Electronic Design

Bob's Mailbox

Dear Bob: I have a little microcontroller circuit powered from a 115-V ac to 24-V dc supply that is then further stepped down to 5 V with a buck converter. An RS-232 converter, whose switched capacitors give 8-V swing, is used to communicate to a PC. The problem I have is that connecting to a grounded PC has blown out the serial port on the PC. We grounded the common in the circuit and have had no problems since.

I'm looking to understand what possibly could have happened. Could the 24-V supply have floated up and accumulated enough charge to blow out the port? If so, how does this happen? I have also seen in many circuits a 1-M½ or larger resistor and a 0.01-µF capacitor in parallel tied between the power-supply common and earth ground. Is there some standard or something else I missed that suggests this? Can you please enlighten me or point me to a reference? Thanks.

  • Jeff Siefke (via e-mail)
  • Pease: I am not an expert on this, but I can make a guess.
    (A) The ground of the RS-232's supply must have floated up to a high voltage, perhaps more than 100 V. There probably was something like a 0.01-µF capacitor, as you suggested.
    (B) When the pins of the connector were connected to the serial port, the ground pin (or pins) did not connect first. (If it had, it would have resulted in no harm.)
    (C) In a well-designed system, the ground pin (or pins) would be longer than the signal pins, so it couldn't help but connect first. If the signal pin connected first, that might cause some harm. And it did.
    (D) A well-designed serial port ought to have 1 k½ in series with the inputs, going over to some protect diodes. But these days, everybody leaves protection components out of their computers because "it's too expensive" or because "extra components hurt reliability." That sounds like enough to explain it.

Hi Bob: I was teaching a unit on transistor amplifiers yesterday, and a student asked me why transistors are labeled as Q1, Q2, Q3... in schematic diagrams. So as an old timer with a wealth of trivia, why the letter Q for transistors? Love your Electronic Design column.

  • Michael Halbern (via e-mail)
  • Pease: When vacuum-tube circuits were developed, all the other letters were used to identify Transformers, (Vacuum) Tubes, Diodes, Rs, Cs, Ls—and Q was left over for " miscellaneous." When transistors came along, they were about as miscellaneous as you could get, so they became Qs.

Hello Bob: Last night I was replacing the CR2032 battery in an inexpensive digital tire-pressure gauge, when the small liquidcrystal display fell out. On closer inspection, I found it had no wires or electrical contacts whatsoever. I was able to sandwich the LCD and two associated foam pieces back between the PC board and the housing, and it worked. How do they do that?

  • Steve Goss (via e-mail)
  • Pease: The foam is made of alternate vertical stripes of conductive and insulating foam. Both are kinda squishy. The pitch of these is like 10 times smaller than the pitch of the electrodes on the PC board and the LCD. And I bet you can see these, presuming you look real close.

Dear Bob: Read with interest your experiences and recommendations on napping (ELECTRONIC DESIGN, May 12, 2005, p. 20). When I am caught by my boss with head on desk later this afternoon, I shall be sure to have on hand your invaluable guide to use in self-defense.

  • Howard Jones (via e-mail)
  • Pease: No, all you have to do is lift up your head, lift up your eyes, lower your eyes, and say, "Amen." Then stand up and ask your boss what you can do for him. Of course, this doesn't work very well if you're snoring!

See Associated Figure

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