Mystery Solved - Strange Phenomenon on PCB

[Ravaner]

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 ...

 

[dlgoff]

Maybe a charged capacitor? We can't tell without knowing what the PCB looks like.

 

[Bob S]

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

 

[Averagesupernova]

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.

 

[Ravaner]

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.