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Decoupling capacitors behavior on power-off

modelrailwaycat

Hello,

 

I am designing a home control system which contains a 'net' of multiple PCBs. Each PCB has several decoupling capacitors, namely 1 large 100 uF capacitor near the power supply connector (5V), and several 100nF spread on the board close to the ICs Vcc pins (between 6 and 10 per board). The finished system will include about 60 boards like this, separated several centimeters or even meters one of each other, and all of them powered by the same power supply. So at the end all the capacitors will be paralelled to the power rails.

 

My question is about the behavior of the system when the power will be turned off. At this point, I realize I will have an equivalent +6000 uF capacitor. What should be considered regarding the design of the system or the power supply in order to guarantee a safe operation?

 

Thank you very much in advance!

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  • Thank you very much for your answers.

     

    Yes, as you told, I believe the current will drawn through the boards themselves, specially each of them has a "Power On" LED with the limiting resistor, so I think I will last short.

     

    My concerns were more orientated to what has been told regarding one board supplying another board and exceeding the Vcc of the ICs, because at the end all the boards Vcc/Gnd lines are paralleled. I thought in puting the diodes in series next to the board supply in each board, but then obviously I will get a voltage drop of 0,6-0,7 V in all the Vcc, so if my power supply is +5V (which I intended, from a PC PSU), that means Vcc will be at 4,3 V, which can cause the circuit will not behave as expected. Any alternative in this direction?

     

    Regarding the Power On phase, I was thinking in power on the system progressively by separating the Vcc lines in groups of several boards, let's say 3 or 4 groups (max 20 boards per group), dividing the total current needed. Anyway, I will carry some test for the current peak the supply can handle as suggested here.

  • in reply to modelrailwaycat

    I guess you can use Zener diode for each circuitry to limit voltage?
    And don't forget to use the proper rating(voltage) of capacitor. Once, I was building a transformerless power supply, took a wrong value capacitor and Bam. It blew so hard I felt air pressure on my face. Luckily no part hit my face/eyes. But it was fun, I'm looking forward to do that once again, may be twice.. thrice....Hey, It was fun! By the way, I am looking forward to this project of yours, You might wanna video record it if you do something mischevious like blown your roof and hit a bird with cap.

     

    mudz

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  • in reply to modelrailwaycat

    I guess you can use Zener diode for each circuitry to limit voltage?
    And don't forget to use the proper rating(voltage) of capacitor. Once, I was building a transformerless power supply, took a wrong value capacitor and Bam. It blew so hard I felt air pressure on my face. Luckily no part hit my face/eyes. But it was fun, I'm looking forward to do that once again, may be twice.. thrice....Hey, It was fun! By the way, I am looking forward to this project of yours, You might wanna video record it if you do something mischevious like blown your roof and hit a bird with cap.

     

    mudz

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