Some questions about a PCB

[dog80]

I recently swapped my old Rokonet RP-808 alarm panel with a newer one because the old one didn't support ContactID.

Anyway, I was examining the old alarm panel PCB and have some questions.

Here is a pic of the back side at the bottom, where the zones connect



1) The big copper traces are the ground plane. Why does it have this checkered pattern?

2) Why the wiggly trace between positive and negative in the red circle?

3) What's that in the blue circle? I measured it with a MM and looks like a resistor?

4) What is the device in the yellow circle? I tested it and it looks like a diode, but it has 1K resistance when forward biased.


Also what is this mystery device? I couldn't find a datasheet.



The engraved circle at the center says "MEXICO AA"

[SeanB]

Checkered pattern was common as there were worries with older soldermask about wrinkling with large copper areas, so the checkerboard pattern is common. The red circle adds a small amount of inductance, probably designed in to make a fusible track that will arc over to the ground if there are large surges like lightning. Blue is a spark gap to dissipate lightning voltages to the grounded chassis.

Yellow circle is a diode in a MELF package, here used as a catch diode across the output.

The transistor is a TIP110 darlington, nothing exotic.

[AndyC_772]

1) The big copper traces are the ground plane. Why does it have this checkered pattern?

At a guess, it's to reduce the percentage coverage of copper on the PCB so that the PCB plating becomes more even. It's helpful when fabricating a PCB to make the coverage as uniform as possible, otherwise the copper builds up in some areas more than others during the plating process.

2) Why the wiggly trace between positive and negative in the red circle?

I've seen things like this on alarm boards before, and I'd argue that they serve no practical purpose whatsoever. Wiggles like that might have a practical effect at frequencies of 100's of MHz or above, but otherwise they achieve nothing. A bit of an alarm industry urban legend, perhaps?

3) What's that in the blue circle? I measured it with a MM and looks like a resistor?

There's no actual component there, so at a guess maybe it's a deliberate attempt to create a small creepage distance between the pin and ground. Such a small gap might act as a spark gap in the event of a high voltage transient on one of the alarm wires, though I don't see any supplementary protection.

4) What is the device in the yellow circle? I tested it and it looks like a diode, but it has 1K resistance when forward biased.

It's a diode, the resistance you measure will depend on the meter's test current.

Also what is this mystery device? I couldn't find a datasheet.

TIP110, general purpose darlington NPN transistor:
http://www.onsemi.com/pub_link/Collateral/TIP110-D.PDF