | 0-15A | 0-20A |
| Output type | 0-50mA | 0-1V | 0-1V | 0-1V | 0-1V |
| Model | SCT013-025 | SCT013-030 | SCT013-050 | SCT013-060 | SCT013-100 |
| Input current | 0-25A | 0-30A | 0-50A | 0-60A | 0-100A |
| Output type | 0-1V | 0-1V | 0-1V | 0-1V | 0-1V |
The price of all models is similar, and we are looking for international sellers such as eBay, Amazon or AliExpress.
The most common model is SCT013-000, of which the maximum current is 100A, the current output is 50mA (100A:50mA), the maximum current of SCT-013-030 is 30A (30A/1V), and the voltage output is 1V.
Finally, while it is important to have a wide range of measurements, it is important to bear in mind that a higher intensity model will result in less precision. An intensity of 30A to 230V corresponds to a load of 6,900W, which is enough for most home users.
How does the SCT013 work?
The SCT013 sensors are small current transformers, or CTs. Current transformers are instruments widely used for measuring elements.
A current transformer is similar to a voltage transformer and is based on the same operating principles (in fact, they were previously identical). However, they have different objectives and, as a result, are designed and constructed differently.
A current transformer seeks to generate an intensity in the secondary that is proportional to the intensity that passes through the primary. For this, it is desired that the primary is formed by a reduced number of turns.
We can use the current transformer to build non-intrusive current sensors. In the current sensor, the ferromagnetic core can be separated so that the conductor can be opened and rolled up.
So, we have a transformer, it is:
When the alternating current circulates through the conductor, a magnetic flux is generated in the ferromagnetic core, which in turn generates an electrical current in the secondary winding.
The intensity transformation ratio depends on the relationship between the number of turns:
The primary is usually formed by a single loop made by the conductor to be measured. Although, it is possible to wind the driver making this happen more than once inside the “clamp”. The number of turns of the secondary, integrated in the probe, varies from 1000-2000, according to the models of the SCT013.
Unlike voltage transformers, in a current transformer, the secondary circuit should never be opened, because induced currents could damage the component. For this reason, the sensors of SCT13 have protections: resistance burden in the sensors of output by voltage, or diodes of protection in the sensors of exit by the current.
Assembly Diagrams
To understand the connection of the sensor SCT013, we have to understand and solve three problems:
Sensor output in intensity
Adjustment of the voltage range
Positive and negative stress
SENSOR OUTPUT IN INTENSITY
The SCT013 are current transformers, that is, the measurement is obtained as an intensity signal proportional to the current flowing through the cable,. Processors, however, are only capable of measuring voltages.
This problem is easy to solve. To convert the output in intensity into a voltage output, we only have to include a resistance (load resistance).
With the exception of model SCT013-000, all other SCT013 models have an internal load resistance so that the output is a voltage signal of 1V. This is why it will not be a concern to worry about.
Only in the case of SCT013-000, there is no resistance internal burden, so the output is a signal of ± 50mA. A resistance of 33Ω in parallel with the sensor will suffice.
Positive and Negative Tensions
Another problem we have to solve is that we are measuring alternating current, and the intensity induced in the secondary is alternating. After passing through the resistance burden, whether internal or external, the voltage output is also alternating.
However, as we know, the analog inputs of the majority of processed currents, including Arduino, can only measure positive voltages.
To measure the voltage at the transformer output, we have several options, ordered here from least to most recommended:
Voltage Range Adaptation
The last problem is the need to adapt the range of voltages at the sensor output. Arduino can only perform measurements between 0 and Vcc. In addition, the smaller the range, the greater loss of accuracy, so we should adapt to this range.
On the other hand, we must remember that when it comes to AC voltage, RMS values are usually used. Briefly review the peak voltage and peak-to-peak equations:
Therefore, for a sensor with an output of ±1V RMS, the peak voltage is ±1.414V and the
In the case of the SCT013-000, the output will be ±50mA. With an external load resistance of 33Ω, the output voltage is ±1.65V RMS, so the peak voltage is ±2.33V and the peak-to-peak voltage is 4.66V.
Electric Connection
We already have all the components to measure the network intensity with an SCT-013 sensor. We will use a sensor with voltage output ± 1V RMS and internal burden resistance, together, with an ADC like the ADS1115 in differential mode.
Adjusting the gain of the ADS1115 to 2.048V will place it within the range of ± 1.414V. In the case of a 30A sensor we will have an accuracy of 1.87mA, and 6.25 mA for a 100A sensor.
If you use an SCT013-000 with an output of ± 50mA, we will have to add an external load resistor of 33Ω and raise the gain of the ADS1115 to 4.096V to comply with the range of ± 2.33
The connection, seen from Arduino, would only be the power supply of the ADS1115 module as we saw in the entry on the ADS1115.
For the measurement, it is important that we use only one wire in the “clamp” If we use multiple conductors (two conductors for a single-phase installation and three for a three-phase installation), the role of the conductor will be abolished. This produces a zero inductance, and therefore produces an empty measurement.
The SCT013 sensor has a Jack 3.5 connector, which is very common in the audio field but is not sufficient to use in our electronic projects. To be able to connect it, we must cut off the cable or get a female connector off our welding cable. Fortunately, these terminals are easy to get, but do not rule out cutting off the cables.
If you don’t want to use an external ADC, you can also use a more traditional solution that adds a circuit that allows us to add a center offset.
From now on, we will assume that you use an Arduino with Vcc 5V. If you use another processor or an Arduino model with another Vcc (for example, 3.3V), you should correct the part accordingly.
When we added a 2.5V DC offset point, the final range was 1.08V to 3.92V, with Arduino powered at 5V over the analog input range.
Code example.
Assembly with ADS1115
If you use a component with SCT013 with ±1V RMS output and ADS1115, the required code is similar to the code we see in the input to the ADS1115. You need to consult the Adafruit library for ADS1115.
In order to sample the ADS1115 at a higher speed, we need to modify the file, ‘Adafruit_ADS1015.h’.
With this modification, we will be able to reduce the sampling time from about 8-9 milliseconds (about 100 hertz) to about 1.8 milliseconds (about 500 hertz). As we leave the Nyquist frequency, we improve the measurement behavior.
Another version uses the measured maximum and minimum values and then calculates the measured value based on the peak value. The result should be similar to that seen in the square sum example. To do this, you can replace this feature with the following features:
We can see the results in the serial port monitor, and either draw them with a serial plotter, collect it in a larger project to display on a web page, or register it in SD.
In this case, the example is very simple and we only need to measure through analog input:
Rogowski coils are specialized devices for measuring alternating current (AC) or high-speed current pulses, offering unique advantages for various applications. They are particularly useful in scenarios where flexibility, accuracy, and safety are crucial.
Rogowski coils are flexible and can be wrapped around live conductors without disconnection, making them perfect for tight spaces or crowded electrical panels. This non-invasive installation reduces downtime and simplifies setup, especially in industrial or commercial settings.
Unlike traditional current transformers, Rogowski coils use an air core, preventing magnetic saturation. This allows them to accurately measure very high currents, such as those in power transmission or welding, and handle rapid changes, like transients or pulses, making them suitable for high-frequency applications.
They are safer because they don’t produce high voltages when open-circuited, reducing electric shock risks. Additionally, their lightweight and simple design lowers construction and installation costs, offering a cost-effective solution for current measurement.
A Rogowski coil is a specialized electrical device designed for measuring alternating current (AC). It consists of a helical coil of wire wound uniformly around a non-magnetic, air-filled core, typically arranged in a toroidal (doughnut-like) shape. The coil is placed around a current-carrying conductor, and as the AC flows through the conductor, it generates a changing magnetic field. This changing magnetic field induces a voltage in the Rogowski coil, which is proportional to the rate of change of the current (the time derivative, di/dt).
To obtain a direct measurement of the current, the induced voltage is typically processed through an integrator circuit, which converts the voltage signal into one proportional to the current itself. This makes the Rogowski coil particularly useful for applications requiring the measurement of high currents or fast transient currents, such as in power systems, industrial equipment, or research settings.
Air Core Design: Unlike traditional current transformers with iron cores, the Rogowski coil’s air core prevents magnetic saturation, allowing it to measure very high currents without distortion.
Flexibility: Many Rogowski coils are designed to be flexible and can be opened to wrap around a conductor without needing to disconnect it, making installation straightforward and non-invasive.
Wide Bandwidth: The coil can accurately measure currents with high-frequency components or rapid changes, such as those in lightning strikes or switching transients.
Power system monitoring and protection.
Measuring high currents in welding or industrial processes.
Research applications, such as analyzing transient phenomena.
Current transformers (CTs) are essential components in modern electrical systems. They allow for the safe measurement, monitoring, and control of high currents by producing a scaled‐down replica of the current flowing in a conductor. This article delves into how current transformers work, their various types, and the diverse applications in which they are used.
Current transformers are devices designed to measure high electrical currents by generating a lower, proportional current that can be easily and safely measured by standard instruments. They serve as a bridge between high-power circuits and sensitive electronic equipment, ensuring that monitoring and control systems can operate without being exposed to dangerous high currents.
At their core, current transformers work on the principle of electromagnetic induction. When a current flows through the primary conductor (which can be as simple as a single wire passing through the CT’s core), it produces a magnetic field. This magnetic field induces a current in the CT’s secondary winding. The ratio of the number of turns in the primary winding (often just the conductor itself) to the number of turns in the secondary winding determines the scale factor, which in turn sets the output current relative to the primary current.
For example, a CT designed to provide a 1 A secondary output might have a ratio of 3000:1, meaning that when 3000 A flows in the primary circuit, the CT outputs 1 A. In many cases, the secondary output is standardized (such as 1 A, 5 A, or even a voltage output like 0.333 V) to simplify instrumentation and protection system designs.
There are several types of current transformers available, each suited to different applications:
Current transformers have a wide range of applications across various sectors:
CTs are fundamental in electricity metering, allowing utilities and consumers to accurately measure energy consumption. By stepping down the high currents found in power distribution systems to measurable levels, CTs enable precise billing and energy monitoring.
In protection relays and circuit breaker control, CTs provide crucial information about the current flowing through a circuit. This data helps in detecting abnormal conditions, such as overloads or short circuits, and initiates protective measures to isolate and protect equipment.
Industrial automation and control systems use CTs to monitor current levels in motors, transformers, and other equipment. This monitoring helps in managing system performance and efficiency while ensuring that components operate within safe parameters.
In renewable energy installations, such as wind farms and solar power plants, CTs are used to monitor the flow of electricity from generation to the grid. They ensure that systems operate efficiently and safely while providing data for performance optimization.
CTs help in ensuring personnel safety by enabling the use of isolated measurement instruments. By providing a low-level secondary current, CTs allow for the safe operation of meters and sensors that would otherwise be at risk if connected directly to high-current circuits.
Used to characterize dynamic behavior of a transducer, step response time is the delay between the primary current reaching 90% of its final value and the transducer’s output reaching 90% of its final amplitude. The primary current shall behave as a current step, with a given di/dt slope (usually 100A/µs) and with the amplitude close to the nominal current value IPN .
The residual magnetic flux (remanence) of the core varies depending on the sensor type and the magnetic material and causes an additional measurement offset, called “magnetic offset”. The value of the magnetic offset depends on the previous magnetization of the core and reaches a maximum value after the magnetic circuit is saturated.
Magnetization can occur in the following situations:
1. High overload current
2. Power supply interruption (power failure)
The offset caused by magnetization disappears in the following situations:
1. The offset caused by magnetization disappears naturally (the core recovers slowly, the speed depends on the magnetic material)
2. It is eliminated by demagnetizing the sensor, for example by appropriately reversing the primary current or using a dedicated demagnetization cycle. After demagnetization, the sensor can restore its initial performance.
To demagnetize a transducer’s magnetic core, the following steps are typically used:
1.AC Excitation: A low-frequency AC source drives the magnetic core through its entire B-H hysteresis loop (the magnetic field vs. magnetization curve), applying alternating current to cycle the core’s magnetization from positive to negative saturation.
2.Gradual Decrease of Excitation: After cycling the core through the B-H loop for at least 5 full cycles at maximum amplitude, the amplitude of the AC excitation is reduced gradually. The rate of reduction should be no faster than 4% per cycle to ensure a smooth return of the core’s magnetization to zero (the origin of the B-H curve).
3.Time Requirements: Typically, a demagnetization cycle will require around 30 cycles at a low frequency, such as 500 milliseconds at 60 Hz.
For closed-loop transducers (devices that have a compensation coil), additional care is needed because the compensation coil can interfere with the demagnetization process by negating its effects. This interference can prevent the core from reaching its neutral state. Therefore, extra steps may be required to ensure proper demagnetization.
Alternatively, partial demagnetization can be achieved by applying a specific signal of opposite polarity to the core’s magnetization. However, this method is more complex because it requires precisely determining the correct amplitude and duration of the signal to effectively reduce the magnetization without overshooting or undercompensating.
In practice, if a specific application is well understood, it is possible to determine the necessary correction signal empirically and apply this whenever demagnetization is needed.
Demagnetization is essential for maintaining the accuracy and reliability of a transducer’s measurements, especially after strong magnetic influences or long-term use.
Let’s take a product with a supply voltage of +5V and an output of 2.5V ± 0.625V with a precision of 1% as an example.
Our Vref pin serves two functions: one is to act as a reference point for output, commonly used in differential input acquisition devices; the other function is for single-ended input acquisition devices, where it is used to control the sensor output, making the static output (2.5V) more accurate. It only affects the static output (2.5V when there is auxiliary power and no input signal) and does not affect the amplification (0.625V).
Due to the discreteness of product parameters, sensors cannot be perfectly identical and ideal, and there will always be some degree of error. For most of our products, the static reference point error is within ±15mV. This means the static output of 2.5V (with 0V or GND as the reference point) will be between 2.485V and 2.515V, which is considered within the acceptable range.
For example, for a product with a power supply of +5V and an output of 2.5V ± 0.625V with a precision of 1%:
If the output is referenced to 0V (GND), we calibrate it as follows:
First, check if the Vref voltage meets the requirements (standard is 2.5V ± 1%, generally no more than 0.5%).
The second step is to adjust the static output to between 2.485V and 2.515V (i.e., 2.5V ± 15mV).
The third step is to apply the input signal to the rated input and adjust the output to within 2.5V + (0.625V ± 1%).
Finally, check if the output at 10% of the rated input is accurate. If there is significant error, adjust the zero-point component for correction. Then, check if the zero-point and full-scale values meet the requirements. If everything is correct, the product moves on to the next process step.
If the output is referenced to Vref, we adjust it as follows:
First, check if the Vref voltage meets the requirements (standard is 2.5V ± 1%, generally no more than 0.5%).
The second step is to adjust the static output to between 2.485V and 2.515V (i.e., 2.5V ± 15mV).
The third step is to connect the black probe of the multimeter to the Vref point, apply the input signal to the rated input, and adjust the output to within 0.625V ± 1%.
Finally, check if the output at 10% of the rated input is accurate. If there is significant error, adjust the zero-point component for correction. Then, check if the zero-point and full-scale values meet the requirements. If everything is correct, the product moves on to the next process step.
If a product adjusted using the first method is used in a differential input acquisition device, the acquired signal will be inaccurate and may even produce negative output errors.
If a product adjusted using the second method is used in a single-ended input acquisition device, the acquired signal will also be incorrect and may result in overflow errors.
By default, our products are calibrated using the first method (referenced to 0V/GND), as most customers request this.
Example 1: A product calibrated using the first method, with Vref = 2.510V and Vout = 2.505V in the static state. This product is qualified, and the precision is acceptable. However, when used in a differential input acquisition device, since Vout is lower than Vref, the static output will be negative, which is incorrect.
Example 2: A product calibrated using the second method, with Vref = 2.510V and Vout = 2.520V in the static state. This product is also qualified, as the zero-point error does not exceed 15mV. However, when used in a single-ended input acquisition device, the output of 2.520V exceeds the calibrated 15mV error, and the full-scale value will also be higher, which could reach up to 2% in extreme cases, affecting the customer’s use.
Now that some customers have reported this type of error, we are asking them to provide calibration requirements to correct the issue. Sales representatives are also being asked to label products based on different calibration methods to ensure correct production for customers.
To calculate the input current of a transformer, you can use the following formula based on the transformer’s power rating, input voltage, and efficiency. The input current depends on the power demand on the secondary side (load), and this can be calculated for both single-phase and three-phase transformers.
For a single-phase transformer, the input current can be calculated using the formula:
Where:
If the efficiency is not given, it can be assumed based on typical transformer efficiency (for instance, 0.98 for a high-efficiency transformer).
For a three-phase transformer, the input current is calculated with a similar formula but with an additional factor for three-phase power:
Where:
Suppose you have a 500 kVA transformer, with a primary voltage of 11 kV (11,000 V) and an efficiency of 98%.
For a 500 kVA transformer, the power is 500,000 VA (since 1 kVA = 1,000 VA).
So, the input current for the primary side of the transformer is approximately 26.92 A.
This calculation helps determine how much current is drawn on the primary side of the transformer based on the output load.
In a current transformer (CT), the burden refers to the total impedance (resistance and reactance) presented by the connected measuring devices, such as ammeters, relays, or meters, to the secondary winding of the CT. It is usually expressed in volt-amperes (VA) at a specific secondary current (typically 1A or 5A) and can include the impedance of the connecting wires and instruments.
In summary, the burden is the load imposed on the secondary winding of the current transformer, and ensuring it is within the CT’s rated capacity is essential for accurate current measurement and safe operation.
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