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Ideal current sensors would not use any power to detect the current in the wire or trace, but real current sensors require some of the circuit energy to provide the information.

Current sensors are frequently used to measure and control the load current in power supplies, safety circuits and a variety of control circuits. In applications where controlling the current is required, such as in power supplies, accurately sensing the magnitude of the current is a fundamental requirement.

In pulsed-current applications or where it is only required to detect an on condition such as some safety circuits, the precise magnitude of the current may not be required. In other safety circuits, the sensed current can be used to trigger a shut down when the current exceeds a pre-set limit.

Shunt current sensors sample a small proportional fraction of the sensed current. The current is shunted through a parallel resistor and the voltage drop measured. As with the series resistor, the voltage drop is proportional to the current being sensed.

Current sense transformers are typically used for AC current sensing. The circuit can be somewhat simple when using a true RMS-to-DC converter such as the LT1966 from Linear Technology. These current sensing devices may use a single wire from the circuit to act as the primary of the transformer (Figure 1a) or they may have the primary (usually 1-turn) winding provided (Figure 1b).

a. Sensor Only |
b. Transformer |

These AC current sense transformers develop a current in the secondary that is proportional to the sensed current in the primary. The secondary current is measured as the voltage drop across the terminating resistor (R

The worst-case current and frequency determine the highest flux density that will be seen by the sensor or transformer. Exceeding 2000 Gauss for most AC current sensors means that the output becomes non-linear vs. the current being sensed and the output voltage is no longer strictly proportional to the input current. Higher secondary turns helps keep the flux density below this limit.

For the wire-through-hole style current sensors the turns ratio can be dramatically reduced by looping additional turns (one pass through the hole is one turn) if the wire size and hole size permit. This allows a higher input current transformer to be used to provide higher output voltage across the terminating resistor (see sidebar eq. 4).

The tool requires user input of expected maximum sensed current, input frequency (kHz), duty cycle of the primary current waveform, and the desired output voltage. The output voltage is the desired output voltage for the expected maximum input current.

The tool calculates the required terminating resistance (R

R_{T} = Nsec × Vout/Ipri

(Calculations based on a 1-turn primary.)

The tool also calculates the maximum flux density of the secondary, based on the output voltage (Vout), duty cycle, secondary turns, and frequency to make sure it does not exceed 2000 Gauss.

The results, shown in Figure 4, list all Coilcraft part numbers that meet these input conditions and shows a graph of the output voltage vs. sensed current for the calculated R_{T}.

The tool also calculates the maximum flux density of the secondary, based on the output voltage (Vout), duty cycle, secondary turns, and frequency to make sure it does not exceed 2000 Gauss.

The results, shown in Figure 4, list all Coilcraft part numbers that meet these input conditions and shows a graph of the output voltage vs. sensed current for the calculated R

Selecting an appropriate current sense transformer requires knowledge of the expected maximum sensed current, frequency and duty cycle of the sensed current, as well as the desired output voltage corresponding to the expected maximum sensed current. With this information, the Coilcraft Current Transformer Selector Tool provides the appropriate terminating resistor value and a list of current sensors that meet the application conditions.

For any transformer, the turns ratio is proportional to the inverse of the current ratio:

Npri/Nsec = Isec / Ipri

where:

Ipri = primary current, Npri = primary turns,

Isec = secondary current, Nsec = secondary turns

Solving for Isec

Isec = Ipri × Npri / Nsec

If Npri = 1 turn,

Isec = Ipri / Nsec

Solving for the current in the primary (sensed current):

Ipri = primary current, Npri = primary turns,

Isec = secondary current, Nsec = secondary turns

Solving for Isec

Isec = Ipri × Npri / Nsec

If Npri = 1 turn,

Isec = Ipri / Nsec

Solving for the current in the primary (sensed current):

Ipri = Isec × Nsec (eq. 1)

The voltage across the terminating resistor R

Vout = Isec × R

Solving for Isec

Isec = Vout/ R

Substituting into eq. 1

Ipri = (Vout/R_{T}) × Nsec (eq. 2)

gives the sensed current in the 1-turn primary, by measuring Vout, and knowing the terminating resistance R_{T} and the number of secondary turns Nsec.

If Npri is not = 1 turn,

If Npri is not = 1 turn,

Ipri = (Vout/ R_{T}) × ( Nsec /Npri) (eq. 3)

Vout = (Ipri × R_{T}) × (Npri/Nsec) (eq. 4)

- Assistance with Safety Agency Approvals
- Basics of Inductor Selection (from Electronic Design magazine)
- Calibration, Compensation, and Correlation
- Current and Temperature Ratings
- Getting Started: An Introduction to Inductor Specifications
- Hipot Testing of Magnetic Components
- How Current and Power Relates to Losses and Temperature Rise
- Measuring Self Resonant Frequency
- Operating Voltage for Inductors
- Simulation Model Considerations: Part I
- Simulation Model Considerations: Part II
- S-parameters for High-frequency Circuit Simulations
- Testing Inductors at Application Frequencies
- Working Voltage Ratings Applied to Inductors

- PCB Washing and Coilcraft Parts
- Selecting Flux for Soldering Coilcraft Components
- Soldering Surface Mount Components

- Broadband Chokes for Bias Tee Applications
- Inductors as RF Chokes
- Key Parameters for Selecting RF Inductors

- Beyond the Data Sheet: The Need for Smarter Power Inductor Specification Tools
- Choosing Inductors for Energy Efficient Power Applications
- Current Sense Transformers for Switched-mode Power Supplies
- Determining Inductor Power Losses
- Ferrite Vs Pressed Powder-core Inductors
- Forward or Flyback? Which is Better?
- Notes on Thermal Aging in Inductor Cores
- Selecting Coupled Inductors for SEPIC Applications
- Selecting Inductors to Drive LEDs
- Selecting the Best Inductor for Your DC-DC Converter
- Structured Design of Switching Power Transformers
- Transformers for SiC FETs

- Coilcraft LC Filter Reference Design
- Common Mode Filter Design Guide
- Common Mode Filter Inductor Analysis
- Data Line Filtering
- Fundamentals of Electromagnetic Compliance
- Passive LC Filter Design and Analysis
- Selecting Common Mode Filter Chokes for High Speed Data Interfaces

- Applying Statistical Techniques to the Design of Custom Magnetics
- Choosing Power Inductors for LiDAR Systems
- Coilcraft Conical Inductors
- Designing a 9th Order Elliptical Filter for MoCA® Applications
- Measuring Sensitivity of Transponder Coils
- Power-handling Capabilities of Inductors
- Signal Transformer Application
- Transponder Coils in an RFID System
- Using Baluns and RF Components for Impedance Matching
- Using Standard Transformers in Multiple Applications