Electrical – MAX9926 Filtering Issue

When I have a design problem like this, I first think "how would I decode this by hand" and then I try to implement my thought process through use of electronics.

So, when I look at your two diagrams I see only one thing that my brain can use to determine what is data and what is noise, that is the width of the pulse. The problem with passive components in a situation like this is that for one you have a square wave that will have harmonics much higher then the cut off point for your filter. This will result in a not so square wave.

The easiest way to do this will probably be in software. You can still use interrupts, but it will take a bit more work. Essentially what you will have to interrupt on an edge, and then count time until the next edge. If the length of time is long enough then you can call it a logic 1 otherwise you can consider it noise on top of a logic 0.

If you must go with passive components you might want to consider a low pass filter followed by a comparator. You would have to play with the low pass filter to get it just right, but essentially you would aim to have the capacitor charge just high enough to cause the comparator to switch its output when the bit has been on long enough. What makes this difficult is that your "glitches" are very similar to your data and it is difficult to tune your circuit just right to get your data with no glitches. Even if you are able to perfect it on paper your components wont be very precise either. It is because of this that you should seriously consider doing this digitally.

The motor won't cause any harm when it's just running- spikes only happen when it is being switched. The power factor will be on the inductive side most likely. You might be able to get away with good design of your control board and an R-C snubber or a MOV across the relay contacts, but perhaps there is some regulatory requirement to do more in your jurisdiction.

Electrically, the common mode choke you link to is rated for 12A DC, meaning it would be suitable for 12A RMS AC from a heating point of view. I don't see safety agency approvals on it though. The X and Y capacitors and the choke should be festooned with safety agency approval markings appropriate for that application (you must not use X capacitors or general-purpose capacitors for Y capacitors) and have file numbers available or it could be much more expensive to perform testing.

One common method is to use a power entry filter, which would typically carry all the safety agency approvals and would be built into a metal casing. Two Y capacitors, one X capacitor and a common-mode choke.

There are also line filters, which are similar electrically but without the IEC plug.

Again, they have one X capacitor, two Y capacitors and a common-mode choke.