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General purpose of the circuit
This circuit is a TV Set voltage protector based on a window comparator. It continuously monitors the mains voltage supplying the TV (represented by the blinking white lamp) and disconnects the load whenever the line voltage goes outside a safe operating window centered at 120 Vac ±10%.
The acceptable operating range is:
• Lower limit: 108 Vac
• Nominal voltage: 120 Vac
• Upper limit: 132 Vac
Visual indication is provided by three LEDs:
• Yellow LED: Undervoltage
• Green LED: Normal operation
• Red LED: Overvoltage
Only when the voltage remains inside the window is the refrigerator allowed to remain connected to the mains.
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AC sensing and DC scaling
The 120 Vac, 60 Hz mains voltage is applied to a 100:6 step-down transformer, which provides galvanic isolation and reduces the voltage to a safe low-level AC proportional to the line voltage.
The transformer secondary is then:
• Half-wave rectified by the diode
• Smoothed by the 100 µF capacitor
• Scaled by the 10 kΩ–10 kΩ divider
This produces a DC voltage directly proportional to the RMS mains voltage. In this specific design, the correspondence between AC input and DC sense voltage is:
• 108 Vac → 4.20 V DC
• 120 Vac → 4.68 V DC
• 132 Vac → 5.19 V DC
This DC voltage is the signal that is applied to the comparator inputs.
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Reference voltage generation and adjustment
Two adjustable resistor networks derived from the regulated 10 V supply generate the reference voltages for the window comparator.
These references must be adjusted as follows:
• Lower reference comparator: set to 4.20 V, corresponding to 108 Vac
• Upper reference comparator: set to 5.19 V, corresponding to 132 Vac
Once these references are set, the window limits are fixed and stable, and no further adjustment is required.
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Window comparator operation
The circuit uses two comparators to define the voltage window:
Undervoltage comparator (lower limit)
This comparator compares the sensed DC voltage with the 4.20 V reference.
• When the sensed voltage falls below 4.20 V (line voltage < 108 Vac), the comparator switches state.
• The yellow LED turns on, indicating an undervoltage condition.
• The window condition is violated.
Overvoltage comparator (upper limit)
This comparator compares the sensed DC voltage with the 5.19 V reference.
• When the sensed voltage rises above 5.19 V (line voltage > 132 Vac), the comparator switches state.
• The red LED turns on, indicating an overvoltage condition.
• The window condition is violated.
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Normal window condition and green LED
The outputs of the two limit comparators are combined and evaluated by a third comparator, which acts as the window-valid detector.
• When the sensed DC voltage is between 4.20 V and 5.19 V, neither limit comparator is active.
• In this condition, the third comparator turns on the green LED, indicating normal operation.
At the nominal voltage:
• 120 Vac corresponds to 4.68 V DC
• This value lies comfortably within the comparator window
• The green LED remains ON
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Relay control and load protection
The output of the window-valid comparator also drives a transistorized relay driver.
• Inside the window (4.20 V–5.19 V DC):
The transistor is biased ON, the relay is energized, and mains power is delivered to the refrigerator.
• Outside the window:
The transistor turns OFF, the relay de-energizes, and the refrigerator is disconnected from the mains.
This prevents compressor damage caused by sustained under- or over-voltage conditions.
Final description
In essence, this circuit converts the mains voltage into a proportional DC level, compares it against two precisely adjusted thresholds (4.20 V and 5.19 V), and only allows the TV Set to operate when the sensed voltage remains inside the defined window. Any deviation outside this range immediately removes power and provides a clear visual indication of the fault condition.
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