priming by resistance
The simplest method is to include a resistor between the trigger and the power source. If this source is an alternative, which is the most interesting case, we will add in series with the resistance a diode to avoid applying the trigger significant negative voltage during the negative half cycle. This diode must obviously have a higher breakdown voltage to the peak voltage of the power source. The resistance value used to set boot time, but since the voltage applied to the trigger via this resistance is in phase with the anode voltage we will use the first part of the alternation is between 0 and 90 ° as shown in the figure below.
To go beyond 90 ° must be able not only to define the ignition voltage through resistance but ensure the possibility of phase-shift cell latter relative to the anode voltage what we will achieve in placing a capacitor in the gate circuit according to the diagram below.
The capacitor will charge via resistor until the voltage is sufficient to initiate the trigger. It is easily seen that by choosing wisely R and C we can scan the entire constant time range corresponding to 180 ° possible. Note that the diode 2 will allow for the negative half cycle to charge the capacitor to the value of negative peak which guarantees the maintenance of the thyristor during the beginning of the positive half cycle.
The major drawback of the previous assembly is that it depends as much on the typical thyristor priming that the RC circuit and requires a significant power in the control circuit, this is what justifies the pulse montages Also the elimination of these drawbacks are well suited to automated controls. Priming Pulse allows you to send a brutal way a higher trigger current (and even significantly higher) than that causing the boot which will result in reducing the time of establishment of the current in the main circuit and thus ensuring more precise control of the duration of the thyristor conduction phase.
Many devices will help generate momentum we simply consider here the UJT transistors and transformer pulse, the first being driven by the same source as the thyristor and the second allowing the use of an electrically isolated generator.
Recall that the unijunction transistor (UJT) has a special feature with a "negative" resistance area (actually it is a misnomer, it simply is an inverse variation in the sense resistor that of the current flowing through it) which is conducive to the operation oscillator via an equivalent mount that figured below.
It may be noted that the duration of the pulse and its frequency can be easily adjusted by varying the RC circuit. In particular can be replaced by a resistance transistor (collector-emitter) whose conductance will be adjusted by varying its current base command. This is a very technique used in the drives.
The other pulse control channel uses a pulse transformer. It is thus possible completely decouple the pulse generator and the power circuit. Also as will be seen in this diagram we can easily control the device even two thyristors mounted head to tail and thus operate the two waves of the AC current.
AC thyristor locking occurs automatically when the voltage reversal, that is to say at the beginning of the negative half cycle, by cons if the thyristor is supplied with a DC voltage the problem of its locking arises. We can obviously get it simply by placing a switch on the main circuit but it is not very realistic, we generally prefer to place a switch in parallel with the thyristor: blocking will occur when will short-circuit the thyristor.
The simplest method is to include a resistor between the trigger and the power source. If this source is an alternative, which is the most interesting case, we will add in series with the resistance a diode to avoid applying the trigger significant negative voltage during the negative half cycle. This diode must obviously have a higher breakdown voltage to the peak voltage of the power source. The resistance value used to set boot time, but since the voltage applied to the trigger via this resistance is in phase with the anode voltage we will use the first part of the alternation is between 0 and 90 ° as shown in the figure below.
triggering via an RC circuit
To go beyond 90 ° must be able not only to define the ignition voltage through resistance but ensure the possibility of phase-shift cell latter relative to the anode voltage what we will achieve in placing a capacitor in the gate circuit according to the diagram below.
The capacitor will charge via resistor until the voltage is sufficient to initiate the trigger. It is easily seen that by choosing wisely R and C we can scan the entire constant time range corresponding to 180 ° possible. Note that the diode 2 will allow for the negative half cycle to charge the capacitor to the value of negative peak which guarantees the maintenance of the thyristor during the beginning of the positive half cycle.
trigger pulse
The major drawback of the previous assembly is that it depends as much on the typical thyristor priming that the RC circuit and requires a significant power in the control circuit, this is what justifies the pulse montages Also the elimination of these drawbacks are well suited to automated controls. Priming Pulse allows you to send a brutal way a higher trigger current (and even significantly higher) than that causing the boot which will result in reducing the time of establishment of the current in the main circuit and thus ensuring more precise control of the duration of the thyristor conduction phase.Many devices will help generate momentum we simply consider here the UJT transistors and transformer pulse, the first being driven by the same source as the thyristor and the second allowing the use of an electrically isolated generator.
Recall that the unijunction transistor (UJT) has a special feature with a "negative" resistance area (actually it is a misnomer, it simply is an inverse variation in the sense resistor that of the current flowing through it) which is conducive to the operation oscillator via an equivalent mount that figured below.
The operation is as follows: When a voltage is applied U the capacitor charges until the voltage Vp peak UJT. Then he abruptly discharge in the RB1 resistance via the EB1 junction producing sufficient amplitude pulse to start the thyristor. As soon as the voltage across the capacitor is returned to the UJT valley Vv value is blocked again and capacitor begins to charge until Vp again.
It may be noted that the duration of the pulse and its frequency can be easily adjusted by varying the RC circuit. In particular can be replaced by a resistance transistor (collector-emitter) whose conductance will be adjusted by varying its current base command. This is a very technique used in the drives.The other pulse control channel uses a pulse transformer. It is thus possible completely decouple the pulse generator and the power circuit. Also as will be seen in this diagram we can easily control the device even two thyristors mounted head to tail and thus operate the two waves of the AC current.
blocking methods
AC thyristor locking occurs automatically when the voltage reversal, that is to say at the beginning of the negative half cycle, by cons if the thyristor is supplied with a DC voltage the problem of its locking arises. We can obviously get it simply by placing a switch on the main circuit but it is not very realistic, we generally prefer to place a switch in parallel with the thyristor: blocking will occur when will short-circuit the thyristor.
In fact the blocking will be faster if you put the switch in series with a reverse voltage, it follows the design of a device that will allow automatic blocking of the thyristor and, ultimately, an operation similar to that obtained with AC.
As the thyristor is not primed the capacitor charges through the load and the self. As soon as one starts the thyristor is seen that the capacitor will then be discharged via the thyristor but the presence of the inductor induces an oscillation process. If one has chosen the element values we see that from the current reversal Icl the total current in the thyristor Ith may fall below the minimum value of the thyristor holding current, it then freezes and the capacitor charging process resets.
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