As you can see, at 1mA the CTR is around 100% while it increases to more than 200% when the forward current is reaching 10mA for a VCE of 5V. Figure 1 shows an example graph of forward current versus the current transfer ratio of a particular optocoupler device. However, operating at higher forward current will degrade the life of the optocoupler faster, so better consider it as well. The higher the forward current, the brighter is the light and the higher the CTR will be. This is dependent to the forward current applied to the light source which is commonly a light emitting diode. Factors Affecting Current Transfer Ratio 1. Opto coupler current transfer ratio is dependent to the luminous intensity of the light source, operating temperature and continuous operation or aging. As a result, design issue sufficed during mass production. In circuit involving optocoupler, in some cases design engineers failed to fully consider the factors affecting current transfer ratio. Then the current will start to flow in the photo transistor’s collector. When the LED is forward biased it will gives off light and bias the photo transistor. The light source if often times an LED while the detector is an open base transistor or a photo transistor. ![]() If we recall the basics, optocoupler is a semiconductor device with a light source and a detector. CTR is simply the measure of the optocoupler’s transfer efficiency. Current transfer ratio is popular by its abbreviation CTR. This is the ratio of the optocoupler’s collector current to the forward current. Current transfer ratio is comparable to BJT’s HFE or beta (β).
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