Over/Under Voltage Cut-off with On-Time Delay

Here is an inexpensive auto cutoff circuit, which is fabricated using transistors and other discrete components. It can be used to protect loads such as refrigerator, TV, and VCR from undesirable over and under line voltages, as well as surges caused due to sudden failure/resumption of mains power supply. This circuit can be used directly as a standalone circuit between the mains supply and the load, or it may be inserted between an existing automatic/manual stabilizer and the load. 

The on-time delay circuit not only protects the load from switching surges but also from quick changeover (off and on) effect of over-/under-voltage relay, in case the mains voltage starts fluctuating in the vicinity of under- or over-voltage preset points. When the mains supply goes out of preset (over- or under-voltage) limits, the relay/load is turned ‘off’ immediately, and it is turned ‘on’ only when AC mains voltage settles within the preset limits for a period equal to the ‘on’ time delay period. The on-time delay period is pre-setable for 5 seconds to 2 minutes duration. using presets VR3 and VR4. For electronic loads such as TV and VCR, the ontime delay may be set for 10 seconds to 20 seconds. For refrigerators, the delay should be preset for about 2 minutes duration, to protect the compressor motor from frequently turning ‘on’ and ‘off’.

In this circuit, the on-time and offtime delays depend on charging and discharging time of capacitor C1. Here the discharge time of capacitor C1 is quite less to suit our requirement. We want that on switching ‘off’ of the supply to the load, the circuit should immediately be ready to provide the required on-time delay when AC mains resumes after a brief interruption,

or when mains AC voltage is interrupted for a short period due to over-/under-voltage

cut-off operation. This circuit is also useful against frequent power supply interruptions

resulting from loose electrical connections; be it at the pole or switch or relay contacts, or due to any other reason.

Here supply for the over- and under-voltage sampling part of the circuit [marked +12V(B)] and that required for the rest of the circuit [marked +12V(A)] are derived separately from lower half and upper half respectively of centre-tapped secondary of step-down transformer X1, as shown in Fig. 1. If we use common 12V DC supply for both parts of the circuit, then during relay ‘on’ operation, 12V DC to this circuit would fall below preset low cut-off voltage and thus affect the proper operation of the sampling circuit. The value of filtering capacitor C4 is so chosen that a fall in mains voltage may quickly activate under-voltage sensing circuit, should the mains voltage reach the low cut-off limit.

In the sampling part of the circuit, wired around transistor T1, presets VR1 and VR2 are used for presetting over- or under-voltage cut-off limits, respectively. The limits are set according to load voltage requirement, as per manufacturer’s specifications.

Once the limits have been set, zener D1 will conduct if upper limit has been exceeded, resulting in cut-off of transistor T2. The same condition can also result when mains voltage falls below the under-voltage setting, as zener D2 stops conducting. Thus, in either case, transistor T2 is cut-off and transistor T3 is forward biased via resistor R3. This causes LED1 to be ‘on’. Simultaneously, capacitor C2 quickly discharges via diode D5 and transistor T3. As collector of transistor T3 is pulled low, transistors T4 and T5 are both cut-off, as also transistor T5. Thus, LED2 and LED3 are ‘off’ and the relay is de-energized.

Now, when the mains voltage comes within the acceptable range, transistor T2 conducts to cut-off transistor T3. LED1 goes ‘off’. Transistor T5 gets forward biased and LED2 becomes ‘on’. However, transistors T4 and T5 are still ‘off’, since base of T4 via zener D4 is connected

to capacitor C1, which was in discharged condition. Thus, LED3 and relay RL1 or load remain ‘off’. Capacitor C1 starts charging slowly towards +12V(A) rail via resistors R6 and R7, and presets VR3 and VR4. When the potential across capacitor C1 reaches 6.8V (after a delay termed as on-time delay) to breakdown zener D4, transistor T4, as also transistor T5, gets forward biased, to switch ‘on’ LED3 and relay RL1 or load, while LED2 goes ‘off’. Should the mains supply go out of preset limits before completion of the on-time delay, capacitor C1 will immediately discharge because of conduction of transistor T3, and the cycle will repeat until mains supply stablises within preset limits for the on-time delay period.

The on-time delay is selected by adjusting presets VR3 and VR4, and resistor R6. Zener diode D3 is used to obtain regulated 9.1 volts for timing capacitor C1, so that preset on-time delay is more or less independent of variation in input DC voltage to this circuit (which would vary according to the mains AC voltage). To switch ‘off’ the relay/load rapidly during undesired mains condition, the timing capacitor C1 is discharged rapidly to provide complete

control over turning ‘on’ or ‘off’ of relay RL1 (or the load). The functioning of the LEDs and relay, depending on the circuit condition, is summarised in Table I.

Schematics:

Supply Unit

Stabilizer main circuit

Component Required:

R1,                  1K ohm

R2,                 470 ohm

R3,                 5.6K ohm

R4,                  1K ohm

R5,                  270K ohm

R6,                  470 ohm

R7,                  6.8K ohm

R8,                  5.6K ohm

R10-R11,       1K ohm

C1,                  220uF/25v electrolyte cap

C2,                  100uF/25v electrolyte cap

C3,                  2200uF/25v electrolyte cap

C4,                  100uF/25v electrolyte cap

T1-T5,            BC547

T6,                  BC557

D1,                  6.8v Zener Diode

D2,                  5.6v Zener Diode

D3,                  9.1v Zener Diode

D4,                  6.8v Zener Diode

D5,                  4148 Diode

D6-D8,           1N4007 Diode

VR1-VR2,      5K pot

VR3-VR4,      220K pot

LEDs,              Red (2), Green

RL1,                12v Relay

X1,                   230 to 12-0-12 500mA transformer