Reverse engineering Lumenition optical switch

[chris.oates]

While troubleshooting I've picked out all the soft rubber potting compound hiding these sparse components but I'm wondering what the circled package type is called and am I actually looking at the semiconductor material. Its probably a power Darlington and despite my efforts may still be working, at first I thought I had clumsily pulled the top off the package but there were still fragments of the rubber stuck to the surface. I've had a good search but can't find anything that looks like this, only the inside of a TO3 Darlington which has the same pattern etched on it but with the leads attached within the pattern. As its about the same size as a TO-218/TO-247 perhaps its just a regular device without the case?

Chris

[camallison]

I may be way off course, but those markings remind me of an antenna for something like bluetooth level of signal. What is the unit it is in?

Colin

[duncanlowe]

I suspect you are right, a semiconductor. I also suspect you have removed the bond wires that link from the connections to the substrate.

[Uncle Bulgaria]

I bet it's T3/T4 in the patent application circuit diagram you posted, so yes a power darlington. Presumably the pink left wire is the 12V in, with the film resistor between it and the red wire to the optical switch (7.5V) and the bottom red object is the Zener Z1. The black wire(s) look like they're on an earth link, which has the small red diode between ground and the base of the Darlington pair.

Seem legit?

[duncanlowe]

It looks like what I think is known as a bare die package. In other words, it's what's normally inside your metal can or plastic moulding. I suspect you have removed the bond out wires along with the potting compound. There appears to be some evidence of them on two of the three areas, just tiny circular marks where the bonds will have been welded. If you have a digital microscope, or a cmaera with a macro function you could probably get a decent photo of just that area.

[Radio Wrangler]

That's a power darlington transistor pair, you can see the main transistor, and its smaller driver in a corner. It looks to be mounted on a ceramic insulator to stop the collectors connecting to the heatsink.

It is possible that this ceramic is simply Alumina, aluminium oxide, which is safe, BUT there is also the possibility that it could be beryllium oxide BeO, which is very dangerous if chipped, cracked, ground or drilled. Units from this era usually didn't have warning labels about BeO. It is good practice (IE it might save your life) to treat ceramics in this sort of position as dangerous unless proven innocent. It should be safe so long as there is no risk of dust being created.

As Duncan says in removing the silicone rubber, you've likely removed the bond wires linking base and emitter on the die to their respective metal areas on the ceramic and the connection pins. Do you have a microscope? But I don't know anyone with a ball/wedge bonder that could replace the bond wires.

As an alternative, there are power transistors of the necessary ratings in pachkages similar to that. The problem comes in insulating the collector-connected tab. You need to pass heat efficiently, but electricity not at all.

David

[duncanlowe]

Yes, this photo of the insides of a Darlington show quite well, where the two bond wires go when inside a metal case. The third connection in this case is the metal can. In your case, the bond wires would go to the two gold stripes at either end of the ceramic.

[duncanlowe]

Originally Posted by Radio Wrangler

As an alternative, there are power transistors of the necessary ratings in pachkages similar to that. The problem comes in insulating the collector-connected tab. You need to pass heat efficiently, but electricity not at all.

David

With care, I would think a heatsink isolation mount kit comprising an insulating but heat conducting 'tab' and an insulator for the screw hole, after drilling and tapping the heatsink. I seem to remember there are even self adhesive versions around.

[mole42uk]

Here's a later power module:

The three-legged component on the right tests as a power darlington but I don't remember if I managed to find a part number. There are several thick-film resistors on the substrate and there were several conventional leaded components within the silicon pottage.

[mole42uk]

Here's another picture with the wires still attached, compare this with the photo above and the circuit in the Patent application to make sense of the whole:

Connections are:
RD power from ignition
RD to opto module (7.5V)
BLU to opto module (signal)
BK to opto module (GND)
BK to chassis (GND)
BN to coil

[Uncle Bulgaria]

BBC Sounds has a few cracking dramatisations of 'The Saint' with Paul Rhys on at the moment. And I see the sun is just over the yardarm, so a gentle task like digging away the potting compound for the LED appeals. Indeed it is the orange-ish example of the TO-46. The markings indicate an OP133, which I see is a 100mA, 935nm IR emitter. 18 degree viewing angle and a 1.75V forward voltage drop.

[Uncle Bulgaria]

We have success! Here's the circuit I've knocked up to test, which fires reliably each time the gap is broken.

Next question: can I make this in a small enough space to fit into the optical switch housing, and what do I pot it in with? Is there anything that is rather more easily removable than the stuff originally there that required some serious time with little drills and scrapers?

[Radio Wrangler]

Non-acetic silicone.

David

[AC/HL]

Brilliant. Once fettled you won't need to get at it, and sealing will be the most important criteria. It will be in a somewhat hostile environment.

[ms660]

I found that one of the most important things with these was routing the sensor's connecting wires, make sure to allow for any movement due to advance/retard etc, I fitted one of these 25 years ago, it was still going strong last time I saw it a few months back.

Lawrence.

[Uncle Bulgaria]

Thanks. I'll look into the silicone. I assume it doesn't shrink away from the edges of the recess. What is a good general purpose compound? I have never potted anything before. Chris's experience above suggests a soft material in the power module, whereas in the switch I would have said it was a pretty rigid epoxy.

Am I best off manufacturing a new circuit board and using the little SOT packages as in the original (and some replacement for the thin film resistors), or using the through-hole parts I am familiar with (which could possibly fit in nearly the same space as the circuit doesn't have too many parts)?

I suppose heat is the enemy here, so do either of these options have thermal advantages?

[Radio Wrangler]

non-acetic silicone rubber. It's significantly more expensive than the bathroom variety, but that stuff exudes acid which rots metal.

The one I;ve used in the past is by GE (General Electric of America) and is grey coloured. Is works very well and survives terrible environments.

David

[AC/HL]

We used something called Plast2000 for waveguide and coax cables, also a grey silicone sealer. Getting it retail may be a problem though.

[Radio Wrangler]

Farnell or RS

David

[MotorBikeLes]

I have been using Loctite 5920 for around 10 years for sealing, potting etc and it has been faultless. A couple of years ago it seemed to have gone off the market, with a few tubes on Ebay at high cost. However it seems to have been replaced by an identical compound (5990) in slightly smaller tubes st higher cost. It is copper coloured and often referred to as "copper silicone plastic" or similar. If your potting is deep, then apply in thinnish multiple layers, or its setting time will be rather extended. This really is a "master of all trades" in my book. No worry about corrosion, electrical leakage, or high temperatures. Don't get it on your lenses, it is pretty opaque!
Les.