Versatile Auto Cut -Off Unit

An 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 switches on immediately the voltage falls within the calibrated range. Power interruptions are quite frequent at times and the deviation of mains voltage from its normal safe value for a few seconds is also common phenomenon. But it is undesirable to switch off and on equipment frequently, particularly motors and compressors which undergo greater stress during 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.

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 diagram of versatile auto cut off unit

Circuit Description of Versatile Auto Cut-Off Unit

The circuit diagram of versatile auto cut-off unit shown here includes supply section. IC1(LM393, a dual comparator with open collector outputs) senses the voltages and IC2 (555 timer) 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 low.

One input of each comparator is connected to reference voltage of 3.3 volts provided by 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 volt, the output of IC2 goes to high level and the relay remains in the deactuated state. Hence the load is not switched on. When the voltage across the capacitor crosses 2/3 Vcc, the output of IC2 goes to low level, 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, R4 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 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 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, 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 certain duration, say a few minutes. (Such a 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 a 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. 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.

PARTS LIST OF VERSATILE AUTO CUT-OFF UNIT

Resistors (all ¼-watt, ± 5% Carbon)

R1 = 390 Ω

R2, R6 = 1 KΩ

R3, R7 = 100 KΩ

R4 = 1 MΩ

R5 = 220 KΩ

VR1, VR2 = 1 KΩ

Capacitors

C1 = 47 µF/25V

C2 = 2200 µF/25V

C3, C4 = 100 µF/16V

C5 = 0.01 µF

Semiconductors

IC1 = LM393N

IC2 = NE555

T1 = SL100

T2 = BC107

D1 – D5 = 1N4001

ZD1  = 6.7V, 1W Zener diode

ZD2 = 3.3V, 0.5W Zener diode

Miscellaneous

X1 = 230V AC primary to 9V – 0 – 9V 500mA secondary transformer

RL1 = 6V 100Ω DPDT Relay

  • G.D. anthony

    Is there any method to set the VR1 and VR2?

  • amjad raja

    Dear Krishan,
    I have made this circuit but I am unable to understand to set the VR1 & VR2, Pls describe method
    of the correct setting again. How to set.
    Thanks