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  • This overvoltage protection or crowbar protection circuit is used where we need protection against high voltage surge. The circuit has a few components, it is very easy to build and will protect your electric equipment againg overvoltages. The crowbar circuit must be mounted between power supply and the protected device.

    The over voltage protection circuit is based on brute force: when the power supply voltage increases too much a thyristor shortcircuit the output. This mean that the overvoltage is quickly removed from equipment power terminals and F1 fuse will burn.

    The voltage at which the crowbar protection starts is set between 5V and 25V with P1:

    1. adjust P1 at maximum resistance value.
    2. temporarily replace the fuse with a wire and connect the crowbar circuit at a variable power supply.
    3. adjust the current limitation at 1 A si the output voltage at the desired value to activate crowbar protection.
    4. slowly rotate P1 until activation of the thyristor (when the current limitator engages)

    The overvoltage (crowbar) circuit is now set. Replace the wire bridge with a fuse (5A). In repaos state the circuit take 1 mA.

    Over Voltage/Crowbar Protection Circuit Diagram

    overvoltage protection circuit diagram

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    2 Responses to "OverVoltage Protection Circuit"

    1. William Hoff says: on January 20, 2014 at 12:54 am

      Polyfuses are not foolproof, I found the one hidden in the window•winder motor in my GM car. A great application because they are heat sensitive and also cut off the motor when it reaches its limit. But in this case the internal resistance was high and this motor would stop about half way up then I would have to wait several minutes for it to reset. Not fun in the rain. Got the same problem? Look for it besides the brushes in the end of the motor. Take care not to get hurt by the counterbalance spring.

    2. Richard Hankins says: on October 18, 2014 at 10:59 am

      This is a huge improvement over the standard zener diode hanging off the thyristor gate. One obvious problem with that circuit is the (usually) wide tolerance on the zener voltage, and the even wider spread on the thyristor gate threshold. Plus its lack of adjustability.

      This circuit is precise and adjustable.

      Its only defect that I can spot is the lack of transient protection. A brief overvoltage transient, which may be harmless to the protected load would trigger this circuit, so we would have “nuisance triggering”. Inductive loads are good at generating such transients and some regulators can also have their output voltage swing about for short periods with load changes.

      The solution is to reduce the bandwidth of the trigger circuit by adding an R-C filter at an appropriate point. A capacitor across R2 or across T1 b-e junction are possibilities. The value of time constant chosen will depend on how quickly the over voltage protection is needed. If the load will stand a 500uS transient, then a time constant of that orer will suffice.

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