I use 4N35 to isolate a Relay . The opto's transistor's base is pulled down with a 1M resistor ,The problem is when this resistor is even touched slightly with finger the relays chatter . Here is the sch;
sheldonstv , thanks for the ckt , but mine is a finalized PCB .
eric , yes , it chatters like a 50Hz humm 'only on touching it' otherwise not . Is it ok ? I pulled the base down to avoid any spurious triggering . But the PCB is going to be inside a closed cabinet . What to do with the base , should this be left open ckt.
Its not the 1meg thats the problem, its the OP's over active index finger.I don't think anyone's told him why he should remove the 1M: connecting the base to the emitter via 1M will greatly reduce the gain of the transistor.
hi,MikeMl I,Thanks for the idea , 100nF capacitor is too good shunting all the noise like a charm.
I dont know what type of signals you are coupling thru the opto, but be aware adding a 100nF from the base to 0v will significantly reduce the rise and fall times of the signals.!!
EDIT: As you are using a FET to drive the relays, the heat dissipation in the FET's will be increased, also the switching point/times will be delayed.
IMHO it dosnt require a 100nF on the base of the opto, just dont touch the base.
hi dark,Hi ,
I could still see the humm even with the 100nF . Anyways I removed the Capacitor .
If you dont feel easy with the performance of the opto, post the circuit fragment that shows the opto driver and signal, also the FET and relay part.
I can check it with an LTspice simulation.