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Refrigerator Hi - Low voltage protector with delayed restart, built around a window comparator, a time-delay stage, and a relay output that connects or disconnects the refrigerator from the mains.
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1. Voltage sensing
The mains voltage (120 Vac, 60 Hz) is first isolated and reduced by a step-down transformer.
The secondary voltage is then rectified, filtered, and scaled, producing a DC voltage proportional to the AC line voltage.
This DC signal represents the actual mains voltage and is used by the comparator stages.
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2. Window comparator
Two comparators continuously monitor the DC sense voltage:
• One detects undervoltage
• The other detects overvoltage
Together they form a window comparator that defines the safe operating range.
• If the voltage is below the window, the undervoltage comparator activates and the yellow LED turns on.
• If the voltage is above the window, the overvoltage comparator activates and the red LED turns on.
• If the voltage is inside the window, neither fault comparator is active and the green LED turns on.
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3. Window-valid signal
When the mains voltage is inside the acceptable range, the circuit generates a window-valid signal.
This signal indicates that the voltage is safe, but it does not immediately power the refrigerator.
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4. Time-delay stage
The window-valid signal feeds an RC time-delay network.
• When the voltage enters the safe window, a capacitor begins charging through a resistor.
• This creates a controlled delay before the next stage can activate.
The delay ensures the refrigerator is not restarted immediately after a voltage fault or power interruption.
This waiting period allows the refrigerant gas to return and pressures to equalize in the compressor and serpentine, preventing hard starts.
If the voltage leaves the safe window at any moment, the capacitor is discharged and the delay is reset.
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5. Relay driver and load control
After the delay has elapsed:
• A transistor driver energizes the relay coil.
• The relay contacts close and mains power is applied to the refrigerator.
If an under- or over-voltage occurs:
• The relay is immediately de-energized.
• The refrigerator is disconnected from the mains.
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6. Overall behavior
In simple terms, the circuit continuously monitors the line voltage, disconnects the refrigerator during unsafe conditions, and only reconnects it after the voltage has remained stable for a defined time, allowing the gas to return before restart.
This provides both electrical protection and mechanical protection for the refrigerator compressor.
Note: Instead of a 10Vdc supply, in real world you have to use a 7810 3 terminal regulator.
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