How to filter power supply noise and spikes

[scoobyrollz]

I have a circuit driving an SCR and when this circuit is enabled, the 5V rail has noise on it. When the load of the SCR is connected, then there are 200mV spikes of noise on the 5V line. I need to reduce and/or eliminate this noise b/c it caues problems with the connected logic board that is supplied with the same 5V. What kind of filter can be used? Would a pi filter work?

 

[felixc]

Apply the kirkoff law. What goes out must go back somehow. That "somehow" is what you have to make sure of. In other terms figure where the current of your load has to travel through, for the whole loop. Make sure that this loop is not going through the rest of your logic circuitry. After you have controlled that, then you can apply filtering to your logic board.

 

[buzzp]

Sounds like trace locations is an issue with your board as well as no basic bypass caps on the 5V line. All the filtering in the world will not help if the PCB is a mess as far as trace routing.
I would imagine your application could be cured with basic filtering using only an RC circuit consisting of a simple cap tied from the 5V bus to ground (try 10u tantalum)in the proper location (perhaps on the SCR board, at regulator output, and probably at your logic board for any hash picked up on the wires connecting the two).

 

[nbucska]

If I understand correctly, your logic drives the SCR and the SCR's noise gets into the logic.

You may try adding cap, but best would be eliminate
the source of the noise:

I suggest separating the SCR from the logic by triggering
it with an optocoupler. ( LED+phototransistor or photoSCR)
or replace the SCR with a Solid State Switch.

Actual design depends on number ( one or mass production )
and specification.


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032