12/3/2023 0 Comments Opto isolator simulationThe linearity degrades at higher operating currents due to reduction in the CTR of the opto but looks good if the maximum current is kept to no more than a couple mA or so (1.6mA maximum in this simulation). The simulation results are somewhat idealized of course, as determined by the accuracy of the models, and the real linearity will depend upon the actual characteristics of the opto coupler used, but should still be good, adequate for non-critical voice and music applications. (Note the blue trace where the output is graphically superimposed over the input so any non-linearities are more readily observed). If a symmetrical output signal with respect to ground is required, then the output can be capacitively coupled to a load resistor connected to Ocom.Īs can be seen, the output voltage plot appears quite linear with respect to the input. R2 can be increased in value to increase the output voltage in this configuration, the tradeoff being some increase in the distortion. If you have a high impedance load (>1.5KΩ) and are satisfied with an output voltage of about 1/2 the input (depending upon the opto transfer gain), then U5 may not be needed and you can use the voltage across R2 as the output. Op amp U5 is a buffer/non-inverting amplifier that amplifies the voltage across R2 to convert the opto isolator output current back to a voltage. The opto is operated Class A with about 1 mA DC bias current. Op amp U3 is configured as a current driver for the opto input, generating a current proportional to the voltage input. (Ocom is the isolated output common and Rsim is just to allow the simulation to run with an isolated output ground). This eliminates the non-linear LED input voltage to current relationship, with the opto isolator output current typically being quite linear with respect to the input current, especially for low current levels.īelow is the LTspice simulation of such a circuit using the 4N25, a common and representative general purpose opto isolator. Sorry for the long story, I am still learning.There are special purpose (and relatively expensive) optical isolators available with separate feedback connections designed for high analog linearity and fidelity but, for non-critical AC applications, a standard opto isolator may be adequate if it is driven by a current source instead of a voltage. Hence I started searching for it and found one i.e SFH628A-3, however I am not good in understanding their datasheet and need help to see if it fits. Due to all these factors, someone suggested me to find another opto which works at very low current. I am also not able to use a x-capacitor or use a transformer due to size constraints. Now I could reduce the resistance, however the heat dissipated would be more (which I don't want). I calculated the voltage and found that for 145V AC and a series resistance of 440K, the current is just 0.33mA which may to be insufficient to turn on the opto. While testing my circuit at various input voltages I found that the opto will start giving fluctuating output when my voltage is 145V. I had connected a series resistance of 440K (1/4W) and everything seems to be fine i.e the microcontroller is able to sense when there is line and otherwise. The optocouplers I have evaluated are PC814, H11AA1 and SFH620A-3, out of which I found SFH620A-3 to be more efficient (must be due to the better CTR). I am trying to detect line voltage (230V AC, if the supply is ON/OFF) by a microcontroller with the help of an AC optocoupler.
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