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.
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.
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.
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.