The C part was probably the best for the two transistor circuits because the controlling transistor would need feedback in the milliamps. I concluded that the A was the best part when using a pwm chip in the 5vsb circuit because the feedback current needed is down in the microamps. Since the CTR range for the 817 was so broad, I figured it was probably floorsweepings and leftovers. I purchased from Digikey some Fairchild optoisolators to test, ten each of the 817, 817A, 817B, 817C, and 817D. I had one 817A, and it was my star performer. I would have intuitively thought that the higher the CTR, the better the part. I found a data sheet for the Fairchild series and found that the differences were due to the Current Transfer Ratio (CTR). Now, when the current flows on the input side, the LED glows, and puts light on the photo transistor, which turns on and drives the output. Most of the optoisolators tested were either 817 or 817C. An optocoupler is a device which operates due to light, its design is such that, it has LED on input side, and photo driver on output side. All had the same output with no loading, but as additional loading was added, the differences began to show. I had some pull outs in my junk box.but which would be the best to use? I built a 5vsb circuit on perf-board with an eight pin socket for plugging in the optoisolator being tested. I was working on a 5vsb circuit in a Bestec power supply, and thought that perhaps the optoisolator was bad.
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