AC Relay EMI Suppression

[Mike Johnson]

If your final load is inductive, then the best solution is a zero crossing turn-on and a MOV protected zero crossing turn-off is the easiest solution. See: http://www.power-io.com/new/motorstarters.htm
as an example of this.

[NavMicroSystems]

Mike,

thanks for the link,
the price is as high as the heatsink is large
;-)

[Melanie]

Sorry Ralph I wasn't continuing to read this thread as I'd assumed it had already been adequately answered some days ago...

Yes, zero cross is the way to go with inductive loads, plus VDR's (capable of absorbing any transients) across the switching element.

Ensure inductive surges will not penetrate your PSU and it is clean. You've sprinked 100nF Capacitors around your power rails like confetti.

Basic filtering on any other input pins (you'll be surprised what a 10K Resistor and a 100nF Capacitor will stop!).

Site the PIC well away from your switching elements or ensure it is screened and if nescessary has a large ground plane all around and underneath it.

I've not experienced the PIC problems that a lot of folks seem to have, perhaps it's because I build-in all the above recommendations into my PIC designs from the very beginning. It is important to identify what kind of interference is causing your problem... without knowing that you're stabbing in the dark as to a solution. It's overly simplistic saying it's "an inductive load switching". well what's it doing? Is the transient going down the power lines and affecting your PSU? Is it being induced into PCB tracks and coming down other input pins (or being induced locally into the PSU)? Or is it a localised EMP screwing the chip directly. You MUST first identify the root cause before applying a solution otherwise you will just end up wasting hours of your life.

[NavMicroSystems]

Thanks Melanie!

[CocaColaKid]

First, thanks for all the help here.

Unfortunately I was not aware of this problem when switching AC relays. We can rule out the load on the relay because we can get the circuit to reset on the bench with no load on it. This makes me think that there is a nasty little spike travelling down through the ground back to my board. If I insert an MOV at the relay the problem seems to disapear. However this I consider a not so easy to implement solution since there about 400 of these products out in the field right now that need to be corrected.

As for driving the relays I use a small signal relay that switches the 24VAC to the larger relay. The small relay is driven from a transistor connected between it and ground. I then have the base connected to the PIC via a 10k resistor.

As for solid state relay, way too expensive and way to much real estate. Would be nice but not doable.

[Luciano]

Post the model number and brand of the large relay.
(URL where we can see the large relay + datasheet).

Luciano

[CocaColaKid]

I was just looking at some relays and noticed that Panasonic has some SSR that have a zero-cross built-in. This would eliminate the need to use my small signal relay + transistor to fire the larger relays while providing a zero-cross circuit for a cheaper price.