Selecting CurrentSensors andTransformers
Choosing the right current sensor does not need to be guesswork
Applications
Current sensors detect the flow of AC or DC current in awire or circuit trace. They can be used to detect an on/off/pulse current condition or to measure the magnitude of
the current in the wire or trace. This discussion is limited
to AC current sensors.
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 andcontrol the load current in power supplies, safety circuits
and a variety of control circuits. In applications wherecontrolling the current is required, such as in power supplies, accurately sensing the magnitude of the current isa fundamental requirement.
In pulsed-current applications or where it is only required
to detect an on condition such as some safety circuits, theprecise 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.

TechnologyA resistor can be used to sense current by measuring thevoltage drop across the resistor. By Ohm’s law, the sensed
current I = V/R. Using a low value resistor in series withthe measured current keeps the voltage drop, and loss
due to dissipation, to a minimum. This sounds simple,
but because the voltage drop is low across such a small
resistance, amplification of the voltage may be needed todetect it, creating additional circuit complexity.
Shunt current sensors sample a small proportional fractionof the sensed current. The current is shunted through aparallel resistor and the voltage drop measured. As withthe series resistor, the voltage drop is proportional to thecurrent being sensed.
Current sense transformers are typically used for AC current sensing. The circuit can be somewhat simple whenusing 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 primaryof the transformer (Figure 1a) or they may have the primary(usually 1-turn) winding provided (Figure 1b).

a. Sensor Only

b. Transformer

Figure 1. Types of current sensors
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 (RT)as shown in Figure 2. By using a low turns ratio current
transformer (pri/sec <<1), the current drawn through theterminating resistor is minimized. This also reduces thevoltage produced across the terminating resistor, which
may then require amplification if that output voltage is toolow. Choice of transformer turns ratio and terminating resistor must balance the desire for low current draw against
the need for sufficient output voltage.

I pri

Pri
(1 turn)

Sec(N turns)

RT

Vout

Figure 2. Typical application schematic

Selecting AC Current Sensors /Transformers
Selecting an appropriate current sensor requires thefrequency range and current rating of the sensor for yourapplication conditions. The sensor type, mounting (surfacemount or through hole), turns ratio and overall dimensions
are additional considerations. Sensor type can be either a“sensor only”, in which a conductor integral to the application serves as the primary (Figure 1a) or it can be a current
transformer in which the primary is included (Figure 1b).

Document 1288-1

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The worst-case current and frequency determine thehighest flux density that will be seen by the sensor ortransformer. Exceeding 2000 Gauss for most AC current
sensors means that the output becomes non-linear vs. thecurrent 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 additionalturns (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).

Coilcraft Current Transformer Selector
Tool
Coilcraft provides an online tool* to aid in selecting the rightcurrent sensor or current transformer for specific application
conditions (Figure 3).
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 (RT)based on the maximum input current (Ipri), the number of
secondary turns (Nsec) and the output voltage (Vout) by:
RT = Nsec × Vout/Ipri(Calculations based on a 1-turn primary.)
The tool also calculates the maximum flux density of thesecondary, based on the output voltage (Vout), duty cycle,
secondary turns, and frequency to make sure it does notexceed 2000 Gauss.
The results, shown in Figure 4, list all Coilcraft part numbers that meet these input conditions and shows a graph ofthe output voltage vs. sensed current for the calculated RT.

Conclusion
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 theexpected 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.

Figure 3. Current sensor selector input screen

Figure 4. Current sensor selector results screen* https://www.coilcraft.com/CurrentTransformerFinder
Document 1288-2

Revised 07/27/20

How is the primary current measured by acurrent transformer?
For any transformer, the turns ratio is proportional to theinverse 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 = Isec × Nsec

(eq. 1)The voltage across the terminating resistor RT is by Ohm’s
Law:
Vout = Isec × RT
Solving for Isec

Isec = Vout / RT
Substituting into eq. 1Ipri = (Vout / RT) × Nsec

(eq. 2)

gives the sensed current in the 1-turn primary, by measuring Vout, and knowing the terminating resistance RT
and the number of secondary turns Nsec.
If Npri is not = 1 turn,
Ipri = (Vout / RT) × ( Nsec / Npri)(eq. 3)

Vout = (Ipri × RT) × (Npri / Nsec)(eq. 4)Document 1288-3

Revised 07/27/20