Crowbar (circuit)

A clamping circuit (English crowbar ) is an electronic circuit which is used to protect against surge, especially in a low voltage DC power supplies. In this case, when a certain voltage, the supply voltage is short-circuited low impedance, so that as a result can cause a fuse and the power supply interrupts.

Crowbars are primarily used to protect expensive modules whose operating voltage is close tolerances (eg, 5 - volt supply of digital circuits ), against a failure of the voltage regulator in the power supply. Due to the relatively long operating time ( 10 microseconds ), they are not suitable for protection against surge transients as they occur for example due to lightning.

The clamping circuit comprises a switching element usually a thyristor or triac which is connected with its two main terminals ( cathode and anode) directly to the supply voltage. When a critical level of overvoltage, the thyristor is ignited via the gate by means of a trip circuit and short-circuits the supply voltage. The occurring as a result of short- circuit current triggers a safety item, usually a fuse in the supply line, and permanently interrupts the power supply. This fuse must trigger fast enough to avoid damage to the thyristor.

The functional difference in a surge suppressor diodes or varistors made ​​is that for reaching the trigger voltage of the power supply is directly short-circuited, so the voltage drop and therefore the power dissipation is kept low on the thyristor. Almost the entire power loss then occurs at the fuse, thereby ensuring the triggers and the circuit interrupts.

For overvoltage protection diodes on the other hand there is the problem that prolonged overvoltage resulting in a high thermal load on the protection diode, with the consequence of the failure of the thermal diode. When fuses are used they must be replaced after triggering. Mostly, however, there is a fault in the power supply, so this needs to be replaced or repaired anyway. Therefore, such clamping circuits are rarely used in conjunction with self-resetting fuses ( PTC fuses ). In PC power supplies a response of the clamp circuit on the 5V rail is usually not the response of the primary-side fuse result, but the control circuit of the power supply detects an overcurrent and thereby regulates the voltage down. The power supply remains in this state until it is turned off. After switching on the power supply again cause it works again without problems, but goes back to the overcurrent mode, if the reason the response has not been eliminated. The operations of a PC power supply without consumer may result in the response of the clamp circuit, as these power supplies are not designed for operation at idle.