PoE: Raven

Curtains!

I’ve bought an apartment “off the plan”, which allowed me to make (some) changes to the layout before settlement. They were happy to put in extra Ethernet points, but one of the places I asked them to put one confused them no end: up high behind the pelmet for the main window. You can guess what I’m doing there; they just acceded to the request.

Design

So the curtain motor is 12V at 0.833A, and is controlled by a 433MHz RF “garage door opener” signal. That meant that I had to be careful with EMI/EMC for any PoE adapter – so I’m using the Silvertel AG9912‑MT. Its most important feature? To me, it’s that it has thermal cutout: if it overheats, it simply stops providing current.

Datasheet

According to the datasheet, the barest minimum of external components is two bridge rectifiers on the PoE input, and one electrolytic capacitor on the output. For safety, they recommend an 80V TransZorb zener across the input. They also say that EMI is an issue, so suggest six ferrite beads on the input, an inductor and two extra capacitors on the output, and two 2kV decoupling capacitors straddling the input and output. Oh, and one application note also had a 2kV capacitor between Protective Earth (the shield) and ground.

Heat

For heat dissipation, they recommend using a thermal pad between the module and the PCB, so there needs to be a solder-mask-free void where the module goes on the PCB. Annoyingly, there’s also a 1,500V separation requirement between input and output, so there needs to be keepout areas too. Thus there isn’t as much copper available for heat transfer, but what is available should have vias to the other side of the PCB for more dissipation area.

All of these ameliorate the heat issue – but again, the module has a thermal cutout, so it’s still safe.

Schematic

To simplify things, and allow as small a board as possible (only 1″ x 2″), I found the Würth Elektronik PoE MagJack, which has all the bridge rectifiers and magnetics inside the housing. Expensive, but worth it!

PCB

OSH Park

Flexibility

So that just left the other external components, and a layout that was as flexible as possible: the space for the AG99xx module has both THD and SMD pads, and all the places for external components are there: I can either fit, omit, or solder across each place, depending on the end use case. Unfortunately, with that small area I haven’t got access to the actual Ethernet data lines – not that this project needs them!

Whine

My pet hate with switched mode power supplies is the coil whine that many have. And the module is reported to suffer this too at low current draw; under 100mA. The motor will draw plenty when it’s working, but 99% of the time I didn’t want the thing whining away.

Solution

Solution: increase the base current load! First of all I added connections to both the MagJack’s LEDs: 15mA each with 680Ω resistors. They’ll stay illuminated permanently if the resistors are fitted. I also added a 2W 150Ω resistor, which at 12V is nicely 80mA and 0.96W.

Note that both of those resistor values were calculated on the 12V output of the module – other voltage modules will need different values.

But I didn’t want to simply waste that power if I didn’t have to, so I also added a 10kΩ potentiometer. Before installing I’ll turn the pot full on for minimal load, and expect a whine. I can then decrease the pot, increasing the load, until the whine goes away.

Heat

Assuming that adjustment will still be most of the documented 100mA, then with the motor running also I’m pretty much maxing out the AG9912-MT. Heat dissipation even for that short time is still a concern, so although I’m 3D printing a shell to hold the board, I won’t use a lid to help with ventilation.

But if all of those measures fail, it will simply cut out. So what if the blind doesn’t go all the way down (or up)? Of course, I don’t want that – but it’s a safety thing.

Components

All of the above is completely theoretical, designed off datasheets and some napkin mathematics. But that’s what I love about OSHPark: $10 for three boards is cheap; the components will cost more! Below is the Bill of Materials for the PCB, with DigiKey part numbers and pricing – at the quantities I bought them at, anyway.

Note the one thermal pad will be sufficient for over 300 of these!

All of the components’ placements should be obvious, either by the silkscreen or pad shape (careful that the AG9912-MT goes in “upside down” – check for Pad #1). The only unlabelled pads are the six (vertical) 0805 ferrite beads just above the MagJack.

Options

You can choose to not include most of the components:

• The 2W 150Ω resistor and trimpot are in series, so either omit both, or bridge the missing trimpot with a fixed resistance – note omitting just the 2W resistor would be a mistake, since the trimpot is only ¼W!
• The ferrite beads are also in-line, so rather than omitting them they need to be solder-bridged, or 0805 0Ω resistors used.

Bill of Materials

Description	Option?	Value	Size	Manufacturer		Qty	DigiKey (mostly)
				Name	Part number		Part number	Each
PoE module	No	12V	21×14 mm	Silvertel	AG9912-MT	1	Symmetry Electronics	$7.93
Thermal pad	Omit	0.1”	16”x8”	Bergquist	GPVOUS-0.100-01-0816	1	211-GPVOUS-0.100-01-0816-ND	$36.45
PoE MagJack	No	10/100	21.8×15.9mm	Würth Elektronik	7499210121A	1	732-4974-ND	$10.23
Zener	Omit	80V	DO-214AC	Vishay TransZorb	SMAJ58A-E3/5A	1	SMAJ58A-E3/5AGICT-ND	$0.295
Inductor	Omit	1u8H	6.3mm	Murata	1255AY-1R8N=P3	1	490-10804-1-ND	$0.440
TrimPot	Omit / Bridge	10kΩ ¼W	5×4.5mm	Ohmite	‎SMUB103PET‎	1	SMUB103PETCT-ND‎	$1.410
Resistor		150Ω 2W	1225/2512W	Vishay Dale	RCL1225150RJNEG	1	RCL150FCT-ND‎	$0.480
Resistor	Omit	680Ω ¼W	1206	Panasonic	ERJ-8GEYJ681V	2	‎P680ECT-ND‎	$0.028
Capacitor	No	100uF	8.0mm	Panasonic	EEE-1EA101AP	1	PCE4593CT-ND	$0.170
Capacitor	Omit	10uF 25V	1210	Samsung	‎CL32A106KA9LNNE‎	2	1276-2882-1-ND	$0.391
Capacitor	Omit	4n7F 2kV	1210	Kemet	C1210X472KGRACAUTO	3	399-16765-1-ND	$1.671
Ferrite bead	Bridge	1kΩ	0805	TDK Corp	MMZ2012S102ATD25	6	445-172862-1-ND	$0.053
TOTAL							(Extended)	$27.27