Versatile Auto Cut -Off Unit

A versatile auto cut-off facility is necessary for all the mains operated equipment and voltage stabilizers (automatic as well as manual) to switch them off when the voltage shoots above or falls below a certain safe level. The cut-off facility available in most commercial stabilizers is so abrupt that they switch off the moment the voltage goes beyond a predetermined window (normally, 180V to 250V) and switch on immediately after the voltage fall within the calibrated range.

Introduction to Versatile Auto Cut-Off Unit

Power interruptions are quite frequent at times and the deviation of mains voltage from its normal safe value for a few seconds is also a common phenomenon. But it is undesirable to switch off and on equipment frequently, particularly motors and compressors which undergo greater stress during the start-up period than during their run time. Hence it is highly desirable to keep the switching on and off processors down to the minimum.

The introduction of on-time and off-time delays into the cut-off unit proves helpful under such circumstances. Such a cut-off circuit will switch off the load only when the mains voltage deviates from the safe value and remains there for a few seconds. If the mains voltage returns to its safe value within this time limit, the load will not be switched off. Similar is the case when the load is switched on.

Circuit Description of Versatile Auto Cut Off Unit

The circuit diagram of the versatile auto cut-off unit shown here includes the supply section. IC₁(LM393, a dual comparator with open collector outputs) senses the voltages, and IC₂ (555 timers) introduces the delay during switch-on and switch-off. The outputs of both the comparators in LM393 are tied to the common pull-up resistor R₃. Hence the output of any comparator goes low the voltage at point X will also be below.

One input of each comparator is connected to a reference voltage of 3.3 volts provided by the R₂-D₅ combination. The other inputs are connected to presets VR₁ and VR₂. Preset VR₁ sets the upper cut-off level and VR₂ sets the lower cut-off level, the output of the corresponding comparator, and hence the voltage level at point X will also go low.

Initially, when the unit is switched on, and assuming that the mains voltage is within the limits set by VR₁ and VR₂, the outputs of both the comparators will also be high. So capacitor C₄ will start charging through R₃ and D₆. Since during switch-on capacitor starts charging from zero volts, the output of IC₂ goes to a high level and the relay remains in the deactivated state. Hence the load is not switched on. When the voltage across the capacitor crosses 2/3 Vcc, the output of IC₂ goes to a low level, and the relay gets actuated and switches on the load. This constitutes the power-on delay and is about 10 seconds for the values shown.

At the same time pin, 7 of IC₂ is grounded by the internal transistor of IC2. Now capacitor C4 is discharged by the parallel combination of R₄ and R₅, but at the same time, it is being charged by R3. The overall combination of R₃, D₆, R_4, and R₅ is selected in such a way that the voltage across the capacitor is maintained above ½ Vcc (say V). This is the steady-state condition.

IF the mains voltage shoots above the level set by VR₁ or falls below the level set by VR₂ the output of the corresponding comparator and hence the voltage level at point X will go low. Now diode D₆ is being reverse biased by isolates points X and Y. Hence there is no charging path for C₄ and it discharges through the parallel combination of R₄ and R₅. When the voltage across the capacitor goes below 1/3 Vcc the output of IC₂ will go high, switching off the load. At the same time, the internal discharge transistor in IC₂ is cut-off. (Now the only discharge path for C₄ is the high valued R₄.)The time taken by the voltage across C₄ to fall from V (its steady-state value) to 1/3 Vcc is the delay introduced by the circuit to switch off the load after sensing an error in mains voltage.

If at any time before the load is switched off the mains voltage returns to its normal value, the charging process of C₄ through R₃ and D₆ is again started. And since the resistance in the charging path is lower than that in the discharge path, the voltage across the C₄ is taken to its steady-state value of V without much delay and the load is not unnecessarily switched off.

When the mains voltage returns to its normal value after the load has been switched off, the capacitor C₄ starts charging through R₃, and once the voltage across it reaches 2/3 Vcc, the output of IC₃ goes low, actuating the relay, thereby switching on the load and returning the circuit to its steady state. Again, the time taken by the voltage across C₄ to reach 2/3 Vcc is the delay introduced in the circuit. But before the voltage across C₄ reaches 2/3 Vcc, if the mains voltage goes out of the window, then the charging of C₄ is stopped and the load is not switched on. And C₄ slowly discharges through R₄.

Now consider a peculiar case. If mains voltage fluctuates in such a way that it is within the window defined by VR₁ and VR₂ for a few seconds and outside the window for a few seconds, both time durations being less than the delay introduced in the circuit and such a fluctuation continues for a certain duration, say a few minutes. (Such behavior of course has a very low probability of occurrence.) According to the standards set for this cut-out circuit, discussed so far, during such behavior of mains voltage the load should not be switched off it is in on and vice versa.

After detecting a deviation in the mains voltage, to switch off the load, the circuit calculates the time by discharging C₄ and by charging it to switch on the load when the mains voltage returns to its normal value. Always the resistance in the charging path is lower than that in the discharge path. Hence when the load is on and mains voltage fluctuates as seen above. Since C₄ is replenished at a faster rate. The voltage across it will not go below 1/3 Vcc and the load will not be switched off-conforming to the standard set earlier. But if the load is off, and the mains voltage fluctuates as seen above because of the same reason that c₄ is charged faster than it is discharged.

Check out other voltage stabilizers and protector circuits posted on bestengineeringprojects.com

1. Auto Cut Circuit for Over and Under Voltage
2. Auto Cut for Manual Stabilisers using 741 IC
3. Automatic Cut off Power Supply
4. Single Phase Preventer with Under/Over Voltage Cutout

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