Optocoupler Help?!?!?
Optocoupler Help?!?!?
(OP)
I have to interface 120V or 240V ac into a parallel port. The voltage varies mattering on what country the product will be installed. My question is, if I have an optocoupler with reverse diodes to activate for ac, what size resistor do I use for current limiting the input? I know I can use ohms law for calculating the resistance based on current, but what wattage do I need so I do not burn up the IC? I calculate about 12W, which seems a little excessive.





RE: Optocoupler Help?!?!?
RE: Optocoupler Help?!?!?
RE: Optocoupler Help?!?!?
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RE: Optocoupler Help?!?!?
RE: Optocoupler Help?!?!?
Additionally, this would isolate your measurement circuitry from the actual AC main.
TTFN
RE: Optocoupler Help?!?!?
Operahouse and/or sdmays can you explain this capacitor idea in more detail?
RE: Optocoupler Help?!?!?
If you're trying to detect if the power is on or off, then just plug the PC into the same outlet. If it's on, it's on.
More seriously, one very common method for making an LED work directly from AC is by means of a series capacitor.
If it is a one-off project, then use a simple AC adapter to run the LED.
RE: Optocoupler Help?!?!?
The capacitor has an impedance - reactance actually - to AC which is 1/(2Pi.freq.C) where C is the capacitance in Farads. The reactance limits the current in much the same way as a resistor, except that the voltage and current have a 90 degree relative phase angle. With a 90 phase there is no power dissipated in the capacitor, but current is still limited.
When you first apply voltage to a discharged capacitor, a very high current flows for a very brief period. use of the resistor in series with the cap as described above keeps the inrush current within reasonable limits to avoid damaging the circuit supplied through the cap.
If you are choosing a capacitor for direct connection to the AC line, you must use a Class X2 or Class Y capacitor. The Class Y type is usually reserved for live (or neutral) to earth applications, so in this case the X2 type is better suited.
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RE: Optocoupler Help?!?!?
lamp and a photo transistor. ( Mount the R,lamp and
photo transistor in a shrink tube. It is a five minute
work....)
<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032