Mystery Solved - Strange Phenomenon on PCB

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Discussion Overview

The discussion revolves around a strange phenomenon observed on a PCB connected to a PC via a Centronics port, where the board operates with a voltage of 3.5V despite having no power supply. Participants explore potential explanations for this occurrence, considering both theoretical and practical aspects of electronic components.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant describes the phenomenon of a PCB running at 3.5V without a power supply, seeking explanations.
  • Another participant suggests that a charged capacitor might be responsible for the observed voltage.
  • A different viewpoint indicates that if the TTL input on the PC is normally high, it could act as a current source, potentially charging capacitors on the PCB.
  • Another participant notes that it is common for power supply rails to be powered through internal protection diodes in ICs when there is activity on the I/O pins.
  • The original poster expresses surprise at this behavior, particularly with TTL ICs, which are typically high power consumers.

Areas of Agreement / Disagreement

Participants present multiple competing views regarding the cause of the phenomenon, and the discussion remains unresolved with no consensus reached.

Contextual Notes

Participants do not provide specific details about the PCB layout or the exact nature of the connections, which may limit the analysis of the phenomenon.

Ravaner
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Hello,
I just watched at a strange phenomenon. One of my PCB's (Cmos+TTL) is connected to my PC through a Centronics port. No power supply is provided by this wire (only data in and stobe signals). With main power supply off and filtering caps empty, this board runs perfectly during hours and hours with a Vcc at 3.5V coming from nowhere. If someone may explain to me ...
 
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Maybe a charged capacitor? We can't tell without knowing what the PCB looks like.
 
If the TTL input on your PC is normally high, it is a current source, and may charge capacitors on your PCB. The corresponding TTL output on your PCB is a current sinking (pull down) output (think npn open collector).

Bob S
 
This is not uncommon at all. The power supply rail can be powered up through the internal protection diodes in ICs if there is activity on the I/O pins of said ICs.
 
Thanks for yr replies. That was also my own explanation, but anyway if I'm not surprised for CMOS IC's, it's more amazing for TTL Ls and NE585 (not Cmos) which are big consummers.
 

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