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Transformerless power supplies provide a low-cost alternative to transformer-based and switcher-based power supplies.

RESISTIVE POWER SUPPLY

Assume minimum value of VRMS. Assume maximum value of VZ and R.
• VRMS = 110VAC
• VZ = 5.1V
• R = R1 = 2kΩ x 1.1 = 2.2 kΩ (assuming ±10% resistor)
IINMIN = 34.2mA

CAPACITIVE POWER SUPPLY WITH SAFETY CONSIDERATIONS

Capacitive power supply with several UL considerations designed in. A fuse is added to protect the circuit during an over-current condition. Adding R2 in parallel with C1 creates a filter that will attenuate EMI from traveling back onto the line. A varistor, or MOV, provides transient protection.

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