Implementing a relay in Felix's WattMote

[ryanm]

Hi Felix (and everyone),

What an awesome site!  Thanks so much for all of the resources for our projects!

My friends and I are interested in building a WattMote for our garage, with a twist.  We'd like to implement a relay so that we can use our moteino to effectively shut off the outlet when a certain set of conditions apply.  I don't really have much experience with relays, but I'm guessing this might be tricky because the Kill-a-watt as I understand it doesn't supply that much power (ironically), whereas the the relay has to be relatively heavy-duty because our friend sometimes charges his prius there, which I'm guessing draws around 15A.  Does anyone with experience in this area have any recommendations?

Thanks!
Ryan

[Felix]

Hey Ryan,
Cool, thanks for the feedback 

Here's an image I found on hackaday, added some of my notes, basically it would be doable if you could cut the HOT, and rewire it through a relay. However there's an issue, the incoming HOT provides power to the KAW and inherently to anything inside - think Moteino. So you cannot remove that trace. The output HOT is too close to all that HOTness, but if you could somehow separate that from the rest of the HOT and wire that through a relay, that would be ideal. Otherwise you could remove the large trace on the NEUTRAL side, plenty of space there, and place a relay there, but if the appliance uses or sinks current through GND then it's a half working solution.

A idea is to remove the thick HOT tracing anyway, and rewire the HOT INPUT to the internal power supply on the other side (don't really need a thick wire for that, perhaps desoldering the green large cap would help). Then rewire the OUTPUT HOT through thick wiring through a relay. But even so there's not a lot of space so it would be tight, and would be a pain to do it.

Using a latching relay  may be more efficient but those tend to be bulkier (one that only needs a pulse to lock/unlock instead of a continuous HIGH to keep it open).
Solid state relays can be slim but I don't think I've seen one that can handle 15 amps...

These are just some thoughts to get this thread started...

[ryanm]

Hey Felix, thanks for the reply and diagram!

I opened up my Kill-a-watt, and to my surprise, the layout is a little different (perhaps a different model)!  In particular, the output hot seems to be displaced, and it may be possible to cut it without killing power to the KAW and moteino.  I'm going to play around with it and I'll post my results if I can get it to work. 

Also, for the relay, I found one that I hope will be compact enough to fit.  It's a 2-coil latching relay with a relatively small package (2.9×1.3×1.65 cm): http://pewa.panasonic.com/components/relays/power-relays/polarized-power-relays/adj/

Thanks again!
Ryan

[Felix]

Hey Ryan, interesting,
Could you post a picture of the other side too?

I'm not surprised yours is different, I've seen many variations between KAW versions.

So it seems you can cut it where you indicated (I assume at that point the only thing that is connected is the HOT OUTPUT -J2). But without looking at the other side I can only assume...
What does surprise me is why they have the thick solder enforced HOT trace going to the big supply capacitor...?

Nice relay you found - I would play with it on a breadboard - most likely you will need a mosfet or transistor and a protection diode to drive it. Once you get it to work on a breadboard, only then try it in the KAW.

I found that same one on digikey: search results. At $14 +S/H its not cheap at all. There's the TE relay which is much cheaper but I don't know if there's a big difference. Perhaps the power it takes to latch/unlatch the relay which may have a big impact on the stability of the KAW supply. You might have to add a separate large cap that charges slowly through an RC circuit (to supply the current for the relay operation so your KAW doesn't faint).

[ryanm]

Hey Felix,

Thanks for the tip!  Yeah I'm guessing there may be some tricks associated with maintaining the power to the KAW.

Here's a pic of the other side of the board.  It looks like there is just that one connection to the HOT out which may need to also be cut.  What do you think?


Thanks
Ryan

[Felix]

Hmm... I guess I'm confused, I don't see an obvious link between the HOT IN and OUT. The current shunt is on the NEUTRAL.
If you cut the trace where you indicated on the previous picture you would cut power to the KAW. You would have to cut it beyond the power supply so the KAW always has power. I'm not really sure where that point is. Maybe I'm missing something or maybe the trace is not visible in the image. 

[rwormbird]

Rather than hacking the KAW, it would be safer and cleaner to build a box external to the KAW with a power core, an outlet and the relay.

[Felix]

Yeah I almost wanted to say the same thing, but didn't want to crush the hacker spirit 

[rwormbird]

Hackers ultimately need to build things