# Current Sense Transformer Turns Ratio

It is important to note that the turns ratio in a current sense transformer does not represent the input/output ratio as compared to in power transformers. Unlike in power transformers, these convert current to voltage instead of voltage to voltage. Although these output a voltage proportional to the input current, a 10:1 current sense transformer does not mean 10 amps will produce 1 volt.

237-2080-ND is a common example of a 10:1 Power Transformer, where it requires 120vac input and produces 12vac output which makes the input/output ratio proportional to the turns ratio. However, with a Current Sense Transformer, the input/output ratio is dependent on factors other than only the turns ratio. The resistance value of the load, commonly called the terminating resistor (or burden resistor) should be calculated based on circuit design and per recommended datasheet specifications.

Although it is fairly common to see 100 ohms value, they can range from roughly 10 ohms ~ 5k ohms. Generally you want to stick with load resistor values around what they recommend – as that is where they are optimized to operate. Outside of that range, you will get less linear response, and you may risk saturation of the transformer, which will quickly reduce accuracy to a useless level.
This datasheet shows some basic calculations.

Graph of 1000:1 Current Sense Transformer 237-1619-ND with a 100 ohm terminating resistor represents a 10:1 current to voltage ratio:

399-C/CT-0306-ND Datasheet shows differences in output voltage with various resistance values:

## Current Sense Transformer Types

Current sense transformers offer both invasive type and non-invasive type. Invasive type is when the primary coil is in series with the current source:
Invasive Type 237-1103-ND The non-invasive type can be a solid or split core which is housed around a wire. Voltage is produced based on the amount of magnetic flux from the wire:
Non-Invasive Solid Core Type 1295-1102-ND -Although pin 3 (center pin) is normally for mechanical support only, it can be used for center tapped designs.

Non-Invasive Split Core Type 582-1004-ND 