have purchased one coil of 0.5sq.mm wire from the shop and without disturbing it, i connected one end to the 230V AC supply and other end to my SMPS ( whose output is 5V DC).

Use a line filter it will improve more than the above method.

there are 24 digital outputs from the micro driven through '574 and an opto to get +24V 2A as output.

What opto are you using and are you sure it can sink 2A.

i am using this output to switch on a relay ( PCB mounted)

Do you have free wheeling diodes across the magnetising coils of the relays.

my doubt is some spikes are going into the 5V SMPS. how to stop them?. i think i have to go for some good SMPS instead?

For conducted noise the best means is to have a good line filter before the supply enters your SMPS. Have good board level and chip level filtering as suggested by steve.


For protection from radiated noise put the control card i.e. card containing micro and other stuff in a good galvanised iron enclosure. Make sure that the enclosure is continuous one without much openings for the noise to enter the area of openings doesn't matter but the linear dimension of openings matter thus for ventilation etc. use large number of small holes rather than few big holes. Connect the enclusre to the star ground of system with the point where the external cables enter the box. Connect the shields of cables entering at this place.

Keep the relay and other input output boards out side this box. Put ferrites on the cables that connect the control boards with input/output boards for connecting input and output boards with control board use flat ribbon cable with alternate signal and ground arrangement. Put 47-100 ohms resistor in series with the inter board signals to reduce the sharp edges.

Regards,
Prahlad Purohit