Example of Estimated Curve of CTR Change Over Time (Typical Values)įigure 5 shows an example of the estimated life of a photocoupler according to differences in the LED input current (I F) and the ambient temperature (T A).įigure 5. The photo-electrical conversion efficiency and DC amplification (h FE) of the photo-transistorįigure 4.The optical coupling efficiency between the LED and the photo-transistor.The CTR of a photocoupler is based mainly on the following factors. Example of CTR Temperature Characteristics Change of CTR over Operating Time The diagram below shows an example of an actual product.įigure 3. Mechanism of CTR Dependency on Temperature Therefore, the CTR dependency on the temperature is a combination of these two parameters.Īs shown in Figure 2, the CTR dependency on the temperature is normally realized by synthesizing the above two temperature coefficients.įigure 2. Whereas the LED luminous efficiency has a negative temperature coefficient, h FE of the transistor has a positive temperature coefficient. Conversely, in the large current area, the output current I C does not become as large as expected even if the value of I F increases, so the value of I C should be designed less than your expectation. In other words, the value of I F should be designed larger than necessary because the output current I C becomes substantially smaller as the value of I F decreases in the small current area. input current variation slope differ between the small current area (around I F = 1mA) and the large current area (around I F = 20mA). It is particularly important that the positive and negative curves of the CTR vs. The CTR depends on the LED input current (I F) as shown in Figure 1, the CTR decreases from a maximum point when the input current is both increased and decreased.įigure 1. CTR Dependency on LED Input Current (I F) Sufficient care must therefore be taken with the CTR when designing if design is not executed making enough allowance for these points, the output may be too small, causing malfunction.Īdditionally, an AC (alternating current) input capable photocoupler has two LEDs (light emitting diodes) on the input side, so a CTR will also exist for each of these LEDs.Įven if I F with the same positive and negative value is input, the value of the output current I C will differ for each polarity of I F, so care must be taken on these points as well. It changes with the operating time (age).It depends on the current input to the light-emitting diode (LED) (I F).The CTR has the following characteristics and is therefore as important as the insulation tolerance among the characteristics of a photocoupler. The current transfer ratio is a parameter similar to the DC current amplification ratio of a transistor (h FE) and is expressed as a percentage indicating the ratio of the output current (I C) to the input current (I F).

Optical isolator definition