Using the CA10 Current Sensor Adapter With a Current Transformer

Introduction

The CA10 Current Sensor Adapter is programmable and customizable to work with third-party current measurement devices that output voltage or current signals proportional to measured current. After programming and configuring the CA10 for a given current transducer/transformer, a compatible Teledyne LeCroy oscilloscope or Motor Drive Analyzer recognizes the device as though it were a native Teledyne LeCroy current sensor. Examples of devices that can be used with the CA10 include Pearson current transformers, Danisense current transducers, PEM-UK Rogowski Coils, or any conventional turns-ratio current transformer. This technical brief will walk through the setup and use of a Murata Power Solutions current transformer with the CA10, which outputs a voltage relative to the sensed current flow.

Murata Power Solutions DMS Current Transformer

The DMS-current transformers are commercial grade transformers that provide an isolated output from 0 to 5 A. More information on this product can be found at: http://www.murata-ps.com/en/7020-01036-0.html.

The model used in this example is Murata Power Solutions part number 7020-01-36-0, as shown in Figure 1. This current transformer has a peak current rating of 50 A and outputs a current at a ratio of 10:1 to the measured current. No external power supply is required to operate the current transformer.

Figure 1:

Murata Power Solutions 7020-01-36-0 Current Transformer

Setting up the CA10

Because this current transformer has an output in Amps, we must install a suitable shunt resistor onto the CA10 circuit board.

• 1. Remove the two screws on the connector end of the adapter (shown at the bottom of the image below).
• 2. Remove the front face and pull out the PCA board.

Figure 2:

Opening the CA10 Case

• 3. Install the shunt resistor in location R14. In this case, we install a 1 Ω shunt resistor. This value is chosen based on the current being measured and the transformer’s rated maximum burden Volt-Amperes of 1.5 VA (from the transformer datasheet). In our application, 5 Amps is the maximum current the transformer will see. Using a 1 Ω resistor ensures that we do not surpass the maximum burden VA while still having an output voltage that exceeds the minimum sensitivity of the oscilloscope. The resistor value may need adjustment for your particular application.

Figure 3:

CA10 Customizable PCA Board

• 4. If necessary, use locations C7, C8, and L1 on the PCA to build a low-pass filter to remove higher-frequency noise that might be present at the current transformer output. The specific components to install depend on the desired frequency response as well as the instrument and transformer impedance. Some possible combinations are shown below:

In this case we did not perform any hardware filtering on the input signal.

• 5. After modifying the signal-path components, replace all CA10 parts as shown in Figure 2. Note: The screws on the front of the case are installed with 2 in-lbs. torque.

Upon connecting the CA10 to a Teledyne LeCroy oscilloscope or Motor Drive Analyzer, a prompt will ask if you want to program the device. Selecting Yes opens the CA10 dialog (Figure 4).

Figure 4:

CA10 Setup Dialog

The current sensor information, scale factors, and channel definition settings can all be defined in the CA10 dialog. The setup for the 7020-01-36-0 current transformer is as follows:

Model: 7020-01-36-0

This field changes the name of the CA10 setup dialog tab to the model of the third-party sensor (Figure 5).

Serial Number: 004048116

The serial number is specific to the particular sensor being used. It can usually be found on the sensor device.

Vertical Unit: A

The vertical unit will always be Amps with the CA10

Units/V(slope): 10

This is the Amps/Volt setting, which is the multiplication factor by which the input will be scaled. In this case it is 10 because of the 50:5 ratio of the current transformer and the 1 Ω shunt resistor installed.

$$Rescale Factor=Transformer Ratio  x  \frac{1}{R_s}$$

Where R_s = Shunt Resistor

$$Rescale Factor=\frac{50}{5}x\frac{1}{1}=10$$

Y-Intercept: 0

This number is added to the rescaled Amps/Volt result. In many cases, this value is zero unless a specific offset number is required.

BW Limit: 20 Mhz

Choices include Full, 200 MHz, or 20 MHz. This applies a filter at the bandwidth specified to the input signal.

Input Termination: 1 Mohm

Choices include 1 Mohm or 50 Ohm. The current sensor’s manufacturer specifies the required termination.

Input Coupling: AC

Choices include AC, DC, or Both. Choosing either AC or DC forces the CA10 to always couple to that particular setting. With Both chosen, the user can choose either AC or DC in the Channel dialog each time the device is used. AC is chosen for this transformer because it does not measure down to DC.

Figure 5:

CA10 setup dialog with saved settings

After entering all the settings, press the Save to Adapter button to program the adapter. The CA10 is now programmed to be recognized as the 7020-01-36-0 current transformer. With subsequent usage on any channel on any compatible Teledyne LeCroy instrument, waveforms will automatically be correctly scaled in Amps. The CA10 is provided with heat-shrink tubing to apply to the BNC cable connection so that the current sensor can be semi-permanently attached to the CA10. This helps ensure correct and consistent results using the current sensor, eliminating the possibility of human error that can arise from manual rescaling. Figure 6 shows the CA10 connected to the 7020-01-36-0 current transformer with application of the clear heat-shrink tubing provided with the CA10.

Figure 6:

7020-01-36-0 Current Transformer with the clear heat-shrink tubing

Conclusion

The CA10 Current Adapter provides an alternative to expensive current probes. This relatively low-cost adapter can be used with a wide variety of third party current sensing devices. The ability to program the current sensor information directly to the CA10 allows seamless use of the third-party sensor without the need to manually rescale the waveforms with each subsequent use.