Stealth naked

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Stealth naked

Postby hyphenated » Sun Sep 06, 2015 1:32 pm

Hi guys

Plugwise has made it to Oz, and I dipped my toe because of the coordination announcement with SMA, allowing SMA Home Manager to control PlugWise sockets with the next software release (I hope).

If you are not aware, Home Manager can link to a 3-phase high current measurement device at grid entry, do the math according to inverter output from solar, and show a real-time usage balance, amongst other things. I have a Stretch, a few Stings, and a few Stealths – the Stealths aren’t in yet.

At this stage I have attempted to bring myself up-to-date on the community, and it certainly appears that a number of talented programmers have been looking at opening up the protocols for inspection and adoption into other systems. I haven’t found much investigation of the hardware.

I’m interested in two things:
• A PlugWise-friendly ZigBee repeater with a little more grunt;
• A Stealth variant with 32A measurement and switch capacity.

The former is to get past the meshing problems in a long building without overkill on units (let’s face it, knowing the current draw of the LED desk light at any one time is not on my priority list). The latter is to make sure I cover the big-ticket power items so that I can load-shed when I have a solar energy deficit. However, a 16A limit does not cover dedicated line devices (Pool Pumps, Spas, Some Air Cons et cetera) and knowing consumption is potentially a big deal.

Plugwise Stealth hardware
Plugwise naked.JPG
Plugwise naked.JPG (104.22 KiB) Viewed 2036 times

Open the box (not that I would do that, of course; instead I employed my X-Ray camera), and you would find a small PCB with a smaller daughterboard. This could be a 14-pin standard PICtail interface design in which case RC0 is expected to be the default output to timer1 clock through an opto-isolator (speculation on my part – it’s a custom daughterboard).

Plugwise Daughterboard.JPG
ZigBee and Flash
Plugwise Daughterboard.JPG (90.47 KiB) Viewed 2036 times

The main chip (Ember EM250) is a now-outdated ZigBee SOC. It has a 12MHz 16-bit processor core, 128k Flash, 5k RAM and a 128 AES engine in addition to the transceiver. Immediately adjacent is a 4 Meg serial firmware flash memory.

Main Board
Plugwise bottom.JPG
Power metering chip
Plugwise bottom.JPG (120.11 KiB) Viewed 2036 times

The metering function is all on-chip, contained within a MCP3905A metering IC. This little bunny will measures ingle-phase active (real) power at 0.1% error and employs the IEC standard with very low shunt resistance (<200 microOhms) providing a proportional frequency output. The shunt appears to be a 1 millliOhm (0.001 Ohm) PMR25 resistor (marked 1L0 between live in and live out). These are rated as 1W.
The mains relay is a high-dielectric (5kV) 5V miniature relay with single pole single throw 16A connectors.

So – the relay contacts are obviously a vulnerable area, but if they were replaced or bridged, what is the maximum current that can a) be put through the shunt and b) be measured and transmitted safely? The shunt will be dissipating 1W at 37A AC RMS, so putting more than 30A through is obviously getting to be a concern (about 0.03V drop). Spec says 16A; I wonder if there is headroom in the frequency and/or parameter transfer for a higher current if the shunt will take it.

The 3905 reference design is specified for 10A with a 40A limit, using a shunt resistor of 250 microOhms (one quarter of the Plugwise implementation). If the current design doesn’t have a headroom for 32A measurement, one possibility (albeit a workaround) is a new shunt, an upgraded relay, a new device type in firmware, and use of a multiplier in the calculation software. I would be interested if anyone has got into the guts, as it were.
Plugwise top.JPG
Stealth assembly
Plugwise top.JPG (90.19 KiB) Viewed 2036 times
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Re: Stealth naked

Postby raymonvdm » Mon Sep 07, 2015 9:30 pm

I think you want to use another type of device when measuring loads higher than 16A using ferriet cores

Note: They also have a DPM-3 for connecting max 3 ferriet cores
Running HS3PRO on PC with Z-Wave / OpenTherm / Plugwise / RFXcom / MQTT / XAP400 / Logitech Media Server and Squeezelite on PI`s
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Re: Stealth naked

Postby hyphenated » Tue Sep 08, 2015 12:37 am

Yes, I can find other means of controlling the higher amperage circuits, but I wish to be able to integrate these higher demand circuits into the SMA home Manager displays, and load-shed when operating entirely on battery (later) or when solar energy drops (now). A Pool Pump is exactly the sort of load that one would want to shed when the sun goes in (even if I don't have one :)). A Spa has an always-on recirculation pump, but you might want to reschedule the cleaning cycle when the main pumps kick in. Air Conditioner loads - you might want to control an AC using infrared, but measure the load current. Refrigeration loads (we have coldrooms) are also outside the 16A limits.

There are work-arounds for controlling a high-amperage circuit set - the obvious is to use Plugwise to control a heavy-duty contactor without current measurement, and I may end up doing this - no messing with the internals. An expensive option is to employ a meter and a Smile, or another SMA meter, but there are potential problems with these approaches beyond the high costs. A more intrusive approach is to relocate the shunt and relay elsewhere, if the measurement protocol allows higher recorded current.

The shame is that the plugwise contains a metering chip which can happily handle any amount of current registration with the appropriate shunt. The unit as manufactured can probably handle higher than 16A, although the relay contacts are an obvious limiting factor. Tweaking the shunt alone should make it handle 40A peaks. Sure - I would re-evaluate the Stealth spring clips in such a case and the relay would have to be replaced, but a repackaging into a DIN Rail unit with standard screw terminals would solve a multitude of problems and open up some new options, even if the contactor is made external.
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