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220V AC Digital Input conditioner

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simce

simce

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digital input conditioner

Hi there.
I must design 24 channel digital input card on size of single europa. I need solution that does not use relays that are for 220V where coil is placed as input because there is not enough space for this solution, nor solution with optocupler and current limiter with resistor since this way it will disipate about 0,6Wx26ch=15.6W (led current is 3mA/channel).
I need some other solution (if there exists one)!
Or maybe someone can tell me how this problem is solved in industrial PLC controllers.
Some schematic is also welcomed.
 

By all means you can use optocouplers but instead of current limiting resistor use ≈150nF capacitor rated for 250Vac. This capacitor will reduce current to ≈10mA and will not generate heat. Just remember about connecting another diode (reversed) to protect the input diode of an optocoupler ..
 

Yes, you are right capacitir does not disipate heat because of phase of 90degrees between current and voltage on it.
I'm planning to use optocuplers with two diodes in parallel connected in oposite directions this will reduce need of protective diode.
 

If you want to drive diodes with 3mA current the caps value will be 3 time smaller, somewhere around 47nF (smaller in size) .. and one diode, connected in opposite direction, will be just fine ..
 

    simce

    simce

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