thyristor trigger interlock

Sample Circuits

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

1

go to top      next      previous A negative-going edge comes into the input node, and is differentiated by the input circuit to pin #2 of the 555. The input brings pin #2 below 1/3 of Vcc, to which the 555 reacts to bring its output pin #3 to logic "1" and its open-collector pin #7 out of conduction. This action at pin #7 will case the resistor to charge the timing capacitor connected to pin #6. The capacitor charges beyond 2/3 of Vcc, causing pin #6 to sense the fact and force pin #3 to logic "0" and the open-collector transistor at pin #7 to conduct, discharging the capacitor and bringing the capacitor voltage near the negative rail. The process will be repeated with the coming of the next input of a negative-going edge. Note that there will be need to provide a reverse-biased diode at pin #2 to Vcc to prevent this input structure from undergoing unnecessary stress from the positive pulse component of the differentiated signal, i.e., when the input signal becomes a positive-going edge.

2

go to top      next      previous The logic "1" output of the preceeding stage above will bring pin #2 and pin #6 high, causing pin #7 of this 555 to conduct and bring the cathode of the LED down causing current to stimulate the emission of light to the base of a light-sensitive transistor, causing it to conduct, as well. With this process, current from the emitter will be sent to the gate of a thyristor, causing it to conduct for the rest of the alternating-current cycle. The process will be repeated with the introduction of another negative-going edge at the input. Note that, once triggered, the thyristor will maintain conduction until the current falls below the physical "latching current of the thyristor.

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