Within an optocoupler you will find a light emitter and a light sensor. It just wouldn’t seem right to me to ask for help with a production project without at least stating so.Optocouplers An optocoupler IC, also known as an opto-isolator or photocoupler, is a semiconductor device which is used to couple isolated circuits. Rest assured if I were designing a product I would have stated such in my post. I appreciate all the help the folks in the forum supply. The folks operating the machine would often remark the machine stopped after she just touched the machine, ![]() Clearly the ESD when right through this board to the main controller resulting in a lockup. Well he did not follow any rules of isolation and had input and output traces in close proximity but they were “isolated”. Whomever designed the machine had an Isolation board made using your standard garden variety optoisolator. Out products consisted mostly of plastic parts which were stored in plastic bags what were in plastic bins. The issue was that the controller would lockup several times a day. ![]() Some years ago I was asked to checkout issues with a piece of production equipment (I was in design engineering). The isolators are to allow me to monitor the mains voltage (at worst). I am simply building a hobby data acquisition for my furnace. Rest assured I’m not designing anything for production or other countries. But later I read that people more often use SMPS with higher capacitance (like 100pF) and short isolation with capacitor in range 1…4.7nF (forming that capacitive voltage divider). When I first time designed isolated RS485 (before I understood that the way I described it here) I used SMPS with 4kV and 4pF trafo and without that 2kV capacitor. If your trafo has capacitance of 20pF and you add 2nF capacitor you reduce disturbance voltage at output 100 times. If you add capacitor between output and GND (in fact in parallel to that 150 ohm) you get capacitive divider. In such circuit harmonic levels would be semi equal up to high frequency (the decrease in harmonic amplitude with frequency is compensated by the decreasing impedance of the capacitor). Generator connected to GND and capacitor to isolated output.įor EMC measurement that output is loaded with 150 ohm resistor to GND and disturbance at that resistor are measured. I imagine SMPS (for common mode) as AC generator in serie with its internal capacitance. SMPS circuits often have a capacitor (rated for 2kV or more) between the primary and secondary side. In high voltage generation / distribution optically triggered thyristors are sometimes used. ![]() “Medical grade” power supplies have extra demands for failing in a safe state if they fail and for lower capacitance and who knows what else.īottom line is you first have to know what to protect against before you can build something. I think it’s to ensure that the capacitance in the optocoupler is not the dominant factor in picking up noise, which could lead to faulty switching. The audio versions (S/PDIF) are the cheapest.īut for normal mains voltage related applications the ubiquitous DIP4 sized optocouplers are apparently sufficient, but would you trust a bag of cheap PC817 optocouplers from Aliexpress? If you want to go extreme you can use fiber (or plastic) optical cable. It also has other hardened features such as a 3mm internal distance between the LED and the detector, with 8kV AC and 14kV DC test voltages. Pin spacing between primary and secondary side is 15.24mm
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