Thursday, September 19, 2013

Let's Tear Up: LM465 x10 Lamplinc module

I have a spare x10 module that is starting to get flaky, and I find myself using the basic non-INSTEON x10s less and less. So, I thought it might be fun to pop it open and see what's inside.

Disclaimer: We're dealing with a 110v AC device with some decently-sized capacitors. If you choose to do something this silly in the privacy of your own home, do be careful, and maybe probably don't plug it into the wall with the case off or anything ridiculous like that.

Let's start with the exterior. This module is held together by two screws and a grooved latch at the bottom, so it's very easy to get open. Applying just a bit of pressure near the prongs did the trick.

I appreciate the design here; there are no electronic components on the top of the shell that are hard-wired to the bottom, which makes it easy to get in and mess about. Note the four-brush conductor bar on the inside of the wheels. 

The underside of the circuit board includes a neat trick: the metallic wheel-plates interface to the selector wheels, and are shaped so that the hot input and the logic pins of the F78561 integrated circuit are connected in different patterns based upon the wheel orientation.

The backside of the circuit board with the wheels resting on it.
Most of the circuitry on the board is care and feeding of the F78561 IC. I know very little about this one; some Googling suggests it's custom-made and blends some digital logic with the analog logic necessary to read the X10 signals. I'm not actually interested in popping that bit open; I trust it works.

Over-side of the x10 controller circuit board.
(note: the big blue pouch behind the tunable resistor labeled "0050" is probably a 250v capacitor. Full label reads "0.68k (newline) 250v x10." I suspect the top means "0.68 microfarad, 10% tolerance..." the lack of a unit designator is really throwing me, but caps of this form-factor tend to be microfarad range, based on some Googling. Assuming it operates at 110v in this circuit, it should be able to store 8.23 millijoules, so probably no chance of a nasty shock from it).

For my purposes, the more exciting part lives under the metal plate. This plate is a heat sink holding a pretty large inductor and what (at first) appears to be a big transistor. Interestingly, the transistor is actually holding the plate; it's screwed to the transistor, which "floats" it in the air, mechanically anchored to the board by the transistor's three leads. Once you have the box closed, I suppose that's less of a concern.

Here's the star of the show: the transistor-looking component is actually a little triac. Triacs are (conceptually, at least) transistors for alternating current; they allow current to flow in both directions along the terminals, with the flow regulated by positive or negative voltage between the gate and terminal 1. However, unlike transistors (where the current across the terminals is proportional to the voltage at the base), the triac begins conducting once the voltage at the gate exceeds a threshold, and it ceases to conduct when the current across the terminals drops below a threshold. One can think of a barrel of water; exceeding the gate voltage is like tapping the barrel, and the water flows until it drops below the level of the tap (then the tap magically reseals and the barrel refills with water; okay, the metaphor breaks down :) ). To change the voltage that gets to the load, you can phase-shift the alternating current applied to the base (for example, by using a pulse-width-modulated signal triggered by the AC current). For example, if the base turns on when the AC signal across the terminals is at the peak, only half of the wave (crest to trough) actually gets through; essentially, only 50% of the current of the AC source (assuming the base is triggered on both max positive and max negative from the AC source).

 The part number of this particular triac is Q010L4, which shows up in this data sheet with the following interesting properties: 
  • Max 10 amps (that's fine; the box is rated for 300W, so it should never flow more than 2.7 amps anyway).
  • 400 repetitive peak blocking voltage (basically how much AC voltage the triac can sustain safely; well below the 110AC this device should operate at).
  • Triggering signal is generally applied between the gate and MT1 (as done here).
  • Triggering happens at 25 mA of current to the base (in-phase with the current across the terminals) and current continues to flow until it drops below 35mA from MT1 to MT2.
So now we know; the logic in the x10 controller is probably generating a digital switching signal to the gate, synchronized off of the AC voltage. Since the x10 circuitry should also be triggering from signals coming along the AC line, this is to be expected.

Some questions I still have that I might investigate further:
  1. How is the 110vAC stepped down to a reasonable operational voltage for the integrated circuits in the controller circuitry?
  2. What is the purpose of the big inductor?
  3. Is the controller core itself running on AC or DC?
I can probably figure out quite a bit more from the components on the board without needing to understand the IC itself.