100V DC Line Switch controlled by logic levels

Discussion in 'The Projects Forum' started by gdylp2004, Feb 24, 2012.

  1. gdylp2004

    Thread Starter Member

    Dec 2, 2011
    Hi guys,

    Could anyone suggest a suitable switch for the following operation?


    SW1 is normally closed (N.C.) since buck #1 is the main operator. SW2 normally open (N.O.) because it is used as a backup.

    Both SW1 & 2 to be controlled with 8051 logic level, that is, 0V for LOW and 3.3V for HIGH.

    Average current through each switch is about 1A, therefore I'm selecting a 2X1A = 2A rated current SW for tolerance.

    Absolute voltage = +100V DC with respect to GND, but potential difference (P.D.) across the switch terminals really depends on the R(on).


    When a fault occurs at Buck #1 (short or open circuit), the 8051 MCU detects and turn off SW1 and on SW2 simultaneously. The step-down operation is transferred from Buck system #1 to #2.

    I've searched through the internet and has narrowed down to 2 options.
    1. Mechanical switch (relay)
      • Slower response but totally cut off operation with no leakage current
      • Not sure if a relay could be controlled by 3.3V logic levels output? Even if it could, would it be safe as 1 terminal = +100V, and the other to 3.3V?
    2. Electronic switch (MOSFET)
      • Any off-the-shelves unit where I could conveniently connect this SW with my existing system? Perhaps with 3 pins; PIN 1 -> +100V DC input; PIN 2 -> Buck convertor; PIN 3 -> logic level?
      • Or do I need to design from scratch an appropriate switching circuit with the use of MOSFET? I did see a few schematics online but those are like for lower voltage system in the range of 12V to 20+ V?
    I am certain someone must have done this before and I hope I could get some good suggestion overhere!

    Attached is my operation layout to facilliate better understanding.

    Thanks in advance!
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    1. This exists, example: http://search.digikey.com/us/en/products/IM01GR/PB1060CT-ND/1209988
    It is safe.

    2. That's an optorelay or solid state relay. example: http://search.digikey.com/us/en/products/CPC1916Y/CLA209-ND/654909
    NC version should also be available. However I think your system can also work with NO only.

    You can of course build the whole thing from scratch. A few transistors , maybe an optocoupler etc.

    The advantage of the solid state relay: it is faster.
    If you want to detect a failure of the converter, e.g. a short circuit, a mecanical relay maybe not fast enough to open before your fuse burns.

    Your major challenge will actually be to detect a short circuit fast enough in order not to damage anything, there could also be a short circuit at the output (load) you have to consider.