Mystery Solved - Strange Phenomenon on PCB

In summary, the conversation discusses a strange phenomenon where a PCB connected to a PC through a Centronics port is able to run for hours with a voltage supply of 3.5V despite having no power source. It is suggested that this could be due to a charged capacitor or the internal protection diodes in the ICs. The conversation also mentions the difference between TTL and CMOS ICs in terms of power consumption.
  • #1
Ravaner
134
3
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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  • #2
Maybe a charged capacitor? We can't tell without knowing what the PCB looks like.
 
  • #3
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
 
  • #4
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.
 
  • #5
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.
 

What is the "Mystery" on the PCB?

The "Mystery" on the PCB refers to a strange or unexplained phenomenon that has been observed on the printed circuit board.

What are some common examples of strange phenomena on PCBs?

Some common examples of strange phenomena on PCBs include unexpected circuit behavior, sudden failures or malfunctions, and abnormal thermal patterns.

What causes these strange phenomena on PCBs?

There can be a variety of causes for strange phenomena on PCBs, including design flaws, manufacturing defects, environmental factors, and electromagnetic interference.

How can these strange phenomena be solved?

Solving the mystery on a PCB typically involves thorough testing and analysis, identifying the root cause of the phenomenon, and implementing corrective actions such as redesigning the circuit or improving manufacturing processes.

Are strange phenomena on PCBs a common occurrence?

While they are not extremely common, strange phenomena on PCBs can occur and can be a source of frustration for engineers and scientists. However, with proper testing and troubleshooting techniques, they can usually be solved.

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