OBJECTIVE

Here is a simple and easy to build electronic gadget that can protect your TV or VCR or any other similar mains operable system from the hazards of operand under voltages. In our countly, Voltage fluctuation as large as ±40% is not unusual   with the result that most of the faults in these systems are due to power supply failure. While TVs and VCRs are more likely to be damaged due to excessive voltage, Refrigerators or Fridges are severely affected by extremely low voltages.

Thus it is important to ensure that your system gets the correct AC mains voltage, of course, within a certain tolerance range which could be something like ±10%Though most of the modern TV sets and VCRs have a power supply section that can take care of fluctuations in the AC mains over a reasonably large range, yet it is not advisable to operate your system (TV or VCR) under stress for a long time. If an excessive voltage for instance Persists for a long time, the power supply section of the system in question will be under stress for a long time. Not only this, the fluctuation could also go beyond the capability of the system’s power supply section. Under these circumstances, the best alternative would be to switch off the mains supply to the system if the mains voltage exceeds or falls below

predetermined limits. This is what this gadget precisely does. With this gadget, you can choose the upper and lower limits and it ensures that the mains reaches the system only when its amplitude is within the window selected bayou. The gadget however also has a provision to bypass the gadget if you wish to do so.

Circuit Description

The heart of the system is the window comparator constituted by two opamps (IC-2 and lC-3) along with peripheral components. The window comparator functions an follows: The output of each opamp depends upon the voltages present at its two inputs. The output is at GND potential when the negative input (voltage appearing at inverting input) is greater than the positive input voltage appearing at non-inverting input) even by a few millivolts. The output voltage goes to +Vcc when the non-inverting input voltage is greater than the inverting input voltage. In the circuit shown, the voltage at the inverting input of the IC-3 and non-inverting input of lC-2 is equal to unregulated stepped down mains voltage.

Infact the voltage at this point is representative of fluctuations in the mains voltage. The other inputs of the opamps (inverting input of IC-2 and non- inverting   input of lC-3) are applied reference   voltages generated from regulated +12V. The

reference voltage to 1C-2 is set at +8v and the one to IC-3 is set at +6V. R9 and R10 are so adjusted   that for the nominal mains input of 230 volts, the voltage at Point (Y) is +7V. Obviously, initially the output of IC-2 as well as IC-3 is at GND.

Transistor Q1 as a result is in saturation i.e. it is fully conducting. Relay RL-l istherefore actuated and its normally open contact closed. The relay contact opens when the voltage at point (Y) exceeds +8V or falls below +6V. In the present circuit, the upper limit can be set anywhere between 240V and 300V whereas the lower limit can be set anywhere between 160V and 200V.

The mains voltage exceeding the upper limit or falling below the lower limit is also Indicated by glowing LEDs. Switch SW-2 can be used to bypass   the circuit.     The power supply portion of the circuit is nothing but a conventional transformer, full wave rectifier, capacitor filter configuration followed by a three terminal regulator of 78XX series (Type 7812) +12V ants as Vcc for transistor Q1 and the two opamps.

PARTS LIST

Resistors and Capacitors

R1 : 47K, 1/4W

R2 : 47K, 1/4W

R3 : 47K, 1/4W

R4 :   100K, 1/4W

R5, R6 : 3.3K, 1/4W

R7, R8 : 2.2K, 1/4W

R9 : 100K, 1/4W

R10 : 47K 1/4W

P1, P2 : 47K (Preset)

C1 : 1000pF, 50V (Electrolytic)

C2, C4, C5, C6 : 0.1µf(Ceramic Disc)

C3 : 10µf  25V (Electrolytic)

semiconductor Devices and ICs

Diodes D1 to D6 : IN 4001 or equivalent

LED-1 and LED-2 : preferably of different colours

Q-1 : Transistor type 2N2907

IC-1 : Three terminal regulator, Type 7812C

IC-2 and 1C-3 : Opamp Type 741C

Miscellaneous

SW-1 : Mains ON/OFF switch

SW-2 : Toggle switch (Rated for mains voltage operation)

RL-1 : 12 volts DC relay with at least one SPST normally open contact

T-1 : 15-0-15, 250mA mains transformer

F-1 : 500mA fuse (tubular type) with holder Mains lead, Solder, metal, 3-pin socket etc.

TESTING GUIDELINES

The gadget can be tested under simulated conditions by connecting an electrical bulb (60 watt or 80 watt) across the output in place of the actual system. The input to the system can be fed from a variac. The input can be varied and the input voltage limits which make the bulb go OFF seen. The test can be repeated for different settings of upper and lower limits and the results obtained compared with the ones theoretically expected.

LEAD IDENTIFICATION

Refer to Fig. 11.4 for lead identification of IC-1/IC-2/IC-3and the diode 1N4001.

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