Current Transfer Ratio (CTR) Formula: A Comprehensive Guide

The current Transfer Ratio also known as CTR is the crucial parameter used to evaluate the optocoupler’s performance. This is also recognized by optoisolators.

An optocoupler is a device that distributes the two circuits electrically. That allows optical signal transmission from one circuit to another.

This optical signal transmission is completed by the combination of an LED (Light Emitting Diode) on the input side and a photodetector or phototransistor on the output side.

The current Transfer Ratio measures the signal transmission efficiency. That is defined as the output current ratio (Ic) to the input current ratio (If). That is measured in percentage format.

The current Transfer Ratio indicates how the phototransistor communicates with the emitted light by the LED.

What is the Current Transfer Ratio?

The current transfer ratio is the optocouplers parameter. These are also known as optoisolators. That defines the device efficiency and shows the current result that can be shown on the output side.

The current transfer ratio formula can have the following:

Current Transfer Ratio = Output current ratio / Input current ratio *100

For example

An optocoupler has a 10 mA output current ratio and a 5 mA input current ratio. The optocoupler would be considered as

Current Transfer Ratio = 10 mA / 5 mA *100 = 200

It means that the optocoupler is able to amplify 200 present input current.

What are the factors affecting the current transfer ratio?

The current transfer ratio can be affected by many factors, including:

Temperature – The current transfer ratio can impact LED and phototransistor performance, while there’s temperature. The CTR can be decreased in a hot environment.

LED wavelength – The CTR of the optocoupler can also affected by the LED wavelength in the device. Optocouplers can use additional LED power to visible light.

LED Emission – LED efficiency can have directed effects on the current transfer ratio. Higher LED lights can increase the current transfer ratio (CTR).

Phototransistor Sensitivity – The phototransistor sensitivity to light-emitting is very important. Phototransistors will displayed higher sensitivity with higher CTR results.

Distance between Phototransistor and LED – The isolation between the Phototransistor and LED can have affected the LED light reaching the phototransistor consequently increasing the CTR ratio.

How to improve the CTR ratio?

There are a number of ways that can have to improve the CTR performance, including:

Use higher input current – the higher input current can increase the optocoupler CTR ratio.

Use optocoupler with higher CTR ratio –Optocoupler are able to use higher CTR ratio. That’s essential to choose it for the device. That’s appropriate for the application.

Use optocoupler with longer wavelength LED – Optocoupler can used additional power of LED to visible light performance.

What is the significance of the current transfer ratio?

You will see below some significance of the current transfer ratio and optocouplers.

The CTR of the optocoupler is generally measured at the input current and temperature.  For example, an Optocoupler may have 200 present if the current input is 5 mA and the temperature is 25 Degrees Celsius.

The Optocoupler CTR ratio can vary depending on the manufacturer and specific model of the optocoupler.

When the circuit uses an optocoupler, it’s essential to choose an optocoupler with CTR. It’s appropriate for the application. For example, if the circuit requires controlling a high-power device, there should be select the high CTR with an optocoupler.

Application of optocouplers with CTR

Optocoupler can be used in many applications, including:

Isolation – An optocoupler can be used to isolate two devices from each other as can be very valuable especially where required to avoid unnecessary voice or interference going between two circuits.

Switching – An Optocoupler can be also more valuable for switching on and off. This is very important to control high-power to low-power signals.

Sensing – An optocoupler can be used to sense of LED lighting or other electromagnetic radiation. That’s more important in applications such as optical encoders and light detectors.

How do you calculate current transfer ratio?

You will see below the best example of the current transfer ratio formula. This will help to learn how to use the current transfer ratio to design the circuits.

Question – Design a circuit to switch a 12V, 1A LED on and off with a 5V, 10mA signal

Answer – Firstly, select the optocoupler with CTR that provides enough power to drive the LED lighting. The CTR score should be 200 percent or more to generate enough power.

In the second step, calculate the input current to the optocoupler, where the LED requires 1 ampere current. The input current to the optocoupler should be as 1A /200 = 5 mA

In the final step, design the circuit, where the input current to the optocoupler should be a minimum of 5 mA. You will see below the design to perform this operation.

current transfer ratio design

Where R1 indicates the 1kΩ resistor. Q1 defines the Optocoupler with a CTR of 200% or higher and LED has 12V and 1A.

Operation – When the 5V signal is applied to base Q1, the LED will turn on. This is because the optocoupler keeps 5V signal from 12V signals. Until the 5V signal exists, the LED light will turn on. If it is removed, the LED light will turn off.

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The current transfer ratio is the useful parameter of optocouplers. That determines the efficiency of the device and current. That’s controlled at the output. By understanding of this current transfer ratio and optocoupler, the engineer can implement the best on-design circuit.

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