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Actually, I know the polarity too, even though there is no visible marking that directly indicates it. The way I know the polarity is as follows: (1) As you might guess from the notation DZ2, there is another Zener diode, labeled DZ1, and its polarity is clearly indicated by a standard electrical symbol for a diode. So, we know the polarity of DZ1. (2) Looking at the PCB with a 7x monocle shows that one end of DZ1 is connected to one end of DZ2, and the other end of DZ1 is connected to the other end of DZ2. (3) This one-to-one correspondence between the leads of the two Zener diodes tells us the polarity of DZ2. It also tells us that they have the same Zener voltage.
Well, maybe things aren't exactly the way I have just described them. The PCB certainly does have a label DZ1 and, in the rectangular space allocated to DZ1 is a very clear electrical symbol for a diode. However, I don't actually see a diode in that space, which is probably why I am able to see the very clear electrical symbol for a diode there. I do see two little spots of solder filling the holes provided for DZ1, but there seem to be lots of little holes on the PCB that are filled with solder and no components.
So, it is more accurate to say that *if* there were a Zener diode at the site denoted DZ1, we would know its polarity and that its polarity would agree with the polarity of DZ2. Since there is apparently no Zener diode at the site denoted DZ1, we have a situation where a false statement can imply any statement, but that's just mathematical logic: there should be no doubt that if they designed the board so that a Zener diode with that polarity could go at the site DZ1, then any Zener diode whose leads are electrically connected, respectively, to the indicated leads of DZ1 must have that same polarity as that prescribed for DZ1.
So, I'm fairly convinced that I know the polarity of DZ2, the Zener diode that *is* there and which, because it is there, covers up whatever marking it sits on that tells what its polarity is.
Now, there is one more detail that, in principle, should tell us the Zener voltage of DZ2. That detail is that the leads of DZ2 are also, respectively, connected to two of the wires that join the mouse to the mouse port. This mouse has what looks to me like a PS/2 connector. So, the Zener voltage has to be the voltage difference between two pins of the mouse port.
The wires are colored and the colors are all different, but I don't know whether that reflects a color coding. The wires that connect to DZ2 are respectively blue and green. To actually find out what wire is connected to what pin, I either have to test for connectivity (which I'm not equipped to do at the moment) or I have to rip open the cable and see which pins are soldered to which wires, which I'd prefer not to do if I don't have to.
Maybe the only nonzero voltage difference between two pins of the mouse port always has to be 5 volts. Or maybe the only constant nonzero voltage between two pins of the mouse port always has to be 5 volts. Such a fact, if true, would settle the question of the Zener voltage without my having to measure anything.
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Ignorantly,
Allan Adler
* Disclaimer: I am a guest and *not* a member of the MIT CSAIL. My actions and
* comments do not reflect in any way on MIT. Also, I am nowhere near Boston.
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