AC Relay EMI Suppression

[mister_e]

you can do many many many many things...

the intelligent and easy solution is to activate and deactivate the relays when the AC signal line cross the zero volt.

Use separate ground lines for your relay and for the digital section of your design.

Be sure all your PIC pins are tie somewhere. Floating pins cause also many problem in noisy environement. Internal pull-up or set the unused i/o to output like Microchip suggest do absolutely nothing good.

if you can post your whole schematic and layout we can also suggest some things.

[NavMicroSystems]

CocaColaKid,

As Steve has already mentioned, best would be to toggle the relais at zero-crossing only.
This would not only reduce EMI-noise, it would also increase the relays lifetime.

There are many ways to achieve this, but we would need to have a look at your schematic to point you in the right direction.

In any case you'll need a zero-crossing detector for this.

What I have done is:
put a 74x574 (8-Bit D-Latch with Clock) between the PIC port (I2C IO Expander in my case) and the relay driver.
and let the ZeroCrossing detector clock the latch.

[CocaColaKid]

So basically I should determine what needs to be done and hold that information until the zero cross and then set the relays to there proper state. Is this correct?

[mister_e]

yep. As Ralph suggest, the latches will do the job for you. OR if you don't have many i/o, you can do it by software by checking the zero crossing on a specific i/o and once you get it, refresh your i/o states.

[NavMicroSystems]

If your application allows to do it within the software, fine.
But It will be a bit tricky to get the timing right.

I have given up on the software solution because I had to write to more than one I2C controller at the same time.

With the latches it works just fine.
The only thing you'll need is an option the adjust the trigger-level of the detector to compensate for the relays latency.

[Luciano]

Use a solid state relays (SSR) with built-in zero crossing.

http://www.components.omron.com/Prod...nd_Relays.shtm
(Omron)

http://www.celduc-relais.com/uk/okpac.htm

http://www.power-io.com/
(Ultra precise zero crossing for reduced EMI, without the cost of external filters).

http://www.branom.com/literature/scr.html
(SOLID STATE RELAYS AND SCR POWER CONTROLLERS Introduction).


Luciano