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Another way to protect power supply circuits against over-voltage conditions is with a crowbar circuit.
Fig. 1 shows two examples of a crowbar circuit. In these circuits, an SCR is used to sense the over-voltage condition and go into conduction. The SCR is strategically located in the circuit to cause a short circuit of sufficient size to cause the fuse or circuit breaker to open and protect the circuit. The circuit in Fig. 1a uses an SCR that is connected in parallel across the dc terminals of the bridge rectifier. The fuse in this circuit's located in the primary circuit of the transformer. When the over-voltage-sensing circuit at the output terminals of the power supply detects the voltage has exceeded the setpoint, it will provide a pulse to the gate of the SCR, which will cause it to go into conduction. When the SCR goes into conduction, it will create a short circuit in the dc circuit, which will cause the primary side of the transformer to draw excessive current and open the fuse. The SCR is used because it's large enough and fast enough to withstand the short-circuit energy that is developed when it goes into conduction.
The crowbar circuit in Fig. 1b shows the SCR connected across the dc output terminals. In this circuit the fuse is connected in the dc side of the circuit prior to the SCR. When the overvoltage condition is sensed, a pulse is sent to the gate of the SCR and the SCR goes into conduction. When this occurs, a short circuit's developed in the secondary of the power supply and the dc current will increase sufficiently to cause the fuse to open. These types of circuits could also use circuit breakers instead of fuses.
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