lf the voltage of the mains supply of a computer rises too high, components on the printed circuit boards can easily be damaged or even destroyed.
This emergency cutout placed between the mains supply and the load interrupts the supply when the voltage level exceeds a predetermined value. . For many reasons it is possible for the output voltage of a power supply to rise to a dangerous level. The emergency cutout described here has been set to the maximum supply voltage of 5.25 V that is stated by the manufacturers of TTL ICs. Zener diode D1 starts conducting just before the stated zener voltage is reached. A small current flows in the and gate circuit of thryristor Th1; the level of this current can be set with preset potentiometer P1 connected in parallel with the gate cathode circuit of Th1. When the mains supply rises, the current ’through the zener diode becomes large enough to cause the thyristor to fire. The firing level lies between 5.2 . . . 6 V. As soon as the thyristor fires, the mains supply voltage drops substantially because the thyristor virtually short-circuits the mains supply. ln the case of a supply without current limiting, fuse F1 prevents the current attaining too high a value.
The rating of the fuse depends, of course, on the load requirement. During testing and adjusting of the circuit, it is important that the thyristor continues to conduct after it has been fired until its current has dropped to zero. The firing voltage level can be set by means of a mains supply with a current limiter before it is put into use, lf it proves impossible, for instance because of tolerances of the zener diode, to set the firing voltage to the required value, try using a 5.1 V zener diode.
If the voltage of the mains supply of a computer rises too high, components on the printed circuit boards can easily be damaged or even destroyed. This emergency cut-out placed between the mains supply and the load interrupts the supply when the voltage level exceeds a predetermined value.
For many reasons it is possible for the output voltage of a power supply to rise to a dangerous level. The emergency cut-out described here has been set to the maximum supply voltage of 5.25 V that is stated by the manufacturers of TTL ICs. Zener diode D1 starts conducting just before the stated zener voltage is reached. A small current flows in the anode-gate circuit of thryristor Th1.
The level of this current can be set with preset potentiometer P1 connected in parallel with the gate-cathode circuit of Th1. When the mains supply rises, the current through the zener diode becomes large enough to cause the thyristor to fire. The firing level lies between 5.2 . . 6 V.
As soon as the thyristor fires, the mains supply voltage drops substantially because the thyristor virtually short-circuits the mains supply. In the case of a supply without current limiting, fuse F1 prevents the current attaining too high a value.
The rating of the fuse depends, of course, on the load requirement. During testing and adjusting of the circuit, it is important that the thyristor continues to conduct after it has been fired until its current has dropped to zero. The firing voltage level can be set by means of a mains supply with a current limiter before it is put into use. If it proves impossible, for instance because of the tolerances of the zener diode to set the firing voltage to the required value, try using a 5.1 V zener diode.
Circuit Description
Referring to the given circuit diagram of a simple DC short circuit protector for computers, the functioning may be grasped with the following points:
The zener diode along with R1 and P1 forms a voltage reference or threshold level for the thyristor gate which is connected at the the center of the above network.
P1 is adjusted to the desired threshold level at which the thyristor is required to be fired.
As long as the input voltage stays below this set level the thyristor remains inactive and the supply output to the computer undergoes a normal execution.
However if the input voltage tends to drift above the set level, the gate potential of the thyristor reaches the firing point and it fires, shorting the voltage across the output to ground.
The above action instantly drops the voltage to the safe set threshold so that now the thyristor is switched OFF.
The above triggering continues as long as the input voltage stays above the threshold mark, thus rectifying the output voltage to the safe limit, ensuring the computer a safe operating environment.
This emergency cutout placed between the mains supply and the load interrupts the supply when the voltage level exceeds a predetermined value. . For many reasons it is possible for the output voltage of a power supply to rise to a dangerous level. The emergency cutout described here has been set to the maximum supply voltage of 5.25 V that is stated by the manufacturers of TTL ICs. Zener diode D1 starts conducting just before the stated zener voltage is reached. A small current flows in the and gate circuit of thryristor Th1; the level of this current can be set with preset potentiometer P1 connected in parallel with the gate cathode circuit of Th1. When the mains supply rises, the current ’through the zener diode becomes large enough to cause the thyristor to fire. The firing level lies between 5.2 . . . 6 V. As soon as the thyristor fires, the mains supply voltage drops substantially because the thyristor virtually short-circuits the mains supply. ln the case of a supply without current limiting, fuse F1 prevents the current attaining too high a value.
The rating of the fuse depends, of course, on the load requirement. During testing and adjusting of the circuit, it is important that the thyristor continues to conduct after it has been fired until its current has dropped to zero. The firing voltage level can be set by means of a mains supply with a current limiter before it is put into use, lf it proves impossible, for instance because of tolerances of the zener diode, to set the firing voltage to the required value, try using a 5.1 V zener diode.
If the voltage of the mains supply of a computer rises too high, components on the printed circuit boards can easily be damaged or even destroyed. This emergency cut-out placed between the mains supply and the load interrupts the supply when the voltage level exceeds a predetermined value.
For many reasons it is possible for the output voltage of a power supply to rise to a dangerous level. The emergency cut-out described here has been set to the maximum supply voltage of 5.25 V that is stated by the manufacturers of TTL ICs. Zener diode D1 starts conducting just before the stated zener voltage is reached. A small current flows in the anode-gate circuit of thryristor Th1.
The level of this current can be set with preset potentiometer P1 connected in parallel with the gate-cathode circuit of Th1. When the mains supply rises, the current through the zener diode becomes large enough to cause the thyristor to fire. The firing level lies between 5.2 . . 6 V.
As soon as the thyristor fires, the mains supply voltage drops substantially because the thyristor virtually short-circuits the mains supply. In the case of a supply without current limiting, fuse F1 prevents the current attaining too high a value.
The rating of the fuse depends, of course, on the load requirement. During testing and adjusting of the circuit, it is important that the thyristor continues to conduct after it has been fired until its current has dropped to zero. The firing voltage level can be set by means of a mains supply with a current limiter before it is put into use. If it proves impossible, for instance because of the tolerances of the zener diode to set the firing voltage to the required value, try using a 5.1 V zener diode.
Circuit Description
Referring to the given circuit diagram of a simple DC short circuit protector for computers, the functioning may be grasped with the following points:
The zener diode along with R1 and P1 forms a voltage reference or threshold level for the thyristor gate which is connected at the the center of the above network.
P1 is adjusted to the desired threshold level at which the thyristor is required to be fired.
As long as the input voltage stays below this set level the thyristor remains inactive and the supply output to the computer undergoes a normal execution.
However if the input voltage tends to drift above the set level, the gate potential of the thyristor reaches the firing point and it fires, shorting the voltage across the output to ground.
The above action instantly drops the voltage to the safe set threshold so that now the thyristor is switched OFF.
The above triggering continues as long as the input voltage stays above the threshold mark, thus rectifying the output voltage to the safe limit, ensuring the computer a safe operating environment.
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