Crowbar Circuit Using TRIAC and TL431 Precision Programmable Reference

This Crowbar project can be used for preventing an overvoltage or surge condition of a power supply unit from damaging the circuit connected to the power supply. It operates by putting a short circuit and fuse blow across the voltage output, like dropping a crowbar across the output terminals of the power supply. The circuit is implemented using a TRIAC, a TL431 precision programmable reference, and a fuse. The resistor divider of R1 and R3 provides the reference voltage for TL431. The divider resistor is set to 2.268V when the power supply voltage is 5V in normal operating conditions. Since this voltage is below the minimum reference voltage of TL431, it remains off and very little current is conducted through the TL431. If the cathode resistor is sized accordingly, very little voltage will be dropped across it and the TRIAC gate terminal will be essentially at the same potential as MT1, keeping the TRIAC off. If the supply voltage increases to 5.55V, the voltage across R3 will exceed V_REF and the TL431 cathode will begin to draw current. The voltage at the gate terminal will be pulled down, exceeding the gate trigger voltage of the TRIAC, and latching it on and blowing the fuse. The circuit has been tested with a 500mA fuse.

Connections and Other Details

• Connector CN1: Pin 1 = VCC 5V Input, Pin 2 = GND
• Connector CN2: Pin 1 = VCC Output, Pin 2 = GND
• Fuse F1: Blow when voltage exceed 5.5V DC
• LED D1: Input Power Indicator
• LED D2: Output Power Indicator

Features

• Input Supply 5V DC
• Load Current 400mA (Fuse 500mA)
• Trip Voltage 5.55V DC
• PCB Dimensions 61.60 x 28.89mm
• 4 x 3.2mm Mounting Holes

Schematic

Parts List

Connections

Gerber View

Photos

Video

TL431 Datasheet

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