The Crowbar is a high voltage limiting device used to protect the CMS Si/Hybrids from a high voltage power supply voltage surge. The design is similar to that described by F.V. Pavlicek of FermiLab. The crowbar is inserted between the supply and the load making use of a simple fast acting fuse which opens following the triggering of a high current silicon control rectifier from a series zener diode chain set to the desired limit voltage. SCR trip time was measured using a PSpice simulation, which showed 3.3 us after exceeding the voltage limit. For the simulation an 800mV 2 us pulse driving the SCR gate (C1=1nf) was used. The trip time appears to be more dependent on the SCR parameters than the gate circuit time constant (R1C1=100ns).

        Trip Voltage Range:    600 to 720 volts
        Fuse Current:                2 mA
        Leakage Current            less than fuse current when below trip voltage range
        Connectors:                    SHV   male panel mount


1. Pavlicek, F.V.; Bledsoe, S., SVX II Silicon Detector Upgrade Project Readout Electronics, High-Voltage Bias Protection Modules. FermiLab Document ESE-SVX-011211 of 20 March 2002.
    2. 1N5267 Zener rated at 75 volts trips SCR at 70.5 volts
    3.  During voltage decay tests at trip point (70.5v) decay was measured at 3uS down to 20 v.  R3 was 100 ohms during the test resulting in a time constant of  2 uS.  R3 will be increased by a factor of 10 to reduce stress on detector during discharge of capacitance (est to be 0.02 uFd).

4. The 2N5064 Teccor SCR was used in the PSpice simulation rather than the TCR22-8. The turn-on parameters are very similar except the the 5064 data sheet shows a 2.2us turn-on and the 22-8 shows a 3.5us turn-on.