Crowbar Circuit

Discussion in 'The Projects Forum' started by DKH, Sep 25, 2012.

  1. DKH

    Thread Starter New Member

    Sep 25, 2012
    Hi everyone,

    I am designing a crowbar circuit that should operate normally until 10.5VDC, using a Silicon Bilateral Switch triggering a triac. I am putting a fuse on the supply line so the triac will trigger the fuse and blow. My problem is I am trying to use this crowbar to prevent my device from AC mains voltage, so 120 VAC. I have hooked my crowbar to bench ac and dc supplies and it does limit the input voltage when the input exceeds 10.5V However, bench supplies only put out so much current. Is there any triac out there that can take whatever the mains puts out in terms of current? Here is a schematic.

    The supply says 80, but ignore that. The triac is where the scr is and the SBS is where the zener diode is. I switched them because I first designed it to protect only against DC, but then needed AC protection, so I switched those elements out with bidirectional ones.
    • pic.png
      File size:
      20.9 KB
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Crowbars are designed to short out the supply when they go over volts, so it will ALWAYS blow the fuse, you cant use this to discriminate against AC or DC.

    For AC crowbars use a Triac instead of a Thyristor, and a Diac or two zeners back to back on the gate.
  3. DMahalko

    AAC Fanatic!

    Oct 5, 2008
    In what situation would a device designed for 10.5 volts ever be connected to 120 volts directly? Are you trying to protect against a power supply failure such as a shorted transformer?

    Install a reversed diode bridging the positive to the negative wires just past the fuse. In normal operation with DC of the correct polarity, the diode does nothing.

    In the case of AC or reversed DC, the diode conducts full reverse current and blows the fuse immediately.