Talema Current Transformer Series Guide

Current transformers are a low cost and effective method of current measurement as well as providing galvanic separation between the current-carrying conductor and the measuring device.

Talema manufactures an extensive range of 50/60 Hz toroidal current transformers. In this article, we will discuss each of the main series in the Talema catalog to help you make the best selection for your application.

Series Characteristics and Core Construction

Each series ls designed with specific characteristics to suit a wide range of applications.

Special characteristics include high saturation, high output voltage, and high accuracy over a wide range of input currents. Core material determines these characteristics in each series.

Across our standard range, we use ferrite, grain-oriented silicon steel (GOSS), and nanocrystalline cores. All GOSS cores are manufactured and annealed in-house. Our ferrite and nanocrystalline cores are sourced from leading manufactures worldwide.

All parts are manufactured using UL/CSA listed materials and are fully RoHS and REACH compliant.

ASM Series

ASM Series current sensors are designed onto ferrite cores with an internal burden resistor to give a linear response over the rated current range. The conductor carrying the current to be measured functions as a one-turn primary. Measurement accuracy can be improved by using multiple turns.

The ASM series offers a low-cost solution where current monitoring is more important than current measurement.

Output voltage of this series is measured in millivolts and normally requires an amplifier circuit.

There are six current options from 1 A–100 A in three package sizes. The most popular rating is ASM-010 which comes in a compact 9.3–mm wide package. Packaging is fully encapsulated and PCB mountable.

ASM Series transformers can be isolated (primary to secondary) to 2500 or 4000 VAC.


  • Feedback control
  • Branch circuit overload
  • Load drop or shutdown

AX Series

AX Series current transformers are designed onto GOSS cores to give a higher output voltage than ASM series transformers. There is excellent linearity over the rated current range with a high saturation level. There are four current options available from 5 A–15 A each with different turns ratios: 500:1, 750:1, 1000:1 and 1500:1.


AX Series transformers come in the same 9.3–mm wide package as ASM-010 but with higher output voltage, removing the need for an amplifier circuit. The graphs below show output voltage at various burden resistances of most popular types.

Higher current density in a popular space-saving package makes the AX Series an excellent choice where an amplifier circuit is not an option.


  • Sensing overload current
  • Ground fault detection

AP Series

AP series current transformers are high accuracy current transformers designed onto nanocrystalline cores. Nanocrystalline offers improved performance over NiFe with higher permeability and saturation induction compared to nickel-based cores. The combination of higher performance and lower cost makes nanocrystalline the material choice for high accuracy CT’s.


The AP Series is designed into the same compact 9.3–mm wide package as ASM-010 and AX Series. This series offers excellent linearity over the entire current range. There are two current options available and four turns ratios to suit most applications.

Typical accuracy is 0.2%.

Maximum current ratings are 20 A and 25 A. The most popular ratios are 1000:1 and 2500:1.


  • Precision instruments
  • Power measurement

ASM, AX and AP Series Comparison

These three very different current transformers are available in the same compact package. Each series offers different degrees of accuracy and output voltages across a similar current range.

Below we can see the output voltage comparison between ASM-010, AX-1000 and AP-1000.

From the graphs above we can see that each series offers different output voltages. Each series also offers differing degrees of accuracy.

The ASM Series provides excellent linearity due to the internal burden R but does not have a defined turns ratio. Output voltage is in low millivolts and accuracy is not defined. This Series is most suited to applications where current detection only is important — i.e. an on/off indicator circuit.

The AX Series provides excellent linearity and higher saturation due to the GOSS core. This series has defined turns ratio and offers higher output voltage capable of operating switching circuits without an amplifier circuit. Thus it is used in applications where the output voltage is used to monitor input current with a degree of accuracy.

The AP Series provides higher accuracy than either ASM or AX series and is used in applications where accurate measurement of currents down to milliamps is required and where space is at a premium.

Price vs. Performance Comparison

The table below shows price versus performance comparison of ASM-010, AX and AP Series.        

TypeAccuracyOutput VoltsCostApplication

AC Series

AC Series current transformers are the most popular of all our current transformer ranges. High output voltage coupled with excellent linearity make the AC Series an ideal low-cost solution used extensively in EV chargers. AC-1020 is the most popular choice for this application.

AC Series low cost 50/60 Hz current transformers are designed onto high-grade GOSS cores offering excellent linearity, high saturation, and high output voltage.

This comprehensive range of 1000:1 ratio CT’s is available in current ratings from 5 A to 200 A in fully encapsulated PCB mount packages.

Typical accuracy is 1%.

The high saturation level of GOSS cores make this series suitable for switching control circuits in overload conditions. 

5–20 A Package


AC 1005 Response Curve


200 A Package


AC 1200 Response Curve



  • Ground fault detection
  • Current overload

ACX Series

ACX Series current transformers offer a range of high accuracy current transformers with a 2500:1 turn ratio. This series is designed to complement the popular AC Series by offering the same high saturation characteristic but with increased accuracy. There are four current ratings (50 A, 75 A, 100 A, and 150 A) in four compact PCB mount package sizes. Applications include ground fault detection and current overload. Typical accuracy is < 1%.

ACX-1050 and ACX-1075 are widely used in EV charging (32 A, 63 A).


AZ Series

AZ Series current transformers offer high current and high output voltage in a small package with integral 250-mm flying leads. This is the only series with flying leads as standard. There are three current ratings available 25 A, 40 A, and 50 A with turns ratios of 500:1, 750:1, and 1000:1 to give 1.65 V at rated current and 33 Ω Burden R. The high-grade GOSS core ensures excellent linearity.

AZ Series is a practical and space saving option when PCB mounting is not required.

Custom lead lengths and optional internal 33 Ω 0.25 watt burden R can be supplied on request.

AZ Package with 250-mm Flying Leads


AZ Series Input Current vs Output Voltage


AQ Series

AQ series current transformers are high accuracy current transformers designed onto nanocrystalline cores. 

Nanocrystalline offers improved performance over nickel-based cores with higher permeability and saturation induction. The combination of higher performance and lower cost makes nanocrystalline the material choice for high-accuracy CT’s.

AQ Series is a follow on to the popular AP Series, offering higher current options with high accuracy and close to zero phase shift. There are four current ratings available in three package sizes.

Typical accuracy is 0.1%–0.2%.


The AQ Series uses existing AC and AZ packages as well as introducing a new 4-pin PCB mount package for 63-A and 125-A versions AQ-2563 and AQ-25125.


Two most popular options from this series are AQ-1020 and AQ-2532, with the most common application being EV charging.



  • Metering
  • Ground fault detection
  • EV charging

FSD Series

FSD Series 5-A current transformers are designed to accurately measure AC currents from 60 A to 2500 A in conjunction with moving coil ammeters or voltmeters up to 5 VA, Accuracy Class 3. Primary current is proportionally transformed to give full-scale deflection on measuring instrument at 5 A.


This series is designed onto high-grade silicon iron cores which are wound and annealed in-house. Windings are double enamel Class H round copper wire; outer insulation is Class B 130°C black polyester film. Leads are flexible equipment wire 3 meters long with 10-mm tinned ends.

Maximum system voltage is 720 V; overload withstand is 1.2 times the continuous rated current. 

1-A versions and accuracy classes 1 and 0.5 are available as custom designs.


  • Generator sets
  • Switchgear
  • Control panels
  • Overload protection


The Talema standard range of 50/60 Hz current transformers are suitable for most applications where current monitoring, current measurement, overload, and fault detection are required. All series are designed to offer an effective and low-cost solution over a wide range of applications.

The use of different core materials and a wide range of packages make Talema current transformers a popular choice. Most ranges are available for next day delivery through distribution outlets such as RS Components and Schukat enabling design engineers’ quick access to samples.       

For applications where standard parts are not suitable, Talema can offer custom design solutions in a wide range of packages as well as assisting in tooling of a custom package if required.  


All packages can be supplied as PCB mount or with flying leads. IDC or 2-way connector are optional.


  • Hugh Boyle

    Hugh Boyle is a Senior Design Engineer with Nuvotem Talema and has been with the company since 1986. Prior to joining Nuvotem, Hugh was an engineer with British Telecom and Telecom Eireann, and studied City and Guilds Telecommunication Engineering at Stow College of Engineering in Glasgow, Scotland.

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Hugh Boyle

View posts by Hugh Boyle
Hugh Boyle is a Senior Design Engineer with Nuvotem Talema and has been with the company since 1986. Prior to joining Nuvotem, Hugh was an engineer with British Telecom and Telecom Eireann, and studied City and Guilds Telecommunication Engineering at Stow College of Engineering in Glasgow, Scotland.
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